Joint Sealants  



The following information on joint sealants has been designed as a supplement to existing joint sealant standards for North America. The approach is to provide high level information on joint sealants and a supportive narrative on seals and brief mentions of alternate technologies like pressure sensitive adhesives (PSAs) in bonding structural glazing. The intent is to provide the reader with design and selection considerations and lead the reader to existing standards that provide the comprehensive information and performance criteria for joint sealants.

Materials used for sealing have evolved over thousands of years from relatively low performing (mortar and tar or pitch as a sealant) to high—performance sealants. Naturally occurring bitumen and asphalt containing materials have been widely accepted as sealants for many centuries. Prior to the 1900's most sealants evolved from vegetable, animal, or mineral substances. The development of modern polymeric sealants coincided with the development of the polymer industry itself, starting in the early 1930's.

Joint sealants are used to seal joints and openings (gaps) between two or more substrates and are a critical component for building design and construction. The main purpose of sealants is to prevent air, water, and other environmental elements from entering or exiting a structure while permitting limited movement of the substrates. Specialty sealants are used in special applications, such as for fire stops, electrical or thermal insulation, and aircraft applications.

Sealants are used for a variety of commercial and residential applications. Common sealants include silicone, acrylic, urethane, butyl, and other polymer types. Various formulations have been developed over the years, which meet performance specifications established by industry standards, as well as for the specific and unique needs of the end user.

nine different tubes of sealant

Figure 1. Sealant Selection is important. Contacting the manufacturer in the design process is recommended.
Source: Fine Homebuilding

It is important for anyone specifying sealants to understand the particular set of conditions to which the sealant will be exposed. This includes at a minimum, the amount of joint movement, temperature extremes, substrate types and whether the sealant will be subject to immersion in water, and UV exposure. Failure to understand these basics could lead to the wrong sealant selection and ultimately failure of the sealant in use.


Selection of Joint Sealants

The proper application of a sealant involves not only choosing a material with appropriate physical and chemical properties, but also having a good understanding of joint design, substrates to be sealed, performance needed, and the economic costs involved in the installation and maintenance of a joint sealant.

illustration of joint design and placement
worker applying sealant to exterior of window

Figure 2. Proper Joint Design and Placement are critical in managing water and air infiltration.
Source: ASC

Figure 3. Installation and knowing sealant compatibility with substrates is critical to maximize adhesion.
Source: Albion Engineering

Typical considerations for selecting a sealant type for use in the construction industry include:

  • Joint Design: The specifics of a joint design must match up with a sealant's movement capabilities for the installed conditions. The practicality of installation of the sealant and other joint elements and the desired aesthetic appearance also need to be considered.

  • Physical and Chemical Properties: Properties of the sealant such as, modulus of elasticity, its stress/strain recovery characteristics, tear strength, and fatigue resistance are all factors that influence sealant performance in a joint. The chemical makeup of the polymer used to prepare the sealant along with additives such as fillers and plasticizers will affect the performance of the product.

  • Adhesion: The ability of the sealant to stick to the various substrates is critical to the performance. This is especially true in moving joints where an early adhesion failure can lead to loss in performance of the sealant. In certain situations, a primer can be recommended to improve this critical property.

  • Durability Properties: The preservation of a sealant's performance to a specific substrate(s) over the design life of the sealant. The sealant durability relates to sealant's resistance to environmental strains among others ultra-violet radiation, moisture, temperature, cyclic joint movement, movement during curing, and biodegradation which can profoundly influence the service life of the installed sealant.

  • Application/Installation Properties: Important considerations include the consistency of the sealant (pourable or gunnable), pot life, and skin over time (tooling time), tack free time, application temperature range, and low temperature "gun ability" (i.e., ability to be dispensed easily by sealant gun). Sealants used for interior applications, even in high-rise or light commercial structures, will have properties and needs different from those used in other applications, such as structural sealant glazing or exterior building facade seals.

  • Compatibility with other Materials: The sealant must be able to be applied in contact or near another construction material without compromising the sealant's cure profile, its ultimate performance, or the other materials' (substrate) performance. Negative performance indicators can include color change, swelling, hardening, staining, gloss change, softening, and cracking.

Key Features of Sealant Chemistries

Joint sealants are available in two forms: Liquid-Applied and Pre-Formed. The following references to Class indicate movement potential (e.g., Class 25 indicates ± 25% movement). The following table lists the main types of liquid applied sealants followed by a brief description.


VOCs are generated by sealant formulations generally from two distinct sources. The first are solvents that are added to the formulation. The second are small molecules that are generated during the cure. The levels of these VOCs are controlled by the manufacturer. Check with the manufacturer for VOC compliance for various parts of the United States.

Reactive Sealant Chemistry Type Subgroup
Polyisobutylene (PIB) Sealant n/a
Polysulfides n/a
Polyurethanes n/a
Silicones n/a
Oil-Based Caulk n/a
Silane Modified Polymers Silane-modified Polyethers (includes silyl-modified polyethers)
Silane-modified Polyisobutylene (SPIB)
Silane-modified Polyurethanes (Includes Silyl-modified Polyurethanes)
Silicone-modified Polyacrylate
Polyurea backbone
Mixed polymer
Others (not included)
Water-Base & Solvent-Base Evaporative Chemistry Types Subgroup
Acrylic / acrylic copolymers Water based latex
Vinyl acetate Copolymer
Butyls (polyisobutene) Solvent based PIB sealants
Gun-dispensable and pumpable sealants
Hot applied PIB sealants
Preformed PIB strips
Elastomeric polymers Synthetic rubber
Synthetic block copolymer
Others (not included)
  • Oil Based Caulk
    • This is an older technology that has been generally replaced by more modern joint sealants.
    • Clean up with mineral spirits or naphtha.
    • Oil-based asphalt caulk is suitable for some venting, flashing and outdoor applications.
    • Use caution when working with oil-based asphalt caulk as it is very thick and dries quickly.
  • Polysulfides
    • First "high-performance" sealant chemistry; mainly used in industrial applications.
    • Products rated for movement meet (ASTM C920, Classes 12½ and 25).
    • Poor recovery limits their use in joints with moderate cyclic movements.
    • Can be formulated for excellent chemical resistance (especially for aviation fuel).
    • Good performance in submerged applications.
    • Require a primer on almost all substrates.
  • Polyurethanes
    • Used in industrial and commercial applications.
    • Very good movement capabilities (ASTM C920, Classes 12½, 25, and 50).
    • Not used in structural glazing applications (avoid direct contact to glass if sealant is not UV shielded).
    • Excellent adhesion, generally without a primer for many substrates (see manufactures recommendations for specific substrates).
    • Can be formulated for good UV resistance.
    • May stain some types of porous materials such as concrete and natural stone
    • Paintable
    • Some formulations may contain low levels of solvent
    • Polyurethane sealants do not give off a VOC during cure if no solvent is added to the formulation
    • Polyurethanes can be formulated for excellent durability
  • Silicones
    • Structural sealant glazing of glass to metal framing systems
    • Excellent joint movement capabilities (ASTM C920, Classes 25, 35, 50, and 100/50)
    • Excellent UV resistance and heat stability
    • Good adhesion to many substrates especially glass; a primer is recommended on some substrates, particularly cementitious substrates
    • Most formulations are not paintable
    • Used in industrial, commercial and DIY applications
    • Used in vandal resistant, missile impact, and blast resistant glazing systems and to insulate glass to improve thermal performance (reduce heat loss).
    • Acetoxy chemistry based sealants have an acidic acid odor while curing, but newer chemistries have very low odor
    • High, medium and low modulus materials available
    • May stain some types of porous materials such as concrete and natural stone
    • Non-staining and non-bleeding formulations are available where aesthetic considerations are important
    • Silicone sealants have a high moisture vapor transmission rate and thus not an appropriate as a primary seal in insulated glass units
  • Polyisobutylene (PIB) Sealants
    • Polyisobutylene (PIB) is a fully saturated, aliphatic polymer of high commercial importance due to its gas barrier properties and can be formulated with high chemical/oxidative stability
    • A principal use is a primary seal in high grade Insulated Glass Units
  • Silane Modified Polymers
    • Silyl-modified polymers (SMP; also, silane-modified polymers, modified-silane polymers, silane-terminated polymers, etc.) are polymers terminating with a silyl group.
    • Used in industrial, commercial and Do It Yourself applications
    • Excellent adhesion to non-porous substrates
    • Good UV performance (resistant to yellowing)
    • Generally require primer on porous substrates
  • Acrylic (solvent containing)
    • Used in residential and light commercial construction, mainly for exterior applications
    • Generally have a maximum of ± 7½% movement (ASTM C1311)
    • May need special handling for flammability and regulatory compliance
    • Can be painted
    • Short open time; difficult to tool
    • Exhibit some shrinkage after cure
    • Often used for perimeter sealing; low movement joints
  • Latex (water-based, including Ethyl Vinyl Acetate (EVA), Acrylic, Acrylic Co-polymers, Modified Acrylics)
    • Used mainly in residential and light commercial construction applications
    • Interior and exterior uses
    • Premium products rated for movement meet (ASTM C920, Classes 12½ and 25)
    • Products not rated for movement meet (ASTM C834)
    • Excellent paint ability (with latex paints)
    • Some Acrylic Sealants have been formulated for good durability (those that can pass ASTM C920)
    • Exhibit some shrinkage after cure
    • Sometimes referred to as caulk
    • Not used for exterior applications on high rise construction or for applications undergoing cyclic movement greater than ±25%
  • Butyls and Elastomerics (solvent containing)
    • Excellent adhesion to most substrates
    • Generally have a maximum of ± 7½% movement (ASTM C1311)
    • Excellent weathering
    • Good use as adhesives in industrial and packaging applications
    • Sometimes used in curtain wall applications where adhesion to rubber compounds is required
    • Most are stringy and difficult to apply neatly
    • May show some shrinkage after cure; may harden and crack over time on exposed surfaces
  • Synthetic Block Copolymer
    • Elastomeric sealant characterized by the ability to be in a relaxed state when stretched or compressed
    • Excellent paintability
    • Easy to repair with itself
    • Cures in all weather and has good weatherability
    • Excellent adhesion to a wide variety of substrates
    • Can be formulated for clarity
    • Can be applied to wet surfaces
    • Immediately water resistant
    • Typically have high VOC content and low solids
    • Can accommodate joint movement for intended application
    • Freeze thaw stable
    • Principally used in exterior applications such as roofing, siding, and windows
    • Used in residential, commercial, industrial, and DIY applications

Factory Pre-formed

  • Dense and Cellular Gaskets
    • Generally used to seal glass in openings and joints between metal panels and usually installed under compression
    • Dense and cellular rubber products are generally formulated from EPDM, neoprene, silicone, and thermoplastic polymer of
    • Cellular non-silicone products meet (ASTM C864), For dense products most meet (ASTM C509), silicone products meet (ASTM C1115) and thermoplastics meet (ASTM E2203)
  • Extruded and Molded Seals (pre cured seals and sealants)
    • Fabricated primarily from silicone formulations, some polyurethane formulations are also available
    • Silicone formulations meet (ASTM C1518, Classes 12½ to 200)
    • Provided as extruded strips in various standard colors and widths from 1 to 6 inches (25 to 152 mm)
    • Allows the sealing of difficult joints
    • Provided as custom molded components for corners and other transition areas used with joints that are difficult to seal
prefabricated window unit using gaskets and silicone sealants
series of different extruded rubber gaskets
series of water stops

Figure 4. A prefabricated window unit using gaskets and silicone sealants.
Source: Tremco

Figure 5. A series of different extruded rubber gaskets.
Source: Tremco

Figure 6. A series of water stops to seal between pours of construction in water-tight construction.
Source: Sika

  • Compression systems
    • Preformed, pre-compressed products usually fabricated of a polyurethane, acrylic impregnated cellular component with a silicone rubber exterior face
    • After removal from the packaging and insertion in the joint opening the foam expands to be compressed in the joint. The silicone rubber face is sealed to the substrates. Products are available to provide water or water and fire resistance.
    • Typical products are capable of ± 50% (total 100%) joint movement.

Pre-Compressed Foam Tapes

Pressure sensitive tape with a release liner.
Pressure sensitive tape being used as seal with Log Home wall
pre-compressed tape with a pre-cured silicone facer

Figure 7. Typically comes supplied with a Pressure Sensitive Tape on one side with a release liner.
Source: Sika

Figure 8. The tape is being used as seal with Log Home wall allowing for expansion and contraction.
Source: Sika

Figure 9. A pre-compressed tape with a pre-cured silicone facer commonly used in expansion joints with high movement.
Source: Sika


Sealants are used to seal joints and openings in various architectural applications, which can include the following:

  • High- and low-rise commercial buildings:
    • Exterior and interior perimeter of windows
    • Roofing and flashing penetrations and terminations
    • Building and material expansion joints
    • Interior perimeters of doors, baseboards, and moldings
  • Plazas and parking deck joints in traffic surfaces
  • Joints between tilt-up concrete exterior panels
  • Airport runway and apron pavement joints
  • Bridge and highway pavement joints
  • Sidewalks, parking lots, and flat work joints
  • Water and wastewater treatment facility joints (including in submerged environments)
  • Part of a fire and smoke stop assembly for joints and penetrations
  • Structural sealant glazing

Weatherproofing Applications

Weatherproofing is intended to keep rain and other environmental elements from entering a building. To achieve successful weatherproofing sealant joints at least the following parameters must be considered, and where applicable, designed for:

  • "Joint Movement"—Occurs as a result of changes in material temperature, seismic movement, elastic frame shortening, creep, live load, concrete shrinkage, moisture-induced material movement, and inadequate joint design. Joint movements must be evaluated, designed for, and accommodated.
  • "Movement Capability"—The ± percent value that indicates the amount of movement the sealant can take in extension (+) and compression (-) from its original cured joint width. Movement capability and expected joint movement must be coordinated.
  • "Adhesion"—The ability of the sealant to adhere to the specific substrates chosen in the application. It is important to ensure that the proper surface preparation of the substrates to achieve the desired adhesion is evaluated and detailed in the specifications.
  • "Installation Tolerances"—Depending on the substrate materials the joint opening will have a ± tolerance for its designed width which must be considered when establishing width (e.g., for a joint width of 1/2 inch (13 mm) in a brick masonry wall the tolerance for its width could be ±1/8 inch (3 mm) or more)
  • "Sealant Selection"—Select the appropriate polymer chemistry type for the application; however, within a polymeric type there can be a wide range in performance capabilities and properties. All sealants in that type may not perform similarly; therefore, it is important to research product capabilities prior to use.

Joint Types

Moving Joints

Moving Joints experience cyclic movement. They are joints where the shape and size of the sealant joint changes significantly when movement occurs, for example, at control, expansion, and isolation joints. There are three types of moving construction joints.

  • Control joint: A formed, sawed, tooled, or assembled joint acting to regulate the location and degree of cracking and separation resulting from the dimensional change of different elements of a structure. DISCUSSION—The joint is usually installed in concrete and concrete masonry construction to induce controlled cracking at preselected locations or where a concentration of stresses is expected. ASTM C717
illustration of control joint
illustration of control joint with sealant applied

Figure 10: A — Sawed or assembled joint made to regulate cracking from shrinkage.
B — Natural crack formed by shrinkage.
Source: Henkel

Figure 11: A — Sealant
B — Sealant backing
C — Natural crack formed by shrinkage.
Source: Henkel

  • Expansion Joint: A formed or assembled joint at a predetermined location, which prevent the transfer of forces across the joint as a result of movement or dimensional change of different elements of a structure or building. ASTM C717
illustration of expansion joint
illustration of expansion joint with sealant applied

Figure 12: A — Expansion Joint Gap
B — Substrate Edges that Form Expansion Joint
Source: Henkel

Figure 13: A — Sealant
B — Sealant Backin
Source: Henkel

  • Isolation joint: A formed or assembled joint specifically intended to separate and prevent the bonding of one element of a structure to another and having little or no transference of movement or vibration across the joint. ASTM C717
isolation joint
isolation joint with sealant applied

Figure 14: A — Isolation Joint Gap
B — Dissimilar substrates that form the point
Source: Henkel

Figure 15: A — Sealant
B — Sealant backing
Source: Henkel

Sealant Joint Types

Moving joints must not only allow for dimensional changes, but also protect against air, water, and other environment contaminants.

Sealants, when applied to these joints, typically provide protection from air, water, and other environmental contaminates. Weatherproofing the joint is an application used to integrate sealants, backing materials, and joint substrates to support resistance to weathering.

There are several factors to be evaluated when establishing the required width of sealant joint. Paramount among them is designing the sealant joint for the anticipated movement, construction tolerances, and other effects known to influence the movement capability of a particular application.

Under no circumstances should sealant be applied in a joint opening that is less than 6 mm (0.25 in.) wide. It is very difficult and impracticable to install sealant effectively in such a small width and is generally not recommended by most sealant manufacturers. Generally, for a joint width over 50 mm (2 in.) a liquid-applied sealant in a vertical joint may sag before curing. ASTM C1193

There are four types of sealant joints:

  • Butt joint: A joint where sealant is applied within the joint between approximately parallel substrate surfaces that are face-to-edge or edge-to edge. ASTM C717
illustration of butt sealant joint

Figures 16 and 17: Butt Sealant Joint
Source: Henkel

  • Fillet Joint: A joint where sealant is applied over the joint to the face of substrates that are approximately perpendicular to each other. ASTM C717
illustration of fillet sealant joint

Figure 18: Fillet Sealant Joint
Source: Henkel

  • Lap Joint: A joint where sealant is applied within the joint between approximately parallel substrates that are face-to-face. ASTM C717
illustration of lap sealant joint

Figure 19: Lap Sealant Joint
Source: Henkel

  • Bridge Joint: A joint where sealant is applied within the joint between approximately parallel substrates that are face-to-face. ASTM C717
illustration of bridge sealant joint

Figure 20: Bridge Sealant Joint
Source: Henkel

Common Problems

  • Sealants are often the least thought about and contribute the lowest percentage to a project's overall cost (less than 1%); however, they can become a serious or, for hidden or concealed joints, impossible problem to correct when a sealed joint fails.
  • There is both a science (joint design, adhesion, and compatibility testing) and an art (sealant and joint components installation) to successful completion of a functional sealant joint.
  • Sealants cannot make up for poor substrate conditions, poor installation practices, or improper or poor joint design. They must have:
    • Proper joint design especially for joints that experience movement
    • Selection of a durable sealant product appropriate for its function and environmental exposure
    • Proper surface preparation, including the use and complete curing of primers when necessary. This also includes the careful use of any solvents and primers that could interfere with the curing of the sealant. Careful adherence to the manufacturer's instructions should alleviate any issues with the primers.
    • Adhesion to the substrates
    • Proper sealant, sealant backing and joint filler installation, as well as proper overall placement of the sealant including tooling of the joint
    • Be compatible with the various materials of construction of the joint and other materials of construction to which they will come in contact.

Failures in any of these areas can lead to premature failure of the sealant causing leaks and potentially other failures.

Sealant Adhesive Failure
Sealant Cohesive Failure

Figure 21: Sealant Adhesive Failure
Source: Gorman Moisture Protection Inc.

Figure 22: Sealant Cohesive Failure
Source: Gorman Moisture Protection Inc.

General Joint Design

The information in these s should be followed when designing and installing sealant joints:

In general, at least the following should be considered.

Joint Location, Spacing, and Condition

  • The location of joint openings must permit applicator access to install joint components and to properly tool the sealant
  • The spacing of joints contributes to establishing joint width (wider spacing = wider joints)
  • The substrate(s) for sealant adhesion must be sound and free of deleterious materials that would compromise adhesion
    • Window perimeters must provide a minimum 1/4; inch (6 mm) wide surface for the sealant to bond to
    • At the butt joints of exposed aggregate precast concrete panels the aggregate must be deleted and a smooth concrete surface provided for sealant adhesion
    • At every termination detail there must be adequate access and sufficient bonding area for sealant application
    • The sealant and substrates must be compatible with the materials of construction.

Design for Sealant Movement

  • For Weatherproofing, a minimum depth of 1/4 inch (6 mm) for the sealant to substrate bond. A minimum width of 1/4 inch (6 mm) opening is necessary to ensure that sealant applied from a caulking gun will flow into the sealant joint properly.

  • For Moving Joints, also need to consider:
    • Use joints greater than the minimum of 1/4 inch (6 mm width), since wider joints can accommodate more movement than narrow joints and can also result in a greater joint spacing.
    • Use a sealant backing or bond breaker tape to eliminate "three-sided adhesion." The sealant should bond only to the substrates that will be moving.
    • Use a 2:1 width to depth ratio to accommodate movement. Create an "hourglass" shape for the sealant profile.
    • For a joint size larger than 1 inch (25 mm), the depth should be kept to about 3/8 inch (10 mm).
    • Practically, a sealant joint should not be greater in width than 2 inches (50 mm).
    • The number and spacing of joints is critical to performance.
  • Placing/Installing/Applying the Sealant
    • Use sealants that have not reached the end of their package shelf-life.
    • Mix multi-component sealants properly (do not entrain air)
    • Tape the outside edge of joints if necessary to prevent overlapping sealant application or to keep crisp lines and insure no sealant smears on the substrate.
    • Apply the primer as directed by the sealant supplier and be sure the primer has had the minimum reaction time before the sealant is applied.
    • Gun the sealant into the joint opening at constant pressure and flow
    • Prevent overlapping sealant (follow ASTM and SWR International Guides)
    • Dry tool the sealant surface to completely fill the joint opening with sealant thereby wetting the entire substrate adhesion area
    • Check the quality of the work frequently and keep samples
    • Maintain a project log (e.g., sealant lot No., weather conditions, application procedure)

Sealant Selection

  • For movement joints only use a sealant that has a current Validation Certificate from the SWR Institute (Sealants, Waterproofing and Restoration Institute Product Validation). This program provides a listing and certificates for all validated products.

  • Will the selected sealant accommodate the anticipated joint movement requirements? This can be determined from the ASTM C920 Class that a sealant has achieved. Class 12.5 means the sealant can provide ±12.5% joint movement. Class 25 means ±25% joint movement, etc. Please make sure that the joint movement needed and the Class of joint movement for the sealant match. ASTM C719

ASTM C719 specimen shown under varying stages of tension and compression
ASTM C719 test preparation and configuration

Figure 23: ASTM C719 specimen shown under varying stages of tension and compression.
Source: Momentive Performance Materials

Figure 24: ASTM C719 test preparation and configuration.
(View enlarged illustration)
Source: Momentive Performance Materials

  • Will the sealant adhere to the substrate(s) properly? This is probably the most critical element in the selection process. This can be determined from ASTM C920 and ASTM C794 where adhesions to common substrates (Aluminum, glass or mortar) are listed in the certification for the sealant. In all cases where there is any doubt about adhesion a determination by the manufacturer in addition to a field test ASTM C1521 preferred or mock-up of the actual installation is highly recommended to ensure required adhesion is achieved.

  • Will the sealant have the requisite durability for the anticipated movement and environmental exposure? Sealant durability depends on many factors. Proper joint design for the anticipated movement, correct surface preparation, correct installation, and adequate adhesion to the substrates, are all required for a sealant joint to perform. Once these factors have been satisfied then the durability of the sealant depends on its ability to survive in the environmental conditions of the application. That is heat, moisture, UV, and joint movement, all of these factors will influence the durability of the sealant. The best source of information on durability must be the sealant manufacturer. There are several practices to understand this performance but this information is not available for every sealant. Sealant suppliers can also provide case studies that can be used to predict if sealant will perform correctly under a similar set of conditions. Finally, there is warranty information that can be considered.

  • Is the sealant compatible with adjacent materials? Sealant manufacturers should be able to provide information about compatibility of their sealant with various materials that are used on construction sites. If the job being quoted has new materials that have not been tested then a compatibility test by the manufacturer is strongly recommended.

  • Does the joint opening width allow for sufficient placement of sealant and other joint components? This question is first about access for the installer. In addition, the conditions at the time of installation have an impact. It is not appropriate to install a sealant when the joint is at the extremes of its movement! Ideally the sealant should be installed at the midpoint of the design range of the sealant. For commercial construction (thermally driven movement) the installation should not occur at an extreme temperature where the properly designed joint is not too narrow or wide for proper installation. If the joint as installed is too small to handle the expected movement or is in a place where sealant placement or joint cleaning would be difficult at best, consider using a pre-cured sealant strip that is adhered to the face of the substrate and bridges the gap. This is especially useful when the joint has aluminum or glass substrates that can't be cut out to make a larger joint.

  • Will the sealant perform under the anticipated conditions of use? This should be based on input from the manufacturer. This is conditioned on the user or specifier knowing the conditions of use (movement, temperature, light, water, chemicals etc.) exposures expected and these are conveyed to the manufacturer, who should then comment on the ability of the product based on that input.

  • Is there a history of application success for the sealant in the specific application? This should be based on input from the manufacturer.

  • Do the sealant manufacturer and supplier have the necessary in-house resources to support your application should problems develop? Ensure that communication is proper between the manufacturer and applicator and/or user as to what technical service and problem solving is available and how it can be sourced. This should be detailed in the pre-job conferences

  • Has the sealant manufacturer verified by laboratory testing that the selected sealant is compatible with adjacent substrates? For Structural Glazing applications use ASTM C1087. For other applications see the section on compatibility below to determine if adequate adhesion to those substrates has been completed. ASTM C719 (1,2) & ASTM C1635. If aesthetics is a concern that it will not stain adjacent porous surfaces ASTM C1248. Compatibility issues should be resolved in pre-job communications with all relevant parties involved.

  • For high profile applications consider using third-party quality control to confirm product performance.

Substrate Surface Preparation

  • The most common mode of sealant joint failure is loss of substrate adhesion.
  • All weak material must be removed from the sealant adhesion surface of porous substrates including any form release agents.
  • Adhesion surfaces must be clean, dry, and free of dew or frost.
  • To prepare substrates use best practices as recommended by ASTM and SWR International Guides as well as specific instructions from the manufacturer of the sealant:
    • SWR Institute—Sealants: The Professionals' Guide;
    • ASTM C1193 (see section 7 on surface cleaning and section 8 on priming)
    • For porous substrates: oil-free compressed air, abrasive blasting, high pressure water (allow to dry), grinding, or wire brushing
    • For non-porous substrates: use the two lint-free rag method—wipe with solvent on the first rag immediately followed by a dry rag wipe

If priming is required, the primer should be applied and allowed to dry, per the manufacturer's instructions prior to application of the sealant.


  • Priming is required when recommended by the sealant manufacturer. The primer to use will also be recommended by the sealant manufacturer and is substrate dependent. If there are any doubts about adhesion of the sealant to a specific substrate then an adhesion test per ASTM C794 or equivalent needs to be conducted. SWR Institute provides some primer guidance in its Technical Bulletin #7  under "Priming."
  • With few exceptions, prime substrates before insertion of the sealant backing
  • When applying the primer ensure installation is in accordance with manufacturing instructions.
    • Commonly observed mistakes include:
      • over application
      • under application
      • multiple use of single applicator tools (dapper or brush)
      • insufficient time for the cure of the primer
  • Priming will not overcome a poor joint design or poor installation in preventing an early failure of the sealant bond.
  • Many sealants perform well without the use of primers
  • With few exceptions, if any, primers should be used for horizontal and submerged joints. This includes traffic joints, and all horizontal joints open to the elements.
  • Almost all sealants, to achieve adequate adhesion for moving joints, require cement-based substrates to be primed.
  • Primers must be installed properly (e.g., use the correct primer, do not apply to thick or too thin, and require drying or curing time before sealant application). Information on priming should be in the technical literature supplied by the sealant manufacturer.

Joint Filler and Sealant Backing Materials: Why Use Them

  • A joint filler is inserted into joints that are deeper than required for the sealant backing and sealant installation.
  • For moving joints the joint filler and sealant backing must be compressible.
  • Sealant backing establishes sealant depth and helps to achieve an hourglass sealant profile.
  • Sealant backing provides resistance to sealant tooling pressure and helps to attain proper wetting of the substrate when sealant is tooled.
  • Sealant backing and bond breakers prevent detrimental 3–side adhesion for moving joints.
  • Recommended Materials
    • Closed cell sealant backing: primarily a polyethylene foam with a surface skin
    • Open cell sealant backing: primarily a urethane foam without a skin
    • Bicellular sealant backing: composed of both open and closed cell polyethylene or polyolefin foam with a surface skin
    • Bond breaker tape: primarily a self-adhesive polyethylene or Teflon material
  • Not Recommended Materials
    • A rigid joint filler or sealant backing in joints that will experience movement
    • Using braided sealant backing to compensate for a joint opening that is too large for it
Joint Sealant Backing Material Overview (Backer Rod)
Type of Backer Rod Characteristics Common Uses Selection Considerations
Closed Cell

Closed cell foam is distinct in that its cells are totally enclosed by its walls and not interconnecting with other cells. Will not wick moisture.

Ideal in flat or horizontal joint applications. Fenestration perimeter seal backer, Expansion joints, Log Construction, Pre-Cast, Pavement Joints, Partitions, Pavement applications, repairs

For joints susceptible to the presence of moisture prior to joint sealing such as horizontal joints. Backer rod should be installed carefully because if it is punctured and then sealant installed directly over top a void is created. That void, filled with air, will then compress and expel air as it is heated through typical day to day temperatures and sunlight causing unsightly bubbles. This is commonly referred to as outgassing.

Open Cell

Highly conformable, unrestricted air and moisture vapor transmission return to their normal shape once pressure is removed.

Expansion & Contraction Joints, Window Glazing, Parking Decks, Precast Assemblies, Curtain wall, bridge construction

May not be ideal for sealants that have potential to bubble if air escaping the backer material. Ideal for applications where a large amount of movement is expected or compression will be necessary. More airflow allows it to dry smoothly from both sides and can substantially reduce curing times. Open cell backer rod should not be used in flat or horizontal joints that can have water ponding on them as they can wick moisture to the underside of the sealant.


Will not wick moisture due to its closed cell exterior, extremely flexible and can be used with cold-applied sealants.

Irregular joints, horizontal or flat work, expansion and contraction joints, window glazing, curtain wall construction partitions, parking decks, bridge construction, modular home gasketing, and log home chinking

This is suited for specialty applications where standard backer rods are not appropriate and is ideal in irregular joint applications where self-leveling, flowable sealants are employed.

Note: Contact backer rod manufacturer for proper selection and application.

Keys to Success

  • Closed cell sealant backing must be no more than 25 to 33% larger than joint width so it remains in compression and in place during sealant installation.
  • Open cell and bicellular sealant backing must be at least 25% larger than joint width so it remains in compression and in place during sealant installation.
  • Don't poke holes in closed cell sealant backing material during installation, this can cause air bubbles in the sealant compromising its ability to perform.
  • Properly installed sealant backing prevents sealant leakage and creates a proper sealant profile.
  • Sealant backing function ceases once sealant is applied, tooled, and cured.

Compatibility Testing

Compatibility of a sealant with the different materials to which it will come into contact is a critical topic for the long-term success of the sealant joint. There are really three issues when thinking about compatibility:

  • In the short term, does the sealant cure correctly in contact with the other materials used in the construction of the joint?
  • The long-term issue involves whether the sealant will perform as intended in contact with the other materials of construction of the joint.
  • It is critical that the sealant develops enough adhesion to the substrate to function correctly in the joint as designed.

An example of the first type would be a material that when curing generates an alcohol. The alcohol would interfere with the cure of a polyurethane. Therefore, if any of the materials of construction in a joint where a polyurethane sealant is placed generate alcohol during cure, then the cure of that material must be completed before the polyurethane sealant is placed. There are many examples of these types of interferences and the manufacturer of the sealant should be able to supply the types of materials to avoid.

The second type of compatibility issue involves diffusion of some material from the sealant into the other material of construction or from the other material of construction into the sealant. There are several industry test methods to evaluate compatibility.

AAMA/FGIA 713 Voluntary Test Method to Determine Chemical Compatibility of Sealants and Self-Adhered Flexible Flashings is one where fresh sealant is placed in contact with self-adhered building flashing materials and then the assembly is placed in an oven for 14 days and the assembly is examined for discoloration, slump, degradation and liquefaction.

Another is ASTM C1087 which is specific for compatibility in a structural glazing assembly. In this test the sealant along with the material to be tested are placed in an assembly with a reference sealant. Assemblies with and without the material to be tested are prepared and then are aged under UV and heat for 21 days. Visual inspection of the control assemblies, without the material to be tested are compared to the assemblies with the material to be tested. A reference to also consider for compatibility of PIB sealants can be reviewed from ift Rosenheim: ift-Guideline DI-01engl/1, 2009, "The usability of sealants Part 1, Testing of materials in contact with the edge-sealing of insulating glass units," ift Rosenheim, Rosenheim, Germany, 2009. Readers may also consider Part 2 ift Rosenheim: ift-Guideline DI-02/engl/1 "The usability of sealants Part 2, Test of Materials in contact with the edge of laminated glass and laminated safety glass", ift Rosenheim, Rosenheim, Germany, 2009.

Other tests can provide the same information as these two standard test methods. Manufacturers have devised their own way of testing compatibility. Some use a modified version of ASTM C794 where adhesion in peel is tested. The basics require the following: An assembly with direct contact between the sealant and the other construction material; Aging the assembly at an elevated temperature, 50°C (122°F), 60°C (140°F) or 70°C (158 °F), to accelerate any diffusion for several weeks to 1 month; Evaluation of the assembly to look for signs of impact of either the sealant on the construction material or the construction material on the sealant. This can be softening, embrittling, color change, loss in adhesion, etc. A control sample of the sealant and construction material that are not in direct contact also needs to be exposed to the same temperature for the same length of time to make sure any changes noted are due to the direct contact between the two and not just aging of one or the other.

Adhesion testing should be conducted on any of the assemblies used to test for compatibility. When a number can be generated as in ASTM C794 it should be compared to the value that the manufacturer deems minimum acceptable for that sealant. Different sealants will have different acceptable values since the modulus of elasticity and hardness varies with the sealant and thus the force on the bond varies with the sealant. In the absence of manufacturers' data, a reasonable default can be found in ASTM C920. If other types of test have been conducted, then a failure mode of mostly cohesive failure or substrate failure generally indicates adequate adhesion.

Preconstruction Meetings

For large complex projects, a preconstruction meeting is required. In these meetings there should be a review of the details of the sealant installation, including primer application. The drawings need to be reviewed and especially important is clarity around the transition points as these are often the areas where failures occur.

The schedule should be reviewed to ensure that all of the pre-work has been completed before the sealant is applied.

There should also be discussion on what situations would lead to a work stoppage.

In complex applications a mockup is strongly recommended as this can flesh out issues that are not always apparent from the drawings.

mockup building with 3M air barrier and Pella windows installed

Figure 25: Mockup Reference, Project: TITILETOWN Phase 2 Multifamily.
GC: Pepper Construction Air Barrier System: 3M Air Barrier 3015VP.
Photo Credit: 3M

Structural Sealant Glazing

Structural sealant glazing involves attaching glass, metal, or other panel materials to a building's window or curtain wall metal framing system in place of using gaskets and other mechanical attachments. Only structural quality silicone sealants are used since they must be able to withstand ultraviolet radiation, weathering effects, wind loads, and other stresses and transfer these affects to the metal framing system. The information provided in the standards below should be followed when specifying, designing, and installing structural sealant glazing:

  • ASTM C1401—Standard Guide for Structural Sealant Glazing
  • ASTM C1184—Standard Specification for Structural Silicone Sealants
  • ASTM C1249—Standard Guide for Secondary Seal for Sealed Insulating Glass Units for Structural Sealant Glazing Applications

For effective structural sealant glazing joint design, at least the following structural joint parameters must be considered:

  • "Bite"—defined as the effective structural contact dimension of a structural sealant required on both the panel and frame faces to accommodate the required transfer of loads.
  • "Thickness"—defined as the minimum structural sealant dimension between structurally bonded substrates (the panel and frame) to facilitate the installation of a sealant and to reduce stress on the structural sealant joint that results from differential thermal movement.
  • "Deadload"—the weight that a panel places on a structural sealant joint when, for certain applications, no setting blocks are used to support a panel's weight

Job Site Start Up

There are several key items to review at job start up the following list can act as a checklist that should be reviewed prior to any sealant application:

  1. Check to make sure that all the necessary sealants, primers, backer rods, etc. have arrived at the site and have been correctly stored.
  2. Inspect the joints where the sealant is to be applied.
    1. Make sure that the joint dimensions match the drawings.
    2. Make sure that the joints have been properly cleaned. In a repair situation make sure that all of the old sealant has been properly removed. If new construction, make sure that the materials of construction are as indicated on the drawings.
    3. Make sure that there is proper access to allow correct application of the sealant.
    4. Make sure there is clarity around any termination points.
  3. Do not apply sealant if the temperature is below 5°C (40°F) and dropping as frost could interfere with the proper adhesion of the sealant.
  4. Do not apply sealant if rain is imminent. Sealant can be applied after a rain once the substrates have dried.
  5. Check the joint dimensions to determine if the joint is excessively closed or open as this could lead to early failure of the sealant.
  6. Check to be sure that all of the job site safety rules are understood and will be followed by the sealant contractor.
  7. If primer is to be applied make sure that the correct primer is used and allowed to dry prior to placement of any backer rod or sealant.
Sealants in Vertical Wall Assemblies

Figure 26: Sealants in Vertical Wall Assemblies
(View enlarged illustration)
Source: Sika

The primary use of sealants in vertical walls is for every place there is a break in the wall continuity. That is, all expansion joints, all control joints, joints around windows, joints around doors, and any place the weather could run in. These are the normal weather protecting sealants. In addition to those are joints inside the wall, as in flame and fire control joints. These use specially formulated sealants that retard flame and fire egress from floor to floor. Part of those sealant joints is a non-flammable backing material. Then there are the joints in the windows themselves, the sealants used in the IG units.

In structurally glazed walls, sealants fasten the glass and glazing details to the curtain wall assembly. These sealants are both an adhesive and a sealant and these sealants have the greatest need for durability and longevity.

There are other considerations when looking at the sealant for special applications. There might be some high temperature lines coming out of a building and those would need special high temperature sealants. There are sealants for the joints in decks and porches on the sides of buildings and they might also need some degree of scuff resistance or special joint designs to keep the foot traffic off the sealant.

Rain Screen (Pressure Equalization) Walls

The water runs down the walls and if the pressure on the inside of the building is less than the pressure on the outside, that negative gradient will be the vacuum that will pull the water in the building, if there are any holes or voids in the wall. The pressure equalized wall is one that has a façade wall that is not perfectly sealed and a void space behind the outside surface and a second seal deeper into the interior of the wall.

The easiest way to visualize this is to name the outside wall—wall #1 and the inside wall #2. When talking about a two wall system we can have face #1 (outside surface); face #2 inside surface of #1 wall; face #3 (outside surface of #2 wall) and face #4 (inside surface of #2 wall).

The wind can blow in a rain storm and present a higher pressure outside than is had inside the structure. If there is a cavity between wall #1 and wall #2 and vent (weeps) in wall #1, the pressure between the walls will be the same as on the outside. Rain hitting the outside surface will rundown the façade and not be sucked into the structure. If some water does enter and get to the cavity, it runs down face #2 and is leaked through weeps to the outside. Surface #3 is a sealed surface and keeps the air out (and water if water makes it through). Surface #3 typically has a membrane covering either an adhered sheet or a liquid applied coating. Sealants are rarely used on the #3 surface. Sealants are almost always used on the #1 surface as a way of minimizing water infiltration. The sealant bead on the #1 surface has weep holes to let the water out and the pressure to equalize. The sealant on the #1 surface is installed with all the instructions and precautions as any sealant bead in any wall structure.

There is a second type of pressure equalization used when only one wall is used. Conventional thinking has a curtain wall and the sealant at the #1 surface is made as continuous as possible and void free as possible to keep out water and air. The pressure equalized single wall has a bead of sealant installed as close to the #2 surface as possible. Typically, it has a backer rod going out to the #2 surface. The sealant is installed with a deep nozzle to get to the backer and to give a sealant bead that keeps water and air from passing through (as much as possible). There is a void between that deep bead and a bead of sealant on surface #1. A backer is installed just below the #1 surface and a sealant applied to it. It is done carefully, as if it were a single seal, so as to make the wall waterproof from this surface seal, except there are vents and weeps holes in this bead to allow pressure equalization and again not allowing a pressure differential from pulling the water into the building. It is a dual seal within a single wall. The outside joint has a vented space behind it and the inside bead is as continuous and water and air tight as possible. The inner seal, because of the location next to the building, will experience a lower UV exposure. Because of these different exposure conditions, different sealants may be employed for the inner and outer seal. It is important during installation to not cross contaminate either seal.

Sustainability and Green Initiatives

Sustainability of the built environment has become a key topic for the building industry. There are multiple organizations that have put forward ways for building owners to ensure that their building was built in a sustainable manner and also will function in a sustainable way. A short listing of these organizations is as follows:

These rating systems award credits or points to projects for using and applying environmentally friendly materials and approaches to building design, construction, and operations and maintenance. If a project achieves enough credits then it earns a specific rating. These ratings can be published and become a point of pride for the building owner as well as ensuring a good return on investment.

For sealants, all of these sustainability initiatives look at three main topics. The degree to which each is important will vary with the different program. The three topics are: Environmental Impact, Transparency, and Volatile Organic Compounds (VOCs).

Environmental Impact

What is the overall impact of the products being used in the building on the environment? This is measured by preparing an Environmental Product Declaration, EPD, for the sealant being used on the building. Information on EPDs can be found in ISO 21930, Sustainability In Buildings And Civil Engineering Works—Core Rules for Environmental Product Declarations Of Construction Products And Services. To complete an EDP there needs to be a Product Category Rule, PCR, for the product being evaluated. A PCR was prepared and then published in 2016 by the Adhesive and Sealant Council (ASC) (product-category-rule-(pcr)-for-sealants). A PCR sets the rules for the Life Cycle Assessment (LCA) that will be completed to generate the data set for the EPD.


This is a critical topic for both manufacturers and owners and operators of buildings. Manufacturers have a legal requirement under the Occupational Safety and Health Act (OSHA) to provide warnings about the hazards of their products to the people who work with these products. This information is transmitted by a Safety Data Sheet, SDS, or product warning label. The OSHA mandated SDS only requires manufacturers to disclose the hazardous chemicals in their formulas and not the full formulation details.

There has been a request from build industry to have manufacturers disclose 100% of the product formulation to receive credit in the different programs referenced above. A Health Product Declaration (HPD), (The HPD Open Standard is a standard specification—composed of a format and instructions—for the accurate, reliable and consistent reporting of product contents and associated health information, for products used in the built environment.) requests that manufacturers disclose 100% of the formulation down to 1000 ppm. The Living Building Challenge has a "Red List" of chemicals that cannot be used in the products in buildings that they certify. There are other organizations that also promote the idea of hazard only thinking about chemicals.

A better way to think about the topic of transparency is to not think only about hazards but also to think about exposure. Hazard without exposure does not create a risk. In the end it is risk that should be driving decisions. However, there is no agreed upon protocol to calculate risk. Each situation needs to be evaluated on an individual basis. For simplicity, hazard becomes the determining factor even if there is no exposure and therefore no risk.

Volatile Organic Compounds (VOCs)

State regulatory agencies and Non-Governmental Organizations (NGOs) have been encouraging the sealant industry to reduce the use of VOCs in sealants. VOCs tend to have an odor, are flammable, and contribute to smog, and if in high enough concentrations can have negative health effects. South Coast Air Quality Management District in California (SCAQMD) has set the most stringent VOC levels for sealants and sealant primers these can be found in Rule 1168, SCAQMD (Rule 1168 for Adhesives and Sealants). Most manufactures of sealants and sealant primers sell products that comply with these stringent requirements. Some of the sustainable building programs above require that the sealant and sealant primer meet the SCAQMD Rule 1168 requirements. In addition for those materials used inside the building they require that the sealant and sealant primer to pass the CDPH Standard Method v1.2, (Emissions Testing 01350) . Whereas Rule 1168 sets a concentration limit for VOC content in a product, the CDPH Standard Method v 1.2 sets a limit for VOC emitted by the material inside a chamber that mimics a standard room, office or classroom.

Therefore, to achieve the credit for a low-emitting material a sealant would need to pass both the SCAQMD Rule 1168 and well as pass the CDPH Standard Method v 1.2.

Testing, Applicable Codes, and Standards

This section's critical function is identifying the required sealant properties for the application and then matching a product with these requirements. In typical sealant installation, determining the conditions and requirements is important to selecting a suitable product. These conditions may include:

  • The temperature the sealant will see in use and how long will it endure the high heat needs to be known. It is not uncommon to see temperatures on surfaces on the sunny side of buildings in the heat of summer hit 70°C (158°F) or even 80°C (176°F) or in some applications and climates 90°C (194°F). Sealants around hot pipes can see these high temperatures or even higher. Often a sealant specifier, architect or contractor will have to contact the manufacturer to get the performance endurance at higher temperatures. Often manufacturers may not have this data publicly available but might supply it upon request.

  • The movement, or change in gap width, expected for the application is a key value. Later there will be comments on calculating joint movement but simply stated it is a function of the nature of the material times the length of the panel and times the expected temperature change. Please note that thermal expansion coefficient for commercial buildings, other materials might change dimensions might change with other environmental factors, such as wood with moisture. The expected temperature range experienced by the actual materials on/in the application is fundamental. Once the movement is known, and the desired joint size is selected, a sealant must be chosen with the movement ability to satisfy the chosen joint size and movement expected. That is one of the most difficult data points. Most, if not all sealant data sheets indicate the movement ability as tested by ASTM C719. However, this is data from a sealant that is only 6 weeks or less in the sealant test joint. All that being said, looking at ASTM C719 data, validated by the SWR Institute (SWR Institute Product Validation Program), is the only quality data readily available when making a sealant selection decision. Other data if often needed, that that has typically obtained from the sealant manufacturer.

  • The longevity. The building owner, architect, specifier, and applicator need to know how long the sealant will maintain the properties needed to successfully seal the building. Presently there are only a few ways to predict the longevity of a sealant in a given application. One is to look at similar jobs, with similar joints, in a similar climate with the given sealant. This is a difficult to achieve fact base. There is also ASTM C1589 10B and 10C which are outdoor studies with movement while the sealants are weathering. Section 10B has the joints in a continuous movement and 10C has the joints opened or closed manually each season. Many architects, engineers, suppliers, contractors have equatorial facing outdoor racks and move the joints periodically. They have the most pertinent data on long term durability. It can be assumed that many sealant manufacturers also have these kinds of test racks. An additional consideration is that the sealant must have adequate adhesion to the substrates.

Sunlight resistance is included in the total outdoor study mentioned above. However, there are many manufacturers who have data with their sealant in UV exposure, even in artificial weathering machines including water and heat with the UV exposure. However, data from the artificial weathering with no stress induced while weathering is distinctly different from similar data done with stress during the artificial weathering. A sealant that looks like it could last many years in outdoor exposure on a test rack, might fail in less than 2 years in outdoor exposure with movement, or after a few months in artificial weathering with induced movement. Thus, the indicator is to look at the weathering data, obtained with stress and strain induced while weathering. Realize the difference between the usual weathering tests and these new data points from sealants that had strain while weathering.

Tolerance to heat, light, water, and movement for the most common type joints noted above is fundamental to looking for durable products. The specification for sealants in weatherproofing applications is ASTM C920 and it is considered one of the most difficult sealant specifications in the world, however it does not consider in-service performance. It is very important to realize that there is no compulsory sealant testing and certification of sealant performance in the U.S. The closest there is to verified performance, even if it is just short-term data, is validation of an ASTM test. ASTM C719 Durability of Adhesion and Cohesion to cyclic movement in Elastomeric Sealants (the most difficult test in specification ASTM C920) by the SWR Institute.

There are many other characteristics that are needed in sealants for special applications. Compatibility with other substrates must be considered in sealant selection. If the sealant is used in Structural Glazing the strength after 5000 hours of artificial weathering is fundamental and is part of the FGIA/AMMA SSGDG-1-17 Structural Silicone Glazing (SSG) Design Guidelines. It is fundamental that SSG sealants pass this specification but, like most other specifications it contains many, but not all the needed performance characteristics. In SSG applications there were many instances where the sealant is required to hold a dead load creep. It is important to know the dead load tested and for how long the load was held. It is really beneficial if the manufacturer has data on how much load the sealant can hold at various dead loads, and know the point at which the dead load doesn't seem to cause either an adhesive or cohesive (creep rupture) failure. Sloped SSG applications are all dead load applications. Again, as always, get that data in writing.

It is important to note that the ASTM C1184 specification is for Silicone Structural Glazing and not general structural glazing. ASTM C1184 cites only 5,000 hours of artificial weathering. Silicones are quite resistant to UV (sunlight) damage and general weathering but the bond between the silicone and the substrate is what is principally studied with the weathering test in the specification. That is very important to note. Silicone sealants, the 100% silicone sealants, are capable of having only relatively small change after decades of exposure to the weather but the bonds are capable of failure at any time, dependent on the bond chemistry, the substrate and the skill of the applicator. Water deteriorates most bonds. Heat accelerates the bond deterioration and thus the long-term testing of the silicone is mainly to determine the durability of the bond.

A second aspect of the 5,000 hours of testing in ASTM C1184 is that if a sealant manufacturer wants to suggest a sealant is appropriate for SSG applications, because of the possible high liability in case of sealant failure, these sealants need at least 20,000 or 30,000 hours of accelerated testing. In this case, look for changes in the sealant performance as well as its adhesion. A specifier of a non-silicone for structural glazing needs to get data from the sealant manufacturer that gives indication of an expected 20 or 30 years of acceptable performance in such an application where many times the sealant bond to the glass will see full sunlight. Making a small transition from normal joints to structural glazing in this introduction makes it logical to provide some notes about Insulated Glass here as well.

About Insulated Glass Units

Insulating glass (IG) units typically utilize a dual seal system comprised of a polyisobutylene (PIB) sealant that functions as a moisture barrier and a silicone sealant that serves a structural function, protecting the PIB seal from damage due to stress from environmental cycling. In some cases, hot melt butyl, urethane, and polysulfide sealants can serve both these barrier and structural functions in IG units.

Main IGU concerns—keeping moisture out; keeping insulating gas in; managing daily temperature cycling; dealing with UV exposure in curtain wall, or exposed edge conditions. The sealants have to function properly to ensure this performance over the life of the IG units.

Temperature inside IG units can be quite high, sometimes in the 70°C (158°F) and 80°C (176°F) range. Thus, the sealant used in all applications in IG units needs to have data on heat stability testing to be sure it is stable at the expected temperature in those units. However, when the IGU is used in a SSG application, the IG secondary sealant is a structural glazing sealant and needs to be tested as one.

Physical Properties of Sealants

This section deals with the physical properties of sealants examined from the specifications point of view. A specification will give the necessary minimum properties for a sealant in a given class but may not be sufficient properties needed in a given application.

Fundamental to all discussions on sealants is the physical properties of the sealant relative to the physical properties needed in the application. This correctly implies that the person designating the sealant for a job should have some knowledge of what properties the sealant needs to have. It also correctly implies that the sealant manufacturer needs to know the properties of the product they produced and made available for sale. There are many uses for sealants in buildings and thus there are many types of sealants to be considered. Consider first what many consider the most demanding application for sealants in most buildings, the outside, moving joints in the equatorial direction (South facing in northern hemisphere, North facing in the southern hemisphere, this effect is a strong function of latitude as well) of the building. In the equatorial direction, the sealant will get the maximum heat, which also translates to maximum movement and it will see the maximum sunlight (UV radiation). Being outside it will see water from rain, dew, snow, fog, and sometimes sea spray.

A comprehensive general statement on critical sealant performance is in the FGIA/AAMA JS-91 Aluminum Curtain Wall Series on Joint Sealants, which has been replaced with FGIA/AAMA 851-20 Fenestration Sealants Guide for Windows, Window Walls and Curtain Walls. Under the title Critical Sealant Properties, it says "Sealants, like other building materials, have specific physical characteristics which determine how they will react under conditions of use. The design of joint systems must take into account such factors.

The most critical properties of a sealant are its adhesive strength, cohesive strength, recovery ability after deformation, modulus, and durability under the effects of weathering. The importance of adhesive and cohesive strength is self-evident. Unless the sealant bonds securely and continuously to the substrates, when subjected to tensile stress, the sealant will fail. In many cases, depending on the substrate material, a primer may be required to promote the bonding action. Clearly, cohesive strength is equally important. A material which lacks the strength to, "hold itself together" under repeated stress cannot provide a suitable seal.

Sealants may accommodate movement by either of two mechanisms. One of these is internal flow of two mechanisms. One of these is internal flow under stress, a characteristic variously referred to as stress relaxation, creep, cold flow or plastic flow. Subjected to stress, the sealant deforms by flowing in the manner of a viscous fluid. An entirely different mechanism is the accommodation of movement by means of a rubber-like property—the ability of deform under stress but, when the stress is removed, to recover.

The properties of many sealant materials vary with temperature. The temperature range occurring on the surface of a building is sufficient to cause changes in the properties of some types of sealant—changes sufficient to cause failure in some cases. It is essential, too, that the basic properties of the sealant do not change significantly with age. Should such changes occur, the assumptions regarding their behavior which originally governed the joint design are no longer valid.

Is it important that the critical properties—adhesion strength, cohesive strength, and modulus—be kept in proper balance in conjunction with the proper joint design and sealant configuration. If the modulus is high and the adhesive strength is low then adhesive failure will occur. If the modulus is high and the adhesive strength is high, substrate failure may occur. If the modulus is low and the adhesive strength is high, then cohesive failure may occur. Some low modulus sealants will fail in working joints.

This means that there must be some way of testing for the sealant's ability to handle the joint movement, under field conditions, and not experience adhesive or cohesive failure for some acceptable time. There must be tests and specifications for building sealants.

The most rigorous specification for such a sealant (in the US) is ASTM C920 Standard Specification for Elastomeric Joint Sealants. This specification and the tests with in it are often used in many other countries of the world. Within that specification are different types (single component (ready to use) or multicomponent (mix on site)); different grades as P = pourable or self-leveling and NS = gunnable, non-sag; and different movement classes. The movement classes range from +100% / -50%, ±50%, ±35%, ±25%, ±12½%. The movement is tested by ASTM C719. This is often considered the principal test method in the specification. Before going into the details of the specification, it is instructive to study the C719 test method.

Miniature test joints (triplicates are made). The joints are held, without movement for 21 days while they cure. They then go into water for 7 days, taken out and flexed. This is sometimes a tough step since sometimes this water interferes with adhesion but sometimes this step is beneficial since it allows more cure time and enhanced performance. In any case the joint is then hand flexed to approximately 60o and examined for adhesive and cohesive flaws. The joints are then compressed to the maximum compression being tested and put into a 70°C (158°F) for a week. During this period there are a number of possible changes that may be occurring to the sealant including changes to the cross-link density, slow curing, solvent loss, or viscous flow. These changes in compression make the subsequent expansion back to the original dimension and then to the extended state much more difficult. If the sealant is stable before it is taken into compression, it is still a difficult stage since the forces on the sealant and the bond in compression are higher than those created by a similar percent movement in extension. However, the damage is never seen when the joint is compressed and only noted when it is extended and an adhesion loss is seen or a tear is started (cohesive failure started).

Next, the sealant joint is put through continuous extension to compression and back and forth for 10 cycles. They are examined for adhesive and/or cohesive failure.

Next, they are compressed again to the maximum compression being tested and put into an oven for 16 to 20 hours at 70°C (158°F), taken out, allowed to cool then put into the extension machine that is now in a freezer at -26°C (-15°F) where it is moved at 1/8 inch (3mm) per hour until it reaches its maximum planned extension. Block the joint at this extension, take it out of the freezer and examine it for adhesive and cohesive failures (note any other flaws as well). Repeat this hot and cold cycling 10 times.

This is considered the most rigorous test for sealant durability in the specification and probably one of the most rigorous standard tests in the sealant industry. However, note that the joint cures while sitting still for 21 days and in the field. On the job, the joint will move from the moment it is installed. Movement during curing always decreases sealant performance in either adhesion or cohesion and often in both. Note also that this test is done as soon as the sealant is cured. This sealant has not seen weathering with movement, as is seen in the field. This combination of weathering with movement always decreases a sealant's performance relative to what is seen if the sealant is not moved while weathering. There is a synergism between the deterioration factors movement, heat, light and water. However, it is difficult to combine all of these into a reasonably fast test and this has not yet been done commercially in a nationally recognized specification.

An important consideration is the passing criteria from a sealant tested to ASTM C719 for the ASTM C920 specification is 75% of the joint has not failed. Thus, it allows as passing up to 25% joint failure from any combination of adhesive and cohesive failure. Some consider this very liberal pass/fail criteria, and yet it is the most difficult and rigorous standard specification for construction sealants.

The important message here is that if the sealant cannot pass C719 test method to the specified movement being tested, it will almost surely not be able to handle that degree of movement in the field. An equally or more important message is that since the sealant in the joint in this test method had no accelerated weathering there is no indication of how long it will continue to perform in the field application. The specifier or user must contact the sealant manufacturer and ask for data to predict useful life in a given climate and application.

There are some test procedures that give some indication of the ability to handle joint movement while weathering and handle the conditions for extended times. Look to ASTM C1589/C1589M and in there look to Section 10.2 Procedure B—Outdoor Weathering of Building Joint Sealants With Continuous Movement and/or Section 10.3 Procedure C—Outdoor Weathering of Building Joint Sealants with Periodic Manual Extension and Compression. These are not accelerated tests but attempts to simulate the conditions seen and damage done in various climates. Consider these tests as buildings where the movement is precisely measured (in procedure B) and the joint movement is precisely controlled (in procedure C).

The Durability Lab's sealant test racks (ASTM C1589 Procedure C)

Figure 27: One of The Durability Lab's sealant test racks (ASTM C1589 Procedure C). See also: Durability of Elastomeric Sealants by Beth Anne Feero and David Nicastro.
Photo Credit: Building Diagnostics, Inc.

The joints used are similar to those in ASTM C719 (or modified to match joint configurations used on jobs (hourglass as opposed to square cross-sections). The joints are put on racks and the racks put out in various climates in the US, and other places, and the sealants are studied for adhesive and cohesive failures along with cracking and crazing and dirt pickup and any other anomalies seen. It is probable that this test procedure will never be in a specification since it takes a very long time (not an accelerated test) and the damage will vary from location to location. It is a test procedure (see ASTM C1589) that many engineering firms, contractors, distributors, and sealant applicators are conducting this procedure to help select which sealants they want to specify for given applications in various climates. It can be assumed that many manufacturers are also doing outdoor weathering, with movement using one or both of the procedures mentioned. For a more nuanced discussion see the durability section.


There are many other test methods and limits indicated in the C920 specification. Reading and becoming familiar with all of it is fundamental to the specifier, the applicator, the distributor, and manufacturer of sealants. These are discussed briefly below.

Looking further in Specification ASTM C920 is the use designations—uses T1, T2. T1 indicates if the sealant is harder, generally desired for traffic areas, especially pedestrian traffic however there are many exceptions to this and often traffic areas need a lower modulus sealant and in that case a T2 sealant is used. NT is a softer, lower modulus sealant most often used in general sealant applications. Again, there are exceptions that are job specific and one most look the specific application and then determine if, for that specific job, do they want a harder or softer sealant (determined by Shore A hardness testing) but truly it is the modulus of elasticity that is most critical and that is tested by ASTM C1735, ASTM 1135 devices and not part of the C920 specification.

Use I (immersion) is of special interest referring to applications that see continuous submergence. Note carefully that one has to go to the test method ASTM C1247 and be aware of how it is tested and see if that is satisfactory to indicate a utility in a specific application. This Use I has Class 1 which is held at 50°C (122°F) for 6 weeks immersion before testing and Class 2 which is held in immersion for 10 weeks before testing. If the application will have continuous immersion for more than 10 weeks the only acceleration of deterioration is a temperature increase. However, it is reasonable to assume that for each 10°C (18°F) increase in temperature, the rate of failure will double (a maximum approximation). Thus, if the application will see immersion at near ambient (77°F, 25°C) the acceleration of this test at best near 5 times. Thus, 10 weeks in tests is at best a year in service, if the sealant is continuously immersed. Note as well, the sealant is held in a neutral position while immersed. After immersion the sealant is moved in extension and compression cycles as in ASTM C719. If the sealant will see movement while immersed the test might not be as difficult as the actual application. That being noted, it is a good test for a sealant's ability to handle immersion conditions. Note that anyone can modify the test to more closely match their job conditions or modify it to run at higher temperatures to further accelerate the damage, but care must be taken in doing that so the temperature is not so high as to produce a degradation mechanism that would not be seen in actual use.

Peel Test: ASTM C794

Next in the ASTM C920 specification is Use M, G, A, O which means acceptable for use on mortar (M); on glass (G); on anodized aluminum (A) and other materials (O). Note that this means it passes the ASTM C719 test at the indicated movement when tested on these substrates, but these are standard substrates. That is no assurance that a sealant will pass the C719 on job site substrates. Thus, in this specification is another test ASTM C794. It is important to mention that if a manufacturer passes the adhesion tests, C719 and ASTM C794, that are in the specification, by using a primer on some of the standard substrates, like Mortar or Aluminum, but in their data sheets claim unprimed adhesion to these substrates, a primer may need to be considered. The message is that regardless of whether a data sheet of the sealant says generally unprimed adhesion, and passes all the tests in the C920 specification, still ask the manufacturer to test adhesion on job site substrates or substrates that closely resemble the job site materials. The job substrates might not be identical or even closely similar to the standard test substrates.

ASTM C794 is called the peel test. It is a great screening test for adhesion but the user of the test data has to realize that the manufacturer has to tell the user or specifier how to interpret the data from this test relative to their product. In this test the sealant is embedded in a strip of stainless steel or aluminum screen and this is adhered to a substrate surface. The sealant is cured and adhesion is allowed to form, and then the strip of screen with the sealant in it is pulled at 180 degrees in a Tensometer. The force needed to pull the tab is noted and the mode of failure (adhesive or cohesive) is noted.

The sample is then immersed in water for 7 days and the tab is pulled again. If the specification report says the sealant is useable on glass, the test piece is put into an accelerated artificial weathering machine for 200 hours (radiation to go through the glass to the bond line) and then put into water for 7 days, then again put into the machine and pulled to determine the force need to pull it for 1 inch at 2 inches per minute, and the mode of failure is noted.

Note well that the specifications, if passed, means that the sealant passed the minimum condition to be used in conventional glazing, but not in structural glazing. The 200 hours of UV exposure (artificial weathering) represents no more than 6 weeks to 12 weeks of continuous sunlight and does not come near to predicting what happens to the bond after 20 years or 30 years of direct sunlight in a South facing, structurally glazed window.

To pass the C920 specification a sealant in this peel test needs to demonstrate no more that 25% adhesive bond loss, regardless of the strength of the adhesive bond, and the adhesive bond must be at least 22.2 Newtons (5 pounds per liner inch). This test does not show the necessary correlation of the modulus (sealant stiffness) to the required bond strength. Most adhesive specifications demand a minimum strength, but few demand a specific mode of failure. When the adhesive (in our case the sealant) fails, it is of little concern to the user if the failure is adhesive or cohesive. The building will leak in either case.

A consideration with the specification is that the specification of a specific minimum adhesive value of 22.2 Newtons does not consider the stiffness value of the product. The stiffness of the product will dictate what the adhesive strength has to be to handle the forces produced by a given movement (strain). The 22.2 N is probably adequate for a lower modulus sealant but not for a higher modulus sealant. Thus, the test, as used in the specification C920 has some subtleties not well understood by the general user but is part of the required properties to pass C920 and be qualified for use on most major projects.

Adhesion Test: ASTM C1521

The common field test ASTM C1521 for adhesion is to have the user make a test specimen similar to that in ASTM C794 (no screen in the sealant but just a strip of sealant adhered to the substrate). The sealant is allowed to cure then partially undercut to make a tab to pull, and then pulled. The pull is done by hand by the worker (there is no tensometer to measure bond strength on the job site). The result is either cohesive failure (a pass) or adhesive failure (a fail). It is simple and most manufacturers agree with it since there are no other simple tests easily done at the job site. If a few sealants get excluded from use it is considered a low price to pay for keeping a quick, easy test that works most of the time. Some manufacturers will give the customer a letter indicating the sealant has more than adequate adhesion even though it had adhesive failure in the C794 test in the C920 specification. If they also fail the field test they either loose the job or provide some other data to show the adhesion will be adequate with their product on the job substrate.

Pictogram of the field pull test, ASTM C1521

Figure 28: Pictogram of the field pull test, ASTM C1521 Standard Practice for Evaluating Adhesion of Installed Weatherproofing Sealant Joints
Pictogram Source: Tremco
Video of C1521 Method click here.
Source of Video: George D Alan Company

Inspecting the C794 test, 22.2 N is too low an adhesive value for a higher modulus, stiffer sealant but okay for the lower modulus, softer products. Now imagine a company with a medium modulus sealant which pulls a very high value of 35 N, however, at the high adhesive value, it fails more than 25% in adhesion. A perfectly good sealant cannot pass the important C920 specification because even though the sealant adhered to a very high force on the bond line, its final failure is in the wrong mode. A sealant must have or exceed a specific adhesion value and then the mode of failure is irrelevant. If a manufacturer can produce test data showing excellent adhesion in this test, the product could be accepted for a given application, in spite of having more than 25% adhesive failure. Note there is no adhesion value that can be stated as sufficient in this test even though the specification calls for 22.2 N minimum. The proper adhesion force is a function of the modulus and not a fixed value. A stiff, high modulus sealant will need a much higher adhesion value then a lower modulus, softer product. The sealant manufacturer should have data to give to the customer as to the minimum value to achieve with their product.

Example of Cohesive Failure (Left), Adhesive Failure (Right)

Figure 29: Example of Cohesive Failure (Left), Adhesive Failure (Right)
Source: ASTM C24 (ASTM C794)

With the above commentary about ASTM C794 it must be noted that it (or a modified version of it) is probably the most often used test in the manufacturer's labs. It is used as the screening test for adhesion since the strip of sealant can be easily adhered to most job site substrates. It is common practice (especially for major jobs) to send in job type substrates to the manufacturer to determine if adhesion can be obtained and if a primer is needed. Most of the time this is the method that is used sometimes ASTM C1135 is used to determine adhesion in a joint). Above it was noted that appropriate values for adhesion are modulus dependent. It is important to note that the most reliable test for actual modulus is ASTM C1735 Standard Test Method for Measuring Time Dependent Modulus of Sealants Using Stress Relaxation.

Other Tests in the ASTM C920 Specification

There are a few more key tests in the ASTM C920 specification.

  • ASTM C510. The most important part of this test is that a manufacturer has to run it to have his sealant qualified to the C920 specification and allowed on major construction jobs. It uses a standard concrete as the substrate (not marble or other more easily stained surfaces). It requires exposing the sealant to accelerated artificial weathering for only 100 hours with no extension or compression to pump oils/polymers from the sealant. Knowing that acceleration factors from accelerated artificial weathering machines are typically from 5 to 10, it means that this test in the specification means a sealant will not stain a substrate or undergo a color change in the field for perhaps 500 hours or 1000 hours of sunlight or maybe 6 months or a year in use. While this test must be run for qualifications to the C920 specification, most, or all sealant manufacturers run the test for color change by ASTM C1501 and run the test in an accelerated weathering machine, for several thousand hours. To test for staining potential, they will use ASTM C1248, use a job site substrate, and have the sealant in compression between 2 blocks of the job site substrate (at manufacturer's maximum recommended compression) and put it in the oven for 4 weeks and then into an accelerated artificial weathering machine for another 4 weeks. This is not to pass the specifications but to have a more reasonable determination if there is a staining potential. Few, if any, major jobs are done without this stain testing being done since esthetics are important to most jobs. If a manufacturer has kept their formulation constant and the substrate is one that was previously tested, the manufacturer might note that in a letter and decide not to do the test, based on the prior experience.

  • The ASTM C1246 is part of the C920 specification. It keeps a sealant for 3 weeks in a forced air oven at 70°C (158°F) and then it is examined for cracking and chalking and weighed for weight loss. Few of the sealants that qualify for C920 specification have trouble with this test, however, the test is worth mentioning since the method is sometimes used for other sealants like hot melts and PIB sealants used in insulated glass applications. Sometimes done at 70°C (158°F) and sometimes at 80°C (176°F) if that is a temperature that can be expected in the IG units at a specific location (other temperatures are sometimes used). The test lab looks for slumping and/or change in viscosity. Other applications might also use this test or modifications of it. Most manufacturers run some sort of heat stability testing to be able to tell the highest stable temperature for use of their product. Most often look at a hardness change or plasticity change as function of temperature with time and that is not in an ASTM test or sealant specification but is quite fundamental information in determining the suitability of a product in high temperature applications.

  • The specification also has test ASTM C679. This is to determine a typical cure rate. The value from the specification can then be used to compare the cure time of sealants on the job and if they as close to typical. It is an easy, generally fast test and almost always part of the manufacturers lot-to-lot quality control as well as a job site test to determine if the sealant received is typical.

  • The test ASTM C661. It is a measure of hardness and with few exceptions, has as its principal use the production of a typical hardness value that is used to compare lots of sealant at the factory (QC) and sometimes used at the job to determine if the sealant delivered to the job was typical.

  • There is ASTM C639 to determine the amount of slump or sag. The C920 specification puts limits on the amount of sag a sealant can have in a joint, so the sealant doesn't sag out of the joint when it is still in the paste form as it is installed. It also dictates the conditions for a sagging or self-leveling sealant (for some horizontal joints).

  • There is also ASTM C1183/C1183M-13(2018). This test has limits in the C920 specification that requires a sealant be soft and pliable enough to extrude nicely in a caulking gun. It also is a shelf-life determinant with mix-on-site sealants. The sealant is putting into a standard caulking gun with a fixed pressure (40 psi) and the rate of sealant extruded in a minute is noted and a minimum value needs to be met or exceeded.

Examining Specific Conditions

As stated at the beginning of this section, there might be conditions that a sealant will see and properties needed by the sealant that are not included in the tests and specifications on ASTM C920. The person recommending the sealant has to know what properties are needed and know what properties specific sealants have. C920 says specific sealant because there are many silicones, many urethanes and hybrids and polysulfides and acrylics and others and not all silicones have the same properties and not all urethanes have the same properties. The safest, most reliable way to choose a sealant for an application is to know what is needed and then contact the manufacturer or supplier. There are some characteristics unique to some types of sealants and they will be discussed in a later section. An example is that all silicone sealants (silicone sealant is defined in ASTM C717 (under Sealant—Silicone) and essential says the 100% of the reactive polymer is silicone) have very low surface energy which means they wet many surfaces to aid adhesion but also few material will wet a silicone surface and thus none or almost no silicones are paintable. Silicones are very resistant to Ultra-Violet (sunlight) damage. However, they are different in that adhesion to the various substrates varies greatly with each specific silicone. There are many other differences as well. Another example of differences within a generic type some generic types are strongly susceptible to UV and heat degradation but they can be formulated with sunscreens and antioxidants that give it long useful life in sunny and or hot locations. The longevity in weathering applications is totally formulation dependent and varies with each sealant.

One needs to look at what tests they passed but must look at specific sealants and not generic types of sealants (except in Structural Glazing Applications) to determine usefulness in a given application at a given job.

SWR Institute (SWR Institute Product Validation Program)—One more critical comment in properties before we go further into this section. The manufacturer's data sheets are the most convenient way to see some of the key performance properties of a specific sealant. The SWR Institute (a contractor driven organization) has a validation program where they looked at the tests for sealants and decided that the ASTM C719 Standard Test Method for Adhesion and Cohesion of Elastomeric Joint Sealants Under Cyclic Movement is the key short term test of a sealants ability and thus have their program (voluntary), and if a manufacturer wants to have the data from a C719 test verified and published for all to see that is a true, they can have an independent test lab purchase the product, and test it according to ASTM C719, and the independent lab send the properties to the SWR Institute for publication. Many manufacturers have done this with their architectural sealants, and anyone can call the SWR Institute office and ask about a specific product as to what class of movement did it pass and on what substrates was it tested and if a primer was used in the test. It is a unique source of validated properties.

While C719 is the best short-term test for sealant properties (at least initial performance) available and getting validated results is one of the only ways to get reliable data.

The discussion above was based on typical joint sealants, of toothpaste-like consistency and applied to a joint where they cure in place. There are some sealants called "pre-cured sealants". Imagine a band-aid that is typical in shape but not one inch or a half inch wide but many feet wide. They are thin strips of cured rubber (sealant) that are put on top of joints that would be difficult or impossible to clean or seal by conventional techniques. These are shown very well in the pictures in ASTM C1518. This specification exemplifies the comment at that start of this section Specifications show the "Necessary but not sufficient properties a product must have". Note that this specification is for materials that are often only 2 mm or 3 mm thick. The title indicates that it is for silicone materials only. By stating that in the title it doesn't have to include the several months of accelerated, artificial weathering testing since silicones have almost no detrimental effects from sunlight. If other materials are used in this application then one would need to see extensive aging in weathering conditions. However, it does include a minimum of 2500 hours in an accelerated weathering machine because adhesion can be greatly influenced by heat and water (and sometimes light if the silicone formula is such that is allows light to transfer through the sealant to the bond to the substrate) or if the sealant sees a continuance of cure and become a bit harder with time. Fundamentals here are the tests for adhesion and for tears and cracks since the rubber sheet put over the joint is very thin (so as not to be visually obtrusive). The test methods are defined in ASTM C1523.

Some joints are sealed with preformed rubber or foam that is placed in the joint and adhered to the joint sides. (See the FACTORY PREFORMED section.)

In all cases, the specifier has to look at the specific joint sealing situation, the conditions that will be encountered, determine the joint configuration and sealant that will tolerate the expected environment. There are guides to this and some are: ASTM C1850, Table 1; Klosowski,J.M. and Wolf, A.T. "Sealants in Construction—Second Edition" CRC Press—Taylor Francis Group, pg 14 fig 2.1; BelCher, W.E., "Installation of Joint Sealants & Guides" Pg 11–12, The ASTM C1193 in Section 5 General Considerations has comments on a large selection of properties needed in sealant applications.

ASTM C1481 talks of the uniqueness of this application and gives insight as to the properties, especially adhesion and the special considerations needed since the substrate is relatively week. The properties needed for EIFS applications are determined by some of the standard tests mentioned earlier C719 and C794 but because of the unique composite structure of EIFS the test method ASTM C1382 is used and exclusively used for EIFS systems. The methods for conditioning of the sealants before stressing are unique and after the heating and freezing steps and accelerated weathering, the test pieces are strained in a tensometer as in ASTM C1135 (Standard Test Method for Determining Tensile Adhesion Properties of Structural Sealants), but here the tensile strength is measured at 10%, 25%, 50% and 100% (a chart of the secant modulus is prepared). Note as well that low and very low modulus sealants are often preferred in this application because of the weakness of the EIFS composite (although the EIFS manufacturer generally direct the sealant be adhered to the base coat instead of the finish coat, and most often a primer is requested).

There are special tests and specifications generally used with solvent diluted sealants like butyls and the acrylics and similar typically low movement sealants. However, there are some sealants in these classes that are considered for the higher performance architectural applications and in these cases they are required to be able to conform to the ASTM C920 specifications and all the test methods therein.

ASTM C834 Water Borne Sealants

There is a specification for latex sealants. Latex as defined in ASTM C717 is an aqueous dispersion of polymers that can be solidified into rubber. A water-based sealant can be based on a variety of different base chemistries including but not limited to: acrylic (most common), urethane, or silicone. The specification for such sealants is ASTM C834. The ASTM C920 is a specification for sealants intended for difficult situations (like exterior moving joints) and the material formula is irrelevant. The specification ASTM C834 calls out test methods and values that are not designed for exterior moving joints. Latex sealants intended for the exterior moving joints have to pass ASTM C920. The majority of the applications for sealants that are tested to the qualifications called out in ASTM C834 are indoor (less exposure and less movement).

The Specification ASTM C834 has 3 tests in it for durability.

  • ASTM C732. A channel is made of wood on one side, aluminum on the other and polyethylene in the bottom. A latex sealant is put in the channel, allowed to cure (and the water evaporate) for a week. This channel is then put in an artificial weathering machine for 500 hours (sometimes considered equivalent to one year outdoors in some climates) and then examined visually for cracking, slump, washout, discoloration and adhesion loss (caused primarily or exclusively) from shrink forces. Visually determined twenty five percent bond loss or less is still a pass.
  • ASTM C734. Here the sealant is put onto a thin aluminum panel and cured, put into an artificial weathering machine for 500 hours, then put into a freezer and cooled to 0°F (-17°C) or 32°F (0°C). While cold, the panel with the sealant attached is quickly bent over a 1–inch (25.4 mm) diameter mandrel. The difference in the cold temperature of the bend makes for 2 classes. The sealant is then examined for cracking through to sealant to the bottom substrate and for adhesive failure. The pass criteria is no failure.
  • ASTM C736. Note this does not test for joint movement having extension and compression cycles but only has a single extension movement. The test consists of making a joint, curing it then extending the width to +25% holding it there for 5 minutes, releasing the strain and then watching it recover. Examine the sealant for adhesive failure and the % of recovery. The specification allows for 25% adhesion loss and it must have 75% or more recovery. It is a performance test but with milder conditions for sealants to be used in applications that are less demanding.

Another important test in the ASTM C834 specification is the test method ASTM C1241. Many latex sealants have significant water in them, and its evaporation gives significant shrink. There is a class OP (Opaque) that allows up to 30% volume shrinkage and a class C (Clear or Translucent) that allows up to 50% shrinkage. Shrinkage is a key since it can produce internal stresses and/or stresses on the bond line. The shrinkage also causes joint deformation which might or might not be a problem. The other tests in the specification are ASTM D2202 for slump, ASTM D2203, ASTM D2377 and ASTM C1183 for Extrudability. These are to make sure the sealant comes out of the tube without significant force, doesn't slip out of the joint as it is being installed and cures in a reasonable time.

Latex sealants are generally paintable, and with the lesser joint movement the paint tends to adhere. Thus the latex sealants that qualify by this specification are sometimes called "painter's caulk" and similar names that describe their principal use.

There are some latex sealants that pass the ASTM C920 specification based on the 28–day post cure weight loss test method ASTM C1246 referenced in ASTM C920 at 7%). Manufacturers who want to have their product used in demanding outdoor applications will often indicate their sealant passes ASTM C920 class 25. Some applications for these high-end latex sealants are in housing, DIY markets and more demanding applications. The attraction there is the paint ability, the ease of use and ease of clean-up while still getting a durable seal. The difficulty is in determining which latex sealants are truly high-end in performance and have durable adhesion. It is also of interest that a sealant might be paintable, and the paint has durable adhesion if there is minimal joint movement but some of the durability of the adhesion of the paint to the sealant is lost when the sealant has significant, repeated movement. It is appropriate to contact the manufacturer of such sealants and receive a written indicating the appropriateness of use in a given application.

As discussed above and also below one component sealants cure by different mechanisms. Some are reactive, where moisture in the air reacts with a component of a sealant completing the cure; others cure by releasing solvent or water. Most of the water containing sealants also cure through coalescence of the latex polymers contained in the sealant as the water is diffusing out of the sealant. The movement of moisture into a sealant bead and the movement of water and solvent out of a sealant bead is diffusion controlled. This means the process of loss of solvent and or water or moisture cure takes time for the moisture to migrate into the sealant bead or for the solvent and water to migrate out of the sealant bead. The larger the bead the longer this process will take before all of the solvent or water has migrated in the desired direction. Therefore, the larger the bead the slower the cure. Historically, ASTM C920 contained a weight loss specification, based on ASTM C792 that was a maximum of 7%. Current ASTM C920 uses ASTM C1246 as the standard for weight loss, with the same specification of 7%. ASTM C792 and ASTM C1246 differ in some significant ways. ASTM C792 requires curing of the adhesive for 7 days at standard conditions prior to over aging for 21 days at 70°C (158°F) to determine the weight loss. ASTM C1246 allows a 28–day cure before the 21 days in the 70°C (158°F) oven. In addition, the thickness of the bead in ASTM C792 is 6.4 mm whereas the thickness of the bead in ASTM C1246 is 3.2 mm. Based on these curing conditions ASTM C792 would give a higher percent weight loss than ASTM C1246. Some companies also report percent solids for their one component sealant. The percent solids are not measured by ASTM C792 or ASTM C1246. It is either determined by a loss in weight test in an oven or IR balance or is calculated based on the formulation.

The percent solids can also be misleading as some sealants contain small molecules that react into the polymer that would be lost in a percent solids test but would actually be incorporated into the final cured sealant. The reason why weight loss is important for a high-performance sealant is to reduce the stresses that could build up in the cured sealant. As the sealant loses weight it would start to shrink, minor shrinkage might not be negative as it could produce a more hourglass profile of the sealant, however, major shrinkage could induce significant tensile loads on the adhesion of the sealant to the substrate. These tensile loads would become higher as the joint opens. If the tensile load exceeds the adhesive strength of the sealant to the substrate then a loss in adhesion would occur or if the tensile load exceeds the tensile strength of the sealant there could be a cohesive break in the sealant. It is for this reason that high performance sealants that need to achieve high joint movement need to have low weight loss to keep the internal stress as low as possible. It is important for the specifier and architect to understand the nature of the sealant being specified and to work with the supplier to ensure that the sealant being specified is suitable for the application.

One-Component Solvent Release Sealants

There is a family of sealants used in both interior and exterior applications that are generally called "Solvent Release Sealants". These are formulated sealants based on elastomeric polymers that "cure" by evaporation of the solvent. There is no chemical reaction taking place after the application of these types of sealants. These sealants are designed for applications in static joints where there is only a limited amount of joint movement, generally less than 7.5%. These types of sealants are covered by ASTM C1311.

ASTM C1311 has a set of performance criteria that a sealant of this type must meet. See Table 1 below taken from the standard. As with all ASTM performance standards, the criteria are minimum performance that a sealant needs to meet. These criteria are not necessarily the performance that is required in a specific application. It is incumbent on the specifier to be aware of the demands of the application and to make sure that the sealant selected has the required performance. The following is a brief discussion of four of the tests included in ASTM C1311, Bubbling ASTM C712, Adhesions and Cohesive loss, ASTM C1216, ASTM D2203, Accelerated weathering, ASTM C1257.

TABLE 1: Physical Requirements
Property Requirement Test Method
Extrudability 9/s/mL, max D2452
Indentation hardness 50, max C661
Bubbling 25%, max C712
Adhesive and cohesive loss 9 cm² (1.5 in.²), max per substrate C1216
Slump 0.38 cm (0.15 in.), max D2202
Stain index 3, max D2203
Tack-free time 72 h, max D2377
Accelerated weathering 10000 h exposure C1257
   Edge cracking 3, max  
   Center cracking 3, max  
   Adhesion loss 3, max  
   Color Change as agreed  

Figure 30: Standard Specification for Solvent Release Sealants Physical Requirement
Source: ASTM C1311

Note the accelerated aging referenced in Table 1 calls out 10,000 hours of exposure. However, in the body of ASTM C1311 the actual test duration is 1000 hours.

  • Adhesion and Cohesion loss, ASTM C1216, is an adhesion test that uses a modification of cyclic movement. The sealant is first applied in a joint configuration to various substrates, glass, aluminum, and a concrete mortar, allowed to cure. Once cured the sealant is subjected to expansion at low temperature, -12°C (10°F) and compression in an oven, 50°C (122°F). During the low-temperature cycle, the sealant is expanded at a rate of 3.8 mm (1/8 inch) per hour to +12.5% of the initial joint opening. In the high-temperature cycle, the sealant is compressed to -12.5% of the initial joint opening. The sealant assembly is subjected to 5 cycles and then the loss in adhesion or cohesive failure is evaluated. The test requires the amount of loss in either adhesion or cohesive failure to be less than 9 cm² on any set of three substrates used in the test. That is roughly 47% loss in adhesion or cohesive failure will still pass this performance criterion.

  • Accelerated Weathering, ASTM C1257, exposes the sealant to 1000 hours of UV exposure. The sealant is applied into aluminum channels, of the following dimensions 76 mm in length by 19 mm wide by 9.5 mm deep (3 inches x 3/4 inches x 3/8 inches), where the sealant has been applied to completely fill the channel. After 1000 hours the sealant is visually evaluated and compared to standard pictures to rate the amount of failure, see below for Edge cracking rating pictures. 1000 hours is not a very long amount of UV exposure, especially for a sealant that will be in direct sunlight. However, in the UV exposure chambers, the temperature is usually in the 50°C to 60°C (120°F to 140°F), range. At this temperature range for 1000 hours, about 42 days, most of the solvent would have evaporated from the sealant. So, while this test is not an extreme UV test it is a good test for issues due to shrinking of the sealant due to solvent evaporation. The maximum allowed rating is a 3 which does allow some Edge Cracking and Loss in Adhesion.

Edge cracking from ASTM C1257-ratings range from zero for no damage to 5 for severe damage

Figure 31: Edge cracking from ASTM C1257. At the end of the accelerated exposure period the specimens are examined for chalking, color change, center cracking, edge cracking and loss of adhesion versus the unexposed file specimen. Ratings range from zero for no damage to 5 for severe damage.
Source: ASTM C24 (C1257)

  • Bubbling, ASTM C712 looks at the potential for these types of sealants to bubble or blister on application. In the test method, a 25 mm wide x 95 mm long x 3.2 mm tall (1–inch x 3.75 inches x 1/8 inch) thick bead of sealant is applied to the various substrates. Aluminum and concrete mortar is standard substrates in this test method but others can be used. Three replicates are applied for each substrate. Once the sealant is applied it is allowed to cure for 48 hours at standard conditions then it is placed in a 50°C (122°F) oven for 72 hours. After removal from the oven, the samples are allowed to cool for 1 hour, and then the amount of the surface that is covered by a bubble or blister is determined. The passing rate of bubbling in this test method is less than 25% of the surface. Bubbling is more of an issue on porous surfaces and the degree of bubbling on a very porous substrate would need to be measured if that substrate was intended to be used in the application. In these situations, the recommendation of the manufacturer as to whether the sealant was appropriate for this type of substrate would be in order.

  • Staining, ASTM D2203, tests the amount of staining caused by the sealant on the application and in the very early stages of cure. The test method has two procedures one uses filter paper and here the test is run for three days. The second procedure uses the substrate of interest and here the exposure time is only five days. In both of these procedures, the aging is done at room temperature and the sealant sample is not compressed. This test method will only show staining due to the early effects of the sealant on the substrate. It will not show any potential long-term effects. If the substrate being considered is quite porous and there is a concern about staining then there are other ASTM test methods that would be more appropriate, specifically ASTM C1248, where the sealant is compressed between the substrates and exposed to a different set of aging conditions.

The other parts of the ASTM C1311, ASTM D2377, ASTM D2202, and Extrudability, ASTM D2452. While these are all important these tests address application issues and do not address long term performance of the in-place sealant. A properly formulated sealant should be able to be extruded from its packaging, it should not slump/sag and its tack-free time should be reasonable to allow enough application and tooling time but not too long to allow significant dirt pick up. ASTM C1311 does not include any discussion about weight loss with any ASTM C792, or ASTM C1246. While weight loss is not covered in the standard it is an important characteristic to consider when specifying a sealant.

It is important to state one more time that just passing an ASTM performance standard like ASTM C1311 does not guarantee that the sealant will work in all applications. It is important to review the Product Data Sheet and to discuss the application with the manufacturer.

Structural Glazing

The objective of this section is to show proper guidelines and glazing procedures and commentary on them. Key literature for Structural Silicone Glazing (SSG) includes the Specification for SSG ASTM C1184 and the Guide to SSG in ASTM C1401, FGIA/AMMA SSGDG-1-17 Structural Silicone Glazing (SSG) Design Guidelines and commentary in "Sealants in Construction"—Second Edition by Klosowski and Wolf. In these standards are equations and general direction as well as the minimum properties a silicone structural glazing sealant must have to perform.

These references showcase the notation that the manufacturer of the silicone to be used should be consulted for proper application of the product. These include the quality control desired at the site, at the factory and guidance on how to get the sealant to fill all of the joint, even in concealed joints.

Silicone structural glazing is more than just architecturally pleasing in that in earthquake prone areas, inspectors have found that a SSG building to have much less window failure, in that the lites of glass that are glued on place with a flexible adhesive tolerate the movement stresses and strains much better than fixed glazing. The rationale for this is the lite of SSG glass is sitting on a rubber setting block and the sealant around the edges holding the glass in place protects it from hard surfaces and tolerates the stresses of movement. In high wind (hurricane, derecho, and tornado) areas SSG insulted units can be made that are resistant to penetration by flying debris. States with a high incidence of high wind events ASCE/SEI 7–16 require windows either be protected by shutters or some form of physical protection or have this hurricane resistant glazing, most often with strong polymeric sheets inside of insulated glass units.

The continuous seal is also important in that it gives a thermal break, thus special thermal breaks are not required. The possible elimination of glazing gaskets, anti-walk blocks, glazing channels, and the possibly obtrusive exterior covers result in a much cleaner look to the envelope by using a continuous seal.

Also note that the SSG concept is carried to other materials than monolithic glass and insulated glass but also to thin stone panels and aluminum panels. These can be adhered to the curtain wall structures using the same concepts. The advantage here is that the stone can be very thin and lighter weight since the forces on the stone are distributed evenly around the perimeter and across the backside and this is done with a flexible adhesive, not needing fixed, mechanical attachments.

A major motivation is the ability to achieve an aesthetic effect to the building, i.e. a glass cube or vertical or horizontal glass ribbon effect.

The SSG bead is sometimes the weather seal as well but in any case the attention to the weather seal that accompanies the silicone structural bead is often very well thought out and the continuous nature of the weather seal around the entire perimeter of the glazing system helps to ensure a good weather seal around the glazed unit.

There are some other considerations that need to be touched on such as the silicone structural glazing sealant will have a finite useful lifetime and that needs to be evaluated before glass starts to fall. This will be discussed below but it is important to know that there are ASTM standards—ASTM C1392 and ASTM C1394. These guides are fundamental to knowing if the silicone sealants when 20 years old or 30 years old in these applications are still functioning and perhaps when the next test and inspection should be done. The next guide is also important in that ASTM C1487 indicates techniques to repair problems that may be encountered.

It is also important to realize that the sealants inside the insulated glass units (IGU) used in SSG applications are also structural glazing sealants. The secondary sealant in IGUs used in SSG applications must be able to withstand direct, prolonged UV exposure (life of the installation 20+ years). The sealant that holds units to the frame structure of the curtain wall are predominantly silicone sealants but alternative adhesive bonding systems (Acrylic Foam Pressure Sensitive Structural Glazing Tapes) are also in use. These alternative systems should be carefully considered along with proper consulting and guidance from the manufacturer.

These concerns for adhesion performance are addressed in ASTM C1249 or ASTM E330 the entire assembly.

Another consideration is that often SSG systems are not just glass to metal or glass to glass bonding but often bonded to coated or painted systems. This correctly implies that the sealant is adhered to the glass on one side and to paints or coated surfaces on the other; the stresses and strains of this application will be fully transferred to the coated surface. Thus, there has to be rigorous testing of that coating or painted surface to be sure it has the short term and long-term durability to stay adhered to the metal under the forces being transferred to the coating and its bond. Often this data is not available, and the coating or paint is deleted at the place where the sealant needs to be adhered. This applies to the curtain wall frames as well as the IGUs edge deletion, in IGUs with special coatings on them. And IGUs in commercial construction have coatings on or in them more often than not.

An important inconsideration in all of this is that a major part of these specifications and guides is that they only apply to silicone sealants used in these applications. This implies that the sealant has silicone as 100% of its reactive polymer and thus has inherent UV and heat stability and flexibility at cold temperatures and lesser softening at higher temperatures. If a non-silicone is ever considered in these applications, the specifications will have to be rewritten to include rigorous tests for the above-mentioned properties, since they are not inherent in most other polymer systems.

Other technologies (such as Acrylic Foam Pressure Sensitive Structural Glazing Tapes) have been deployed in this application since the 1990. These technologies have a manufacturer developed separate testing regime that is based on several ASTM testing methods combined with a thermal cycling of completed assemblies at sequentially higher pressures ASTM E283/E283M-19, ASTM E331, ASTM E330. These tests simulate weathering resistance with thermal cycling, but do not include any consideration of Ultraviolet (UV) radiation exposure. The specific exposure of an application to weathering and UV should be part of any conversation about the applicability of these alternative systems deployment in structural glazing. The manufacturer should be consulted for more details on the UV durability and long-term performance of the alternative system.

This section is not intended to reproduce the details of the many facets of Structural Glazing but to point to the most significant sources of information and some commentary on design considerations.

As with all or most all ASTM specifications, what is written is the minimum requirement.

Sealants in Structural Glazing—Sealants Used

Historically, all of the sealants used in structural sealants have been silicones. Because of the known UV resistance of these materials, the standards assume the base polymer is UV resistant and the standards only evaluate the adhesive bonding. This assumed UV resistance is shown in the relatively short 5000 hours and in the first few values in Table 1 in ASTM C1184 Standard Specification for Structural Silicone Sealants indicating values or ranges of values for Extrudability, Shore A hardness, Heat aging, Tack Free time (a measure of cure time) and the Rheological values (slump for the joint at time of installation). These are all important as before. The hardness and tack free time measurements have ranges or maximums so that one should not get a product that is too hard or too soft and cures fast enough to be reasonable to work with. The real use of these values, when test data is reported, is to set the standards by which one can judge if the sealant delivered to the job is typical and within the manufacturing sales specifications.

The extrudability and Rheology tests are simply to indicate it had the proper viscosity to be extruded into the joint and stay in the joint as the sealant cures.

The Heat aging test ASTM C792 in this specification has a maximum of 10% weight loss between what it weighed after curing for a week and what it weighed after oven aging (after cure) at 70°C (158°F) for 21 days. The significance is that it indicates if shrink will be a factor. Shrinking after cure sets can produce long term internal stress and can lead to premature adhesion or cohesion failure, or in some cases just esthetic differences. Up to 10% shrinkage is noted as acceptable.

The key part of this specification ASTM C1184 is the tensile value. This is tested by making actual test joints as in ASTM C1135 but make them only 9.53 mm (3/8") inches wide (more consistent with the SSG application) 25 test joints are made and cured 21 days and split into sets of 5. Each set is tested as follows:

TABLE 1: Requirements for Physical, Mechanical, and Performance Qualities of the Sealant
Property Requirements Test Method
Extrudability, max 10 s C603
Hardness, Shore A 20 to 60 C661
Heat Aging   C792
      Weight Loss, max 10%  
      Cracking none  
      Chalking none  
Rheological, max   C639
      Vertical 4.8 mm (3/16 in.)  
      Horizontal no deformation  
Tack-free time, max no transfer in 3 h C679
Tensile Value, min   C1135
      Standard Conditions: 345 kPa (50 psi)  
      88°C (190°F) 345 kPa (50 psi)  
      -29°C (-20°F) 345 kPa (50 psi)  
      Water Immersion 345 kPa (50 psi)  
      A minimum of 5000 h weathering 345 kPa (50 psi)

Figure 32: Standard Specification for Structural Silicone Sealants ASTM C1184
Source: ASTM C24

This specification specifically states the 5000 hours of weathering is the minimum. Most manufacturers of an SSG sealant will have 20,000 hours of artificial, accelerated weathering. This is a risky application because if a sealant fails, either adhesively or cohesively, glass can fall.

There was much discussion about putting a modulus component into the specification. A lower modulus, softer sealant is somewhat desired in that the SSG sealant has to handle the thermal stresses and strains from temperature changes, especially on sunny sides of buildings. The glass can get quite cold in the winter and material can and will contact and put shear stresses on the sealant. Also, the joint width in SSG applications is often 6.35mm (¼") or 9.53mm (3/8") inch and the stresses from these thermal movements are a function of joint width and these can experience a significant force. That force is lesser if the modulus is lower. On the other hand, the wind force on the lee side of the building (a negative wind load with vacuum force pulling the glass from the building) makes a desire for a stiffer sealant to keep the outward movement from the wind load to a minimum (keep the glass on the setting blocks). There is an extensive discussion of modulus for SSG sealants in the Appendix after the ASTM C1184 Specification. When designing for wind loads, consider all that is included in ASTM C1401. In section 8.2 of the Guide, in the comments about wind loads, is a note that using wind loads from standard building codes and then going a step further to ASCE standard ANSI/ASCE 7 which gets into non-standard shapes of buildings. Often in urban areas the wind loads will change by the position of nearby buildings channeling or blocking the winds. Sometimes modeling and wind tunnel work needs to be done.

The ASTM C1401 Structural Glazing Guide in Vol 04.07 is 55 pages long and should be required reading for all in the industry. This guide assists in the identification and development of performance criteria, test methods and industry practices that should be implemented to obtain the required structural glazing sealant adhesion and compatibility with other system components.

Note in the specification is no mention of the dead load. Many times, a SSG system handles some degree of dead load. The dead load capacity of a SSG sealant is lesser than the ability to handle tensile loads. A sustained load produces bond line fatigue and some degree of rubber (sealant) fatigue. Typically, the dead load is held to a design maximum of about 1 psi (7 kPa) while the design load for the tensile is almost always 20 psi (138 kPa).

The 20–psi maximum design load seems unnecessarily low to some when the sealant must always pass the 50 psi (345 kPa) in the specification and most SSG sealants have values of 100 psi or more. However the 20 psi had little to with the ultimate strength of the product in the ASTM C1135 Tensile Adhesion test, since that test is done with a sample that is lab prepared in the best possible conditions (cleaned perfectly, primed (when required) perfectly and cured perfectly, for a long time (21 days) with no joint movement. The structural joint in actual practice, sometimes has a variable joint filling and variable cleaning and less than the optimum 21–day cure time before being moved and shipped. Thus, the design load is most always kept at 20 psi to compensate for the unseen, unplanned non-ideal conditions. The same is true for the 1 psi (7 kPa) design dead load (see section 30.3.2 and of ASTM C1401). Some designers and specifiers will be even more conservative and hold the dead load to ½ psi (3 kPa) maximum because of unpredicted problems. Some hold the design load to 10 psi (69 kPa).

The magnitude of the forces in a structural glazing sealant is the combination of the wind force (principally on the lee side of the building), the thermal forces (expansion and contraction), and dead load forces. There is a material and bond line fatigue induced by each of those forces, repeated strains and stresses, like in the wind loading (present to some degree every time the wind blows at any speed), and the thermal loading happening to some degree every day, and the dead loads produce a constant stress. It is the combination of all of these stresses that produce that actual fatigue on the sealant and the bonds. The strength after fatigue is always lesser than the strength when measured in the tests shortly after cure, and thus is the basis for the conservative design force maximums used in structural glazing. Variations from the 20 psi and 1 psi recommendations in the guide need to be done with consultation with the sealant manufacturer and the curtain wall fabricator and it is good to error on the side of caution.

Fatigue is mentioned throughout the sealant section and a very good summary of this in ASTM C1401 Section 30.3.5. It is somewhat hidden in the section on seismic movement, but it is very general in concept and applies to all structural glazing and should be even considered when looking at the longevity of the weather seals.

It is important to note that the SSG specification does not require a cohesive failure. SSG is looked at as more of an engineering application and thus the strength needed is key and not the mode of failure. It is important to note short term adhesion tests cannot predict long term durability unless the changes with adhesion (adhesion failure) can be noted as a function of time and temperature in given environments. The strength of the bond cannot be determined without measurable adhesive failure. The Arrhenius equation can be used to predict the lifetime of the adhesive bond, but it can only be used with measurable adhesive failure.

In the discussion of adhesion, a further mention of structurally glazing with stone or aluminum or other façade panels is needed. Care must be taken to be sure the adhesion of these panels has been tested for long term dead load carrying capacity since occasionally there will be designs with no underlying fin or other means to carry the dead load.

It is not the intent here to reproduce the specific instructions noted in the references, but some mention of the determination of the bite, or amount of sealant needed in the adhesive/sealant joint can be made. In the guide ASTM C1401 Standard Guide for Structural Sealant Glazing is the formula for minimum required "bite" based on Trapezoidal distribution of forces on the structurally glazed glass. When leaving the rectangular glass also in ASTM C1401 are formulas for non-rectangular shapes based on the radius of the enclosed circle. These are relatively easy formulas to calculate and in studies done by many researchers, such as Chris White and Larry Carbary. These formulas produce answers that are within 15% of that achieved by finite element analysis. Thus, these formulas are most often used. However, the accuracy in either method of calculation depends on realistic values for wind loads.

This paper is not intended to be all inclusive on any topic and that is especially true of structural glazing designs, but some notes a made here. Designs can be divided into two major types: four sided (no external supports on any side—the "ice cube" look) and two–sided (external, mechanical supports on two sides and SSG on two sides—the horizontal or vertical ribbons of glass). There are of course many variations on this theme like an atrium and other "special features". There are other hybrid versions of two-sided systems where very tall glass lites are held top and bottom mechanically and glazed to glass mullions on the verticals. It gives a complete or total vision with no opaque mullions. The SSG concepts give architects a great deal of design freedom but with it comes a great deal of needed caution to be sure it is done correctly.

Design for Movement and Service Life Prediction

There is no topic more difficult in the sealant industry than Service Life Prediction. Dr. Chris White (of NIST) and Jerome Klosowski (Klosowski Scientific Inc.) presented data at many ASTM C24 committee meetings on the need for combining movement with outdoor exposure. It started as far back as 1980 when Dr. L. Bogue Sandberg and Jerome Klosowski had study on weathering where some sealants were compared to those in weathering racks and those in buildings at Mich. Tech. Univ in Houghton Michigan where Dr. Sandberg was teaching. The sealants on the buildings looked worse than those on the outdoor test racks and those in a UV Florescent Artificial Weathering Machine. It was then they strained the samples to match the maximum elongation seen on the building and more realistic damage was seen in the testing.

This was then collaborated by Dr. H. Bolte and Dr. T. Boettger of the Univ. of Leipzig and with Suni Linde of the National Institute of Testing and Materials in Boris, Sweden.

Each bit of research indicated that realistic damage to sealants need to see the 4 primary forces of heat, light, water and joint movement working together. From the early 1980s to today, attempts were made to develop a standard test for service life. However, the efforts are still not complete.

There are many other variables such as:

Movement during cure—Many have worked on this but few as diligently as Dr. Allan Hutchinson of Oxford-Brooks Univ (Oxford England). His work, with his associates, showed that every sealant they tested had lesser performance if there was joint movement during the cure and the severity of the damage depended on the amount of movement and the cure rate of the sealant. Thus, a really good service life prediction would have to take that movement into consideration. Of course, the amount of movement during cure, seen on an actual job would depend on time of day and the season of the year that the sealant was installed and the specific weather the day of installation and the days immediately following. How can that be standardized for realistic service life prediction?

Work done by Jerome Klosowski in developing the Procedure C of ASTM C1589 with Periodic Manual Extension and Compression showed that the durability to weathering with movement was influenced by the schedule. If after curing 28 days, the first outdoor exposure was in compression, for sealants with slow cure or viscous flow for example there would still be some curing taking place while the sealant was in compression and thus act like the joint was installed at a smaller dimension. If the first exposure was in extension, the slow curing sealant would act like it was installed in a wider joint and have less stress on the bond and sealant in subsequent extensions and compression. Thus, a standard test to predict a realistic service would have to take into consideration the nature of the curing done. How can that be standardized for a test method?

Work done by Chris White in developing the Procedure B of ASTM C1589 Outdoor Weathering of Building Joint Sealants with Continuous Movement got around the variable of movement with cure because the joints cure in the rack with movement. This work has demonstrated the ability to link laboratory generated data to data from an ASTM C1589B exposure in Florida. These results can approximate the change in sealant modulus of the studied formulation at any given time and at a given place. However, these results are valid only for the sealant formulation studied. Future studies seek to generalize these results to entire classes of sealants. The data suggests how the sealant will perform in service for 5 of 10 or 15 years after installation. Now imagine the influence of climate change on that thinking, some regions are experiencing record drought, other record days are record breaking high temperatures.

However, both ASTM 1589 procedure B and C can be done with a variety of sealants and those materials can be compared as to the damage as a function of time at the specific location of the test rack. These are long-term outdoor experiments one won't have a perfect service life prediction, but one can be certain that the sealant that has no flaws after 5 years of exposure and movement will undoubtedly have a longer service life than one with major flaws in a year or two. The sealants under test that show no damage or very little damage in 5 years, by either procedure, will most probably last much longer in real buildings. Note that procedure B has the advantage of having continuous movement and does not need anyone to change the joint dimensions each season, but the disadvantage if having more complicated racks and more expensive racks and the actual movement dependent on the weather that year. Procedure C has the advantage of handling a hundred or more specimens on a 4 ft x 3 ft rack made for a few hundred dollars and allows one to choose the movement and control it precisely, but the disadvantage of having to adjust the joint size several times each year.

outdoor rack ASTM C1589 procedure B in a Florida location

Figure 33: Picture of outdoor rack ASTM C1589 procedure B in a Florida location.

There are many factors to consider when doing either of these two procedures. If the data sheet says no primer is needed, should one use a primer since in almost all cases, a primed joint has longer, sustained, adequate adhesion? If the data sheet says the sealant will perform at ± 50% should these procedures be set to produce 50% extension and 50% compression, or should this be done at half the advertised strain? Using half of the data sheet stated movement ability is reasonable since many architects are so doubtful of values in the data sheets, they only use half that value in the joint size calculations, and they want to see how a sealant will perform as they instruct it to be installed. Or some specifiers design all joints for a sealant with ± 25% joint movement ability, so they want all sealants tested in these procedures at +25%, regardless of what is put in the data sheet. It is their test rack, and they are testing to the conditions they instruct to be installed.

These questions confound the problem of making a standard test. These procedures are performed by individual applicators, individual engineering firms, individual architects, and individual distributors. Relative to Procedure C, they put racks out in areas where they do business (their climate), some in two or three different parts of the country (various climates). They typically test only five to ten sealants (the ones they have been using and maybe a new one or two) and get comparative data. At least one contractor and one engineer has over 25 sealants under test with this procedure. Each puts their unique twist to the conditions. Some start in extension, others in compression and some have two racks one starting in extension and the other in compression. One has all sealants moved at ±25%. One has all at the maximum movement noted in the data sheets. One has all samples primed on all surfaces. One has samples primed or unprimed following the dictates in the data sheets.

There have been efforts to match the weathering on the outdoor racks. In Procedure B it is done with the instrumentation at National Institute of Standards & Technology. With Procedure C the samples are put into an UV Fluorescent Artificial Weather Machine using the condition called out in ASTM C1442, with 8 hours radiation at 70°C (158°F) and 4 hours of condensing water at 50°C (122°F) for 8 weeks of exposure and then, the joint is inspected and the joint size changed and the above weathering repeated. The joint size is changed every 8 weeks. Note that this sometimes produces damage more severe than outdoors. This was especially true where the outdoor samples had severe dirt pickup that protected them from some of the UV rays of the sunlight. For many sealants the damage outdoors was very similar to the damage seen in the artificial weathering but the time to do that varied with the sealant because many sealants differ in their primary mechanism of failure. Some always fail adhesively (some got hard and put much added stress with movement) while some sealants rubber properties didn't seem to change, and it seemed the bond deteriorated by chemical changes or simple bond or substrate fatigue.

These advanced techniques to evaluate service life are being used by many organizations, but the interpretation of the data has yet to be standardized into common practice.

Note that these outdoor tests also give an idea of the long-term esthetics. Dirt pickup, cracking, crazing, yellowing, deterioration to a gum like substrate can all be noted.

It needs to be noted that in planning these type tests there is a very useful guide in ASTM C1850. This Guide is very useful, and it also has a table 1 (degradation factors affecting the design life of the sealant systems). It indicates that the user of sealants must look at the expected movement, temperature of application, temperatures expected during use and for what duration, ultraviolet exposure, extra movement from wind, rain, snow, ice, dew (type and kind of wetness and duration), probability of physical damage during use, possibility of chemical damage (nearness to vent pipes or factory pollution or other chemical sources), biological factors (note that fungus and mold growth are common, generally unsightly and for some, detrimental to performance), seismic movements and others.

The conclusion is this section shows that work on service life predictions is being done with some success but not with significantly accelerated tests, and the data from the above procedures are at best estimates, since industry will put all the possible deterioration factors into a single test. The above-mentioned procedures are best used for sealant comparisons as to sustained adhesion and sealant performance under common use conditions. It would be prudent not to expect these procedures to ever be in a specification but be a very useful and perhaps necessary tool for manufacturers to determine the usefulness of their product in specific climates, certain applications, and users and specifiers to have a handle on real data on durability of the products they choose. The data will also be used comparatively and be qualitative, and not truly quantitative, relative to actual expected useful lifetimes of the various sealants on specific buildings.

Insulated Glass Units

Insulating Glass Units (IGUs) provide visibility to the outside and a source of natural lighting while also insulating a structure. IGUs come in many forms with a variety of interior components, spacer designs, and sealant choices (Figure 1). In general, the sealants for IGUs must perform several functions—adhere to the glass and spacer, protect against moisture ingress, prevent loss of insulating gas, and absorb/diffuse mechanical stresses due to environmental changes.

Common IGU Edge Seal Designs

Figure 34: Common IGU Edge Seal Designs
Source: HB Fuller

Traditional IGU design relies upon a dual-seal edge design around a desiccant-filled box spacer, composed of polyisobutylene (PIB) primary sealant and an elastic secondary sealant that protects the primary seal. PIB is an excellent gas and moisture barrier, but is thermoplastic and subject to stresses during the life of an IGU that will fatigue it beyond its ability to recover, hence the need for a secondary sealant to provide structural support. PIB is produced in both black and gray variants, both with unique additive and filler packages to provide UV and thermal stability. Due to PIB's important role in preserving the insulating cavity, it must be subjected to UV and oxidative stability tests, as well as tested for compatibility with any polymeric glazing components that will be using in the IG system (i.e. setting blocks, glazing sealants, etc.). Note that a black PIB sealant is a quite different formula from the Gray PIB and each need testing for durability and compatibility. Excellent test methods for compatibility can be found in FGIA/NGA TB-1700 Compatibility Testing of Glazing Materials Related to the Performance of Polyisobutylene in Insulating Glass Units—(In Preparation), ASTM C1087 Test for Compatibility with Accessories and ASTM C1294 for Compatibility of Edge Seals with Glazing materials are important test. An excellent informational document related to PIB can be found in FGIA TB-1250 PIB Primary Sealant in Insulating Glass Units and ASTM C2149 Guide for Secondary Seals for Sealed Insulted Glass Units for structural glazed applications. PIB adhesion to glass and spacer can be evaluated using FGIA TB-2700 Sealant Adhesion Test.

In IGUs where the edge seal will be exposed to UV light, such as in structurally glazed IGUs (Figure 35), this secondary sealant is almost exclusively silicone due to its superior UV resistance and weathering properties. Since the secondary sealant in a structurally glazed insulated glass unit is also a structural sealant It is critical that the secondary sealant pass all the tests in ASTM C1184 Specification for Structural Silicone Sealant, except for the water immersion tests since excessive water contact is not expected in this application. Look to ASTM C1369 for details for Secondary Edge Sealants for Structurally Glazed Insulated Glass Units.

Structurally glazed IGU

Figure 35: Structurally glazed IGU
Source: HB Fuller

Larger manufacturers will typically use two-component silicones to allow for more rapid cure and shorter hold times in the facility. One-component silicones allow for easier application and less equipment expense, but usually at the expense of cure speed. High modulus silicone is typically chosen for high performance units such as those that are structurally glazed or are subject to high wind loads. The importance of the silicone chosen for a structurally glazed unit cannot be overstated, as it is one of the only components holding the outboard lite of an IGU on to the building envelope. Such sealants should pass all the same tests as in ASTM C1369.

For dual-seal IGUs that have a captured edge where UV exposure is not a concern, such as in a residential or light commercial units, it is very typical for manufacturers to utilize polysulfide or polyurethane secondary sealants. Such sealants must still protect the PIB primary sealant, but the sealant is not as subject to UV degradation as in an exposed edge system. Polysulfide and polyurethane tend to be lower cost than silicone and still maintain excellent physical properties and favorable cure profiles. A comprehensive listing of sealant tests for both quality assurance and characterization is published in FGIA TM-2400 Test Methods of Insulating Glass Sealants.

Regardless of the primary and secondary sealant chosen, the test methods used to determine suitability need to be looked at critically to be certain the high temperature used in the testing is equal to or greater than the temperatures expected to be encountered in the IG unit, and the time at temperature is sufficient to determine if a possible negative effect will be seen.

Other edge constructions detailed in Figure 34 are U-channel systems, wherein the spacer is folded into a channel that contains desiccating matrix (desiccant carried in a polymer); foam spacer systems that have a moisture barrier film along the back of a desiccating foam and acrylic PSA along the sides to help hold it in place; and thermoplastic spacer, a PIB-based spacer that replaces primary sealant and desiccant and can be extruded directly from a drum to the glass surface. Sealant choices for each of these edge designs are unique to the end use, but are commonly single-seal types such as hot melt butyl (HMB) or reactive hot melt (RHM) sealants. Both make varying degrees of compromises between the barrier properties of PIB and the structural properties of elastic sealants. Both classes are applied warm or hot and achieve handling strength immediately upon cooling, while RHMs will cure in the presence of ambient moisture to achieve higher strength and heat resistance than HMBs.

All sealants will still need to meet the standard industry specification, ASTM E2190 with the test methods called out in ASTM E2188, ASTM E2189, and ASTM E2649. Only after units are constructed and meet this specification have the various sealants proven their basic capability to function in an IGU application. Innovative fabricators will find innovative ways of making IGUs and if these units are sealed, the sealants will need to be tested. Thus, knowledge of the standard tests, specifications, and performance criteria mentioned above is fundamental. However, knowledge about how a given application produces conditions more severe than those above is just as fundamental. Knowing these things, one can approach the sealant manufacturer and talk about testing and data that assures the sealant offered will tolerate the IGU's service conditions for the life of the unit.

Relevant Codes and Standards

Guide Specifications

Additional Resources

Trade Associations and Other Organizations

Trade Publications




A more complete living listing of definitions can be found in ASTM C717. The purpose of the definitions below is to give some guidance so the reader can better understand the terms in this document.

  • Creep: The deformation of a material due to a constant applied load. Creep Rupture is the extreme case where the material not only deforms but fails catastrophically under the constant load.
  • Diffusion: A physio-chemical phenomena where mobile chemicals can move from an area of high concentration through a semi-permeable membrane to an area of lower concentration.
  • Durability: The ability of a material to still function as intended after exposure to environmental conditions as well as physical forces. For a sealant this means that after a number of years in service the sealant is still able to seal the joint and prevent air, water and other environmental substances from moving between the inside and outside of the structure.
  • Environmental Product Declaration (EPD): The document that sets forth a product's environmental impacts throughout the various phases of its use from Manufacture, use and end of life. An EPD is calculated based on a specific Product Category Rule. These documents can help earn specific credits in various "green building rating systems".
  • Fatigue: In engineering sense, is crack propagation in a material due to cyclic movement.
  • Green Globes: An online assessment protocol of the Green Building Initiative, rating system and guidance for green building design, operations, and management.
  • International Green Construction Code (IgCC): A model code that provides minimum requirements to safeguard the environment, public health, safety and the general welfare through the establishment of requirements that are intended to reduce the negative impacts and increase the positive impacts of buildings.
  • Joint Movement: The oscillation of the surfaces to which a sealant is adhered. This oscillation is due primarily to thermal factors, but other environmental conditions can also drive changes in dimensions of the substrates. Joint Movement is usually measured in percent of movement. Therefore, a one inch wide joint that has a joint movement of ±25% would open to 1.25 inches and close to 0.75 inches.
  • Leadership in Energy and Environmental Design (LEED): A green building rating system of the U.S. Green Building Council (USGBC) used to "certify" buildings, interiors, and neighborhoods as being constructed in an environmentally preferable manner.
  • Life Cycle Assessment (LCA): The calculation of the impact of a product on the environment throughout its manufacture, use and end of life stages.
  • Living Building Challenge (LBC): A green building standard of the International Living Future Institute that focuses on visionary, but attainable building goals including regeneration.
  • Modulus of Elasticity: The instantaneous slope of the stress strain curve. The higher the modulus the more force is needed to deform the material.
  • Product Category Rule (PCR): The rule set used when an Environmental Product Declaration is prepared. It sets the end point and limitations to be used.
  • Sealant: A material with adhesive properties which is formulated to fill or seal gaps or joints between two surfaces. The main purpose of a sealant is to prevent air, water and other environmental substances from entering or exiting a structure while permitting limited movement of the substrates.
  • Stress Relaxation: The reduction of stress (load) when a material is placed under a constant strain (deformation).
  • Stress Strain Recovery: Measures how much of a sealant's original dimension is retained after the sealant has been stretched sometimes held for a period of time and then allowed to relax. This is usually expressed as a percentage.
  • Substrate: The physical surface to which the sealant is applied. Substrates can be concrete, glass, aluminum, wood, etc., depending on the materials of construction.
  • Tear Strength: The resistance to a tear propagating through a material. There are several generally accepted methods to test tear strength which are covered in ASTM D624.
  • United States Green Building Council (USGBC): A Non-Governmental Organization (NGO), in the "Green Building" area. Their main program is called LEED and is used to certify that new or remodeled buildings, interiors, and even neighborhoods are constructed and maintained in an environmentally preferable manner.


The Adhesive and Sealant Council (ASC) would like to recognize the following contributors to this publication which is positioned to supplement joint sealant standards that are published and on the books. ASC recognizes primary co-authors Steve Rosenberg formerly of Sika and Jerome Klosowski (Klosowski Scientific Inc.) formerly of Dow Corning. ASC would like to additionally recognize Chris White of Exponent (formerly NIST) and Steve Duren of ASC as the primary editors/contributors to this publication. The ASC Wall Interface Task Force is also recognized in contributing images, time with document review, and content contributions, as well as Dan Horner of Applied Adhesives, Dave White of Sika, Paul Majka of Henkel Corporation, Brian White of HB Fuller, and Mark Lund and Peter Golter of 3M.

ASC is the voice of the North American adhesive and sealant industry and has developed online free resources on its building and construction Vertical Wall Landing Page. This resource is a complete library of videos, white papers, webinars, and where to find standards and educational resources for various types of adhesives and sealants used in vertical wall systems.