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Building Envelope Design Guide - Glazing

by Nik Vigener, PE and Mark A. Brown
Simpson Gumpertz & Heger Inc.

Last updated: 06-01-2009

Introduction

Glass has been used for thousands of years to allow daylight into our buildings, while providing weather protection. The development of the float glass process in the 1950s allowed the economical mass production of high quality flat glass and virtually all architectural glass is now produced by this process. The vast majority of new windows, curtain walls and skylights for commercial building construction have insulating glazing for energy efficiency and comfort. This glazing Chapter is complementary to the other fenestration sections of the Design Guide.

 

Description

The following covers brief descriptions of commonly used glass and glazing components:

Architectural glass comes in three different strength categories. Annealed glass is the most commonly used architectural glass. Because it is not heat-treated and therefore not subject to distortion typically produced during glass tempering, it has good surface flatness. On the downside, annealed glass breaks into sharp, dangerous shards. Heat-strengthened and fully-tempered glass are heat-treated glass products, heated and quenched in such a way to create residual surface compression in the glass. The surface compression gives the glass generally higher resistance to breakage than annealed glass. Heat-strengthened glass has at least twice the strength and resistance to breakage from wind loads or thermal stresses as annealed glass. The necessary heat treatment generally results in some distortion compared to annealed glass. Like annealed glass, heat-strengthened glass can break into large shards. Fully-tempered glass provides at least four times the strength of annealed glass, which gives it superior resistance to glass breakage. Similar to heat-strengthened glass, the heat-treatment generally results in some distortion. If it breaks, fully-tempered glass breaks into many small fragments, which makes it suitable as safety glazing under certain conditions.

Laminated glass consists of two or more lites of glass adhered together with a plastic interlayer. Because it can prevent the fall-out of dangerous glass shards following fracture, it is often used as safety glazing and as overhead glazing in skylights. The plastic interlayer also provides protection from ultraviolet rays and attenuates vibration, which gives laminated glass good acoustical characteristics. Because laminated glass has good energy absorption characteristics, it is also a critical component of protective glazing, such as blast and bullet-resistant glazing assemblies. See Building Envelope Design Resource Page Blast Safety for more information.

Coated glass is covered with reflective or low-emissivity (low-E) coatings. In addition to providing aesthetic appeal, the coatings improve the thermal performance of the glass by reflecting visible light and infrared radiation.

Tinted glass contains minerals that color the glass uniformly through its thickness and promote absorption of visible light and infrared radiation.

Insulating glass units (ig units) consists of two or more lites of glass with a continuous spacer that encloses a sealed air space. The spacer typically contains a desiccant that dehydrates the sealed air space. The air space reduces heat gain and loss, as well as sound transmission, which gives the ig unit superior thermal performance and acoustical characteristics compared to single glazing. Most commercial windows, curtain walls, and skylights contain ig units. Most perimeter seals consist of a combination of non-curing (typically butyl) primary seal and cured (frequently silicone) secondary seal. The service life of an ig unit is typically determined by the quality of the hermetic sealants installed between the glass and the spacers, and the quality of the desiccant.

Fundamentals

Thermal Performance (Conduction, Solar Radiation, Thermal Break, Comfort)

Glass and glazing selection play a key role in determining the overall building's thermal performance. Fenestration thermal performance requirements must be integrated with the design of the building's heating and cooling systems. Single glazing has poor thermal performance and is suitable only for applications where thermal performance is irrelevant, such as interior applications or installations where interior and exterior temperatures do not vary substantially. The vast majority of architectural glazing consists of ig units. The thermal performance of insulating glazing depends mainly on the solar energy transmittance through the glazing, the reflectance of the glazing (measured by the shading coefficient—the ratio of the solar heat gain through the glazing to the solar heat gain or loss through a lite of 1/8 in. thick clear glass), the width of the air space, and the material and configuration of the spacer around the perimeter of the unit. Low-emissivity (low-E coatings) limit heat gain through the glazing by reflecting heat energy. Reflective coatings reduce interior solar heat gain by reflecting solar energy.

Thermal performance of glazing is expressed by its thermal conductance, which a measure of air-to-air heat transmission due to thermal conductance and the difference between indoor and outdoor temperature. Conductance is expressed in terms of U-value. A lower U-value indicates reduced heat transfer through the glass. Thermal modeling of specific fenestration assemblies using computer programs such as THERM allow estimation of total U-values for fenestration assemblies and help predict thermal performance.

Moisture Protection (Water Penetration, Condensation Resistance)

Since glass itself is impervious to water penetration, glazing waterproofing performance is determined by the glazing method chosen (e.g. wet glazing versus dry glazing) and drainage details of the framing system. Wet glazing most commonly consists of a gunable ("wet") sealant installed over a preformed tape or gasket. Dry glazing systems utilize extruded rubber gaskets as the glazing seals. This system is also referred to as compression gasket glazing because the system relies on compression of the glazing gasket to seal against air infiltration and water penetration. The systems are sometimes mixed, most commonly with exterior wet glazing and interior dry glazing.

Condensation occurs if the temperature of interior frame or glazing surfaces falls below the dewpoint temperature for the interior air. Glazing strategies for limiting condensation include providing glazing with a low U-value and providing supplemental heat to the glazing to increase surface temperatures.

Visual (Daylighting, Aesthetics)

Glass appearance is influenced by several factors, including tinting (colorants added to the glass batch), reflective and low - E coatings, and opacifiers (for spandrel glass).

Sound (Acoustics)

Acoustic performance of exterior building envelope assemblies is expressed in terms of the Outdoor—Indoor Transmission Class (OITC) rating, which is a measure of the sound transmission loss during standard tests. High sound transmission loss, and therefore good sound insulation, is desirable in most applications. An integrated strategy to limit sound transmission through building walls requires review and testing of the entire wall system, since even small discontinuities in the wall assembly can negate the benefits of a well designed glazing system with a high OITC rating. In general, a higher fenestration OITC rating can be attained by incorporating laminated glass, and insulating glass assemblies (double or triple glazing) because the laminate damps vibration and the air space limits sound transmission. Additional mass in the form of thicker glass lites also helps sound absorption.

Safety Glazing

Fully-tempered or laminated glass is commonly used for safety glazing. Tempered glass limits the risk of injury by fracturing into small fragments. Laminated glass limits the risk of injury by retaining the fractured glass on the plastic interlayer and thereby limiting fall-out of glass fragments. Safety glazing must be identified with an indelible label on the glass indicating its conformance to federal safety standards published by the Consumer Products Safety Commission (CPSC).

Wire glass typically does not meet CPSC requirements for safety glazing but is used for fire-rated glazing.

Health and Indoor Air Quality

Glazing can contribute to health and indoor air quality problems (by supplying water for mold and mildew growth) by allowing water leakage or condensation.

Durability and Service Life Expectancy

Glass is one of the most durable construction materials, substantially resisting the effects of normal weathering for decades. But glass is also a classic brittle material, suffering significant strength degradation from scratched edges or chips. Most glazing durability problems fall into the following categories (in roughly descending order of failure frequency):

Fogging of ig units is caused by condensation of moist air that penetrates into the air space of insulating glass units through or around the hermetic seal of the unit. Seal failure is usually caused by prolonged water exposure of the perimeter seal, such as occurs when ig units are glazed into frames that do not have functional weep holes to drain water leakage. Premature seal failure can also be caused by discontinuities, poor bond or thin applications of the perimeter seals. To assess the susceptibility of ig units to seal failures, representative units are tested by cycling the units through heating and cooling cycles in accordance with ASTM E-774. Units that pass the test are grouped in three performance levels: Class C, Class CB, and Class CBA. Studies have shown that, in the absence of other deficiencies such as water immersion, after about 20 years the failure rate of Class C or CB units will be about 15% and the failure rate of Class CBA units will be about 2.5%. The desiccant contained in the spacer helps condensation resistance by absorbing moisture built into the unit. Spacers with bent, welded, or soldered corners, rather than corners constructed with slip-in corner keys, are more reliable because they provide a stable surface for primary and secondary seal adhesion.

Similar to ig unit seal failure, laminated glass can delaminate when the edge of the laminated glass is in contact with water over extended periods, causing the interlayer to debond from the glass surface.

Glass fracture is typically caused by impact or by weakening of the glass through the development of cracks, chips or surface scratches. Edge and surface damage can result from careless handling or glass-to-metal frame contact. Fully-tempered glass can break spontaneously from Nickel-sulfide impurities.

Maintainability and Repairability

Except for cleaning, glass is generally maintenance-free. Glass used in masonry walls requires frequent cleaning when the building is new to remove alkalis that leach out of the masonry and will etch the glass if left on too long. Some acids used in common masonry cleaners, such as hydrofluoric acid, can dissolve the glass surface and mar it permanently. The deteriorating effect of cleaners is acutest if the glass has a reflective coating on the exterior surface. Cleaning methods for glass should be mild and non-abrasive.

The glazing seals between the glass and framing must be replaced periodically to maintain good performance. Properly installed silicone wet seals should last 10 to 20 years; gaskets 15 to 20 years.

Replacement of failed ig units is facilitated if the units are glazed from the interior, see the discussion in the relevant window, curtain wall and skylight portions of the design guide, but this glazing configuration typically has less reliable waterproofing performance. Fogged i.g. units cannot be repaired.

Sustainability

I.g. units have a shorter service life (most practitioners estimate it at 15 to 30 years) compared to monolithic glass, which, if not physically damaged, has an infinite lifespan. The energy savings afforded by i.g. units usually pays for the replacement cost if the units last more than 15 years. Actual payback periods vary substantially by location.

On the downside, i.g. units are typically not recycled: since they consist of a mix of glass, metallic glass coatings, sealants, and aluminum spacers, i.g. units require significant and costly effort to separate the constituent materials. Furthermore, glass is manufactured from relatively inexpensive and abundant raw materials, which makes glass recycling unattractive. At the end of their service life, i.g. units are generally discarded as general trash. Crushed glass is sometimes utilized as hard fill. Most glass manufacturing plants recover glass discarded during the float glass manufacturing process and combine them with other batch materials for subsequent production. Overall, the most promising strategy to limit the amount of glazing in the waste stream is find ways to extend the service life of i.g. units.

Applications

Improving Waterproofing Performance

The waterproofing performance of the glazing system depends on the following:

  • Details of the framing system that promote drainage,
  • internal framing seals,
  • external (i.e. glass-to-frame) seals, and
  • frame perimeter seals and flashings.

Glazing sealants cannot exclude all water, so providing internal drainage is critical; see the discussion in the relevant window, skylight, curtain wall and door portions of the design guide.

Wet-glazed systems generally prevent water entry around the glass and into the glazing pocket much better than dry-glazed systems. Replacement intervals for dry gaskets and the cap bead of wet-glazed systems are about equal. Table 1 lists advantages and disadvantages of both systems. For maximum watertightness, a wet-glazed system, consisting of pre-shimmed butyl tape glazing tape and silicone cap bead, should be specified.

Table 1—Wet vs. Dry Glazing

Glazing SystemAdvantagesDisadvantages
Wet Glazing
(Gunable wet seal over back-up rod or glazing tape)
• Improved resistance to water penetration•    Requires exterior access for installation, maintenance and glass removal
•  Protects i.g. unit edges and laminated glass from water and premature deterioration•  Highly workmanship dependent (surface preparation, weather, etc.)
•  Reduces glass movement ("walk")•  Costs more than dry glazing
Dry Glazing
(pre-formed rubber gasket)
•  Can be done from interior•  Not as watertight
•  Less dependent on field workmanship and weather•  Gaskets can shrink, creating openings for water penetration
•  Generally less costly than wet glazing•  Gaskets can roll into pocket and place uneven stress on glass
 •  Glass can "walk"

Improving Insulating Glass Durability and Thermal Performance

The durability of ig units is dependent on the quality of the hermetic seal and the level of protection from water afforded by the glazing seal and the window frame system. The following design requirements are critical to ig unit durability:

Dual seals (butyl-based primary seal and silicone secondary seal) are more reliable and durable than single-seal systems. The continuity and uniformity of both primary and secondary seals is critical, and continuous seals should be stipulated in the specifications. The spacer should be filled with desiccant and constructed with bent, welded, or soldered corners rather than corner keys. The units should carry a CBA rating per ASTM E774 to ensure comparable units have reasonable durability.

Other critical glazing features that should be specified to reduce the risk of i.g. unit seal failure include properly sized setting blocks (min. 1/4 inch thick) to raise the edge of the ig unit glass above water level in the glazing pocket. The setting blocks must be wide enough to support the entire ig unit cross section and be notched to allow water to drain toward the weep holes. Setting block material must be chemically compatible with the i.g. unit secondary seal. The frame design must promote water drainage away from i.g. unit (i.e. sloped glazing pockets, large (3/8 in. diameter) weep holes, and drainage within each glazing opening (i.e. do not use vertical mullions as "downspouts"); see the discussion in the window, sloped glazing and curtain wall sections.

Glass manufacturers publish center-of-glass U-values. The perimeter of insulating glass typically has a higher U-value due to heat transmission through the spacer. Fenestration framing will also have a different U-value. Therefore, window and curtain wall manufacturers publish total U-values based on specific glass products glazed into their systems. Heat loss and condensation problems almost always occur near the glazing perimeter. Thermal analysis of the entire window or curtain wall system, including all perimeter conditions, is required for high-humidity applications or buildings where condensation is a concern.

Improving Laminated Glass Durability

Similar to i.g. unit failure, failure of laminated glass by delamination is frequently caused by long-term exposure of the glass edge to moisture. Design recommendations to limit the risk of laminated glass failure include the following:

  • Protect the edges of laminated glass from exposure to water to limit the risk of delamination. In general, glazing installation details that promote good waterproofing performance and i.g. unit durability (see paragraphs Improving Waterproofing Performance and Improving Insulating Glass Durability and Thermal Permformance above), will also result in improved laminated glass durability.
  • Some materials used for laminated glass interlayers, such as polyvinyl-butyral (PVB) are not compatible with many building sealants, so some delamination will occur with butt-glazed joints where the sealant is in contact with the interlayer.
  • Check the track record of laminated glass products that have several added plastic interlayers for increased impact resistance as some combinations of interlayer products adhere poorly and can cause delamination.

Designing for Fracture Resistance

Design recommendations to limit the risk of glass fracture include the following:

  • Avoid glass-to-frame contact. Provide setting blocks and anti-walk pads to separate the glass edge from the metal. Follow GANA glazing guidelines.
  • Use heat-strengthened glass for high temperature applications, such as spandrel glass, and where greater resistance to bending an thermal stresses, compared to annealed glass, is required. Limit the residual surface compressive stress to 7,500 psi to reduce the risk of breakage due to Nickel Sulfide (NiS) impurities. Producing heat-strengthened glass within these limits is difficult and requires tight control of the production process to avoid exceeding the upper limit for residual surface compressive stress and introducing the potential for NiS fracture.
  • Use fully-tempered (FT) glass where required by code, but avoid use in areas where breakage poses a risk to safety due to the potential for spontaneous breakage from NiS impurities. Where the use of FT glass is unavoidable, and where its breakage poses a threat to people or property, heat-soak the FT glass to reduce the risk of spontaneous breakage due to nickel sulfide inclusions. This additional processing step adds cost and time, but is warranted where the consequences of glass fracture are significant. Alternatively, use laminated glass for safety glazing and fall-out protection. In many applications where FT glass is used, heat-strengthened glass is adequate to meet strength demands and reduces the risk of spontaneous fracture.
  • For all glass types avoid edge and surface damage. Such damage concentrates stress from normal wind or thermal loads, especially for tinted glass or spandrel glass enclosing un-vented spaces.

Determining Glass Thickness

Use ASTM Standard E1300—"Standard Load Practice for Determining Load Resistance of Glass in Buildings" to select appropriate glass thickness to resist service loads.

Special Considerations for Overhead (Sloped) Glazing

The design and selection of glass for overhead glazing requires special attention to the following considerations; see Sloped Glazing for additional information:

The high degree of solar exposure and stratification of warm air beneath the glass results in higher temperatures, increased thermal movement and stresses. The increased strength demand generally requires heat-strengthened glass. The inboard lite of sloped glazing should be laminated for fall-out protection.

Dead loads, snow loads, seismic loads, live loads and wind loads must be analyzed in combinations as required by codes and good engineering practice. Service loads typically include maintenance workers walking on the glass and framing. Unlike wind loads, snow loads are long-duration loads. Glass strength diminishes with duration of loading so time of loading must be included in the analysis. The structural strength of glass is time-dependent and decreases with the length of load application.

Designing for UV Protection

Ultraviolet radiation can cause material deterioration. Methods to provide UV protection, e.g. for libraries or museums, include providing laminated glazing (the PVB interlayer absorbs UV), certain applied films, or curtains and shades. Depending on the thickness of the PVB interlayer, laminated glass can filter out more than 99% of the UV radiation. Applied films are easily scratched and eventually experience color changes, so they are less durable than laminated glazing.

Design Considerations for Safety Glazing

The International Building Code (IBC) Section 2406, state and local building codes, and federal safety standard CPSC 16 CFR Part 1201—Safety Standard for Architectural Glazing Materials, require safety glazing in specified hazardous applications, including:

  • Glass in interior and exterior doors and sidelites
  • Glass within 36 inches (horizontal) of walking surfaces, with bottom edge less than 18 inches and top edge greater than 36 inches above walking surface, and glass area greater than 9 sf.
  • Glass in guards and railings

These code provisions are minimum requirements. Prudent design practice may dictate the use of safety glazing for other applications.

Available manufacturing techniques may limit combinations and choices for glass assemblies; e.g. bent, heat-strengthened or fully tempered, and laminated glass is difficult to achieve because the heat treatment warps the glass and makes mating glass surfaces during laminating difficult).

Logistical and Construction Administration Issues

Inspection and maintenance of exterior glazing sealants requires access to the exterior of windows and curtain walls. Provisions for this access (e.g. suspended scaffolding tie-off anchors) must be made during the design.

Primary and secondary seal continuity and uniformity of minimum width requirements for i.g. units is critical to durability and should be spot-checked on a representative number of actual production units, not just mock-up samples, prior to installation. Mock-up or sample installation often save time and money in the long run as it highlights potential production, lead time, coordination, and performance problems. The production quality of the glazing components and their proper configuration must be checked on a statistically relevant sample of production units.

Glass must be handled carefully during transportation and installation to avoid edge damage and reduce the risk of later glass fracture. Refer to GANA manual for appropriate handling techniques and PPG's Technical Bulletin TD112—Handling Do's and Don'ts to Reduce Glass Breakage.

Details

DWG iconDWF iconPDF icon

The following details can be downloaded in DWG format or viewed online in DWF™ (Design Web Format™) or Adobe Acrobat PDF by clicking on the appropriate format to the right of the drawing title. Download Autodesk® DWF Viewer. Download Adobe Reader.

Schematic of Poor Glazing System (Detail 3.1-1)  DWG | DWF | PDF

This detail illustrates commonly found poor design and installation features that contrast with the good design features of the next detail, Schematic of Good Glazing System.

  • The lack of weep holes in the glazing pocket allows water accumulation and promotes i.g. unit and laminated glass failure.
  • Without anti-walk pads, the i.g. unit may "walk" and contact metal frame edges. Glass-to-metal contact may lead to glass edge damage and fracture.
  • Fastener penetrations through the glazing pocket allow leakage into the wall cavity below. If sill flashing is present, fasteners set through the glazing pocket will puncture the flashing and cause leakage.
  • The flat sill allows water to pond and increases the risk of leakage. Missing sill flashing allows water leakage into the wall cavity.
  • Without an integral return edge, the frame provides inadequate bonding substrate for the perimeter sealant. A poorly configured perimeter seal will not be durable and will promote leakage past the window jamb.

Schematic of Better Glazing System (Detail 3.1-2)  DWG | DWF | PDF

The most important features illustrated in this schematic detail are glazing pocket weep holes, sloped-to-drain sill glazing pocket and sill flashing.

  • The glazing pocket weep holes drain water that penetrates the glazing seals. A well-drained glazing pocket prolongs the service life of the insulating glass (by reducing the exposure of edge seals to water), and reduces interior leakage.
  • The sill flashing is sloped to the exterior to promote drainage. The window sill frame is attached through the back to a structural clip angle, to avoid fastener penetration of the horizontal portion of the sill flashing.
  • The wet glazing seal provides better water penetration resistance than dry glazing (gaskets).
  • The anti-walk pad at the window jamb prevents the glass from "walking" in the glazing pocket and contacting the metal frame.
  • The perimeter of the window frame includes substantial return legs that provide adequate bonding surfaces for a properly configured sealant joint at the window perimeter.

Emerging Issues

Self-cleaning or easy-to-clean glass was recently developed and uses titanium dioxide coatings as a catalyst to break up organic deposits. It requires direct sunlight to sustain the chemical reaction and rainwater to wash off the residue. Anorganic deposits are not affected by the coatings.

Photochromic coatings incorporate organic photochromic dyes to produce self-shading glass. Originally developed for sunglasses, these coatings are self-adjusting to ambient light and reduce visible light transmission through the glass. In architectural glass they are typically used to provide shading.

Glass with electrochromic coatings utilizes a small electrical voltage, adjusted with dimmable ballasts, to adjust the shading coefficient and visible light transmission. Like photochromic coatings, they are intended to attain lighting energy savings.

Point-supported glazing is sometimes used in wall systems that are all glass. These systems utilize mechanical anchors at discrete locations near the glass edge, rather than continuous edge supports. Edge-supported glass is typically sized according to glass load resistance charts; see ASTM E1300. These charts do not apply to point-supported glazing, which requires specific structural engineering analysis.

Relevant Codes and Standards

General Glazing Information

Thermal Performance

Acoustical Ratings

Additional Resources

WBDG

Design Objectives

Functional / Operational—Ensure Appropriate Product/Systems Integration

Products and Systems

Section 07 92 00: Joint Sealants, See appropriate sections under applicable guide specifications: Unified Facility Guide Specifications (UFGS), VA Guide Specifications (UFGS), DRAFT Federal Guide for Green Construction Specifications, MasterSpec®

  • Lawrence Berkeley National Laboratory, information on emerging issues and energy efficiency
  • Department of Energy, information on emerging issues and energy efficiency
  • PPG Industries—A glass manufacturer's website with good glossary, general information on glass and glazing technology and useful technical bulletins
  • U.S. Green Building Council, information on the LEED program, news articles related to Green building
  • Oikos® Green Building Source, information on sustainable design and Green building, including glazing, windows, skylights, and emerging technology related to sustainable design