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Building Envelope Design Guide - Windows

by Nik Vigener, PE and Mark A. Brown
Simpson Gumpertz & Heger Inc.
Revised by the Chairs of the Building Enclosure Councils with assistance from Richard Keleher, AIA, CSI, LEED AP and Rob Kistler, The Facade Group, LLC.

Last updated: 06-25-2012

Introduction

Prior to 1900, windows in the U.S. were predominantly wood frame, with some custom metal windows (iron, bronze, steel) in institutional construction. Around 1900, some British manufacturers of custom metal windows adopted the technology of rolled steel shapes to produce special rail profiles for windows. Two of the more prominent British steel window companies opened U.S. manufacturing companies to produce rolled steel windows. The fire resistance of steel windows with wire glass helped popularize steel window use in the U.S. in the early 1900's. Catastrophic fires in Baltimore, Boston, Chicago and San Francisco led to the development of building regulations that restricted the use of combustible materials in many types of construction.

After World War II, the technology of extruding aluminum frames developed and aluminum windows began to gain popularity. By the 1990's, aluminum-framed windows accounted for approximately 65% of the commercial window market. Wood, vinyl, fiberglass and steel-framed windows comprise most of the remaining 35% of the market.

Commonly used window frame materials include aluminum, vinyl, fiberglass, steel wood, and PVC (glass-fiber reinforced epoxy resin laminate pultrusions). Aluminum frames are the most widely used window frame material, and provide design flexibility because of the wide range of available stock systems and the relative economy of creating custom extrusions. Wood, vinyl and fiberglass are the most widely used window frames in the residential market. They provide better energy performance than aluminum and offer welded components that seal the joinery. Steel frames are less common than aluminum; there are relatively few manufacturers who produce high quality steel windows. Design flexibility is generally limited by the available stock rolled shapes. The cost premium for custom shapes is larger for steel frames than for aluminum frames. This section does not further address residential windows.

A critical element of successful window design is integration with adjacent wall components to create a functioning wall system. Reliable wall system design (see the Building Envelope Design Guide page on Exterior Walls) generally includes a water resistant barrier behind the wall cladding, an air barrier, thermal insulation, and sometimes a vapor retarder. The "punched" window openings in the wall system threaten to create holes in the water/air/thermal/vapor barrier(s). Careful detailing is required to integrate water/air/vapor barriers with the window frames and maintain their continuity at the window perimeters.

Description

The following describes commonly used window and frame options:

Perhaps the most important consideration is to understand whether the window system is a rain screen or barrier system. A rain screen system provides internal drainage for water that infiltrates into the glazing pocket. A barrier system assumes that no water will ever infiltrate the perimeter seals and thus does not provide internal drainage.

Window units can be fixed, operable, or a combination of the two. Fixed windows generally offer better air infiltration and water penetration resistance, and require less maintenance, than operable windows. On the other hand, operable windows allow for natural ventilation. Fixed windows typically consist of frame with an infill that are sealed together. Operable windows consist of a frame and sash that are weathersealed by weatherstrips in addition to the infill being sealed to the sash.

There are many configurations of operable window, broadly classified as sliding seal windows or compression seal windows. Compression seal windows generally provide better long-term air infiltration and water penetration resistance than sliding seal windows because they reduce friction and wear on the weatherstripping. Since they can be fully opened, compression seal windows also provide better ventilation potential.

Compression seal windows include the following:

  • Awning (Top hinged, project out bottom)
  • Hopper (Bottom hinged, project in top)
  • Casement (Side hinged, project in or out)
  • Vertically or horizontally pivoted windows

Sliding seal window types include the following:

  • Hung windows
  • Horizontal sliding windows

Pivot windows, jal-awning, and jalousie windows generally offer the poorest resistance.

The basic window types, performance rating, and glossary of window-related terms is contained in AAMA/NWWDA/CSA 101/I.S.2 Standard Specification for windows, doors, and unit skylights.

Fundamentals

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

Overall window thermal performance is a function of the glazing (see Glazing), frame and perimeter details. Typically, the overall goal is to achieve the best possible daylight transmission at cost of the least heat transmission.

Glazing thermal performance mostly depends on how well it can control radiative heat transfer. Radiative heat can be transferred through long-wave infrared radiation and through solar radiation. Either kind of radiation can be minimized by low-E coatings on the glass, which are one of the most effective means of improving window thermal performance. See Glazing for more details.

Window frame conductivity is a function of the frame material, geometry and design (e.g. thermal breaks in metal frames). Wood has low (better) thermal conductivity and inherently provides good thermal performance. Steel has higher (worse) thermal conductivity than wood.

PVC and fiberglass have lower (better) conductivity than the other materials.

Thermally broken steel windows are not generally available in the U.S. Narrower steel sightlines result in higher percentage of glass area than wood or aluminum frames and that factor, combined with steel's lower thermal conductivity than aluminum, may compensate for the lack of a thermal break. The overall U-value of steel frames compares favorably with thermally unbroken aluminum frames, but less favorably with wood frames.

It is common practice to incorporate thermal breaks of low thermal conductivity materials, traditionally polyurethane, polyamide or nylon I beam separators, for improved thermal performance. Disadvantages of thermal breaks include reduced frame strength and stiffness. Polyurethane in "poured and de-bridged" thermal breaks can shrink if not mechanically locked to the frame and in some instances embrittle. Back-up mechanical attachment of the two halves of the frame is recommended (skip debridging or "t-in-a box").

Proper placement of insulation in the voids at the window perimeter and maintaining continuity of the façade functional layers improves performance including reduction of drafts and energy loss around windows.

Buildings in cold climates have struggled throughout the ages with ice and snow formations that slide, fall, or get windblown from their roofs, ledges, and window sills, causing harm to people and damage to property below. Refer to the Resource Page on Considerations for Building Design in Cold Climates.

Moisture Protection (Water Penetration, Condensation Resistance)

Water penetration resistance is a function of glazing details (see Glazing), frame drainage details, weatherstripping (for operable windows) and perimeter details.

Key frame drainage features include slope to the exterior at surfaces that collect water (sloped glazing pocket sill), large (3/8 by 1 in. long) slot holes, two per sill minimum, and drainage at every horizontal frame (i.e. do not use vertical frames to drain past horizontal frames). Design the drainage system to handle condensation as well as rain where condensation is likely. Provide water baffles, insect protection and check for air and water tightness ratings before you start messing with drainage holes. These items affect each other.

High performance windows will generally include dual weather stripping for improved air/water penetration performance.

Window perimeters should have flashings (sill, jambs and head) that are integrated with the waterproofing at adjacent walls (see Exterior Wall). Slope head and sill flashings to the exterior for prompt drainage. Many windows leak at sill-to-jamb frame corners and at sash corners; these corners should be mitered and stiffened by spline inserts that stiffen the connections and prevent seal failures. Vulcanized continuous gaskets and weatherstrips should be utilized, but they seldom are economically available. To compensate for the low quality of windows available on the market, sill flashings with a panned up interior leg and end dams are required to collect this leakage and drain it to the exterior. Do not penetrate the horizontal portion of the sill flashing with window fasteners. Instead, where attachment of the sill frame is required, provide an attachment angle inboard of the window sill and fasten through the upturned leg of the sill flashing into back of the sill frame.

It's a good practice to conduct condensation risk assessment of sensitive applications (museums, swimming pools, etc.) particularly at areas suspect to thermal bridging (window offset from wall insulation, metal studs coupled with metal flashing, etc.) The THERM software is typically a sufficient tool for such an assessment.

Perimeter sealants are useful for limiting air and water penetration through the outermost plane of the wall, but should not be relied upon as the sole air/water penetration barrier.

Visual (Daylighting, Aesthetics)

Key visual features of windows include glazing appearance (see Glazing) and window frame sightlines. Sightlines are a function of both the width and depth of the window frame. Where narrow sightlines are desired, the strength and stiffness of steel frames permits the use of relatively slender frames compared with aluminum or wood.

Sound (Acoustics)

The acoustic performance of windows is a function of framing, glass and joinery (see the discussion in Glazing). Sound insulation of windows can be improved by increasing the mass of the frames and perimeter infill (albeit typically with a negative effect on thermal performance), improving the air tightness of the perimeter construction, placing sound absorptive materials or high mass materials at the perimeter of the windows, increasing the insulating glass (I.G.) unit airspace, using heavy gas infill, using laminated glass, acoustic laminates, and using I.G. units with different glass thicknesses. Providing sound isolators (such as rubber shims) at window attachments is a measure generally reserved for applications such as sound studios.

Safety

A primary factor in window choice is a design wind pressure that is specific for each window location.

Verify your local building code for safety requirements. List the requirements in specifications for comparable bids.

Always consult a structural engineer to obtain information about wind pressures, window support, feasibility of safe installation, and separation of supporting function from other facade functions (weatherproofing, soundproofing, etc.)

Whenever a window is supported indirectly on other cladding systems, consult the structural engineer and specify the reactions and anchorage for both trades.

An important design consideration for operable windows is resistance to wind loads in the open position. Unfortunately, the industry provides little guidance on this issue. Sliding seal windows are always supported on two sides whether open or closed. Projecting windows rely on operating hardware for support against wind loads. The operating hardware for projecting windows may not be adequate for severe exposures.

Impact resistant rated products are deemed necessary by certain authorities having jurisdictions in zones prone to windborne debris; these products are certified as tested following procedures set by a jurisdiction, they should be clearly identified together with the design pressures on the construction drawings.

Fenestration often comes as a design-build (DB) product designed by the manufacturer. In such cases, the design data, performance data, and interface drawings need to be provided to the DB engineering team as a very minimum to assure a proper product engineering. Verify how the fenestration is provided in your project prior to the design. Keep your design within your scope of responsibility (e.g. detail the interfaces among adjacent systems, don't modify the manufacturer's details of an already engineered system, unless you take responsibility for engineering of the system).

Fire Safety

  • For fenestration in fire-rated walls, provide fire-rated steel frames with suitable glazing (wired glass or fire-rated ceramic "glass").
  • Provide knock-out glazing panels (typically fully tempered to reduce shards) for venting and emergency access from the exterior.
  • Emergency egress requirements frequently dictate the geometry and size of operable sash.

Fall-out Protection

  • Limit stops on operable sash, or approved window guards over window openings are used to prevent children from falling out of windows. Insect screens do not provide fall-out protection. Use laminated glass in hazardous locations as opposed to fully tempered glass.

Maintenance Access

  • Window washing tiebacks, if required, are not typically incorporated into window systems.

Health and Indoor Air Quality

Water leakage through or around windows frequently contributes to Indoor Air Quality (IAQ) problems by supplying moisture for microbial growth. This leakage can often remain concealed within the wall system and not become evident until concealed wall components experience significant deterioration and microbial growth requiring costly repairs. See the section on moisture protection above on how to integrate water barriers with window frames and maintain continuity at window perimeters. On the other hand, excessive air tightness (e.g. afforded by modern casement windows) may require provision of additional ventilation, due to build-up of excessive relative humidity on the inside.

Durability and Service Life Expectancy

Window durability problems depend to a large degree on the window framing material and its assembly details.

Aluminum frames are inherently corrosion resistant in most environments if anodized and properly sealed or painted with baked-on fluoropolymer paint (other paints generally offer less durability). Aluminum frames are subject to deterioration of the coating and corrosion of aluminum in severe (industrial, coastal) environments and galvanic corrosion from contact with dissimilar metals. Frame corner seals constructed using sealant are prone to debonding from prolonged contact with moisture and from thermal, structural, and transportation movements.

Steel windows depend on an applied coating for corrosion resistance. Coating systems that include a galvanic protection primer (zinc-rich paint, hot dipped galvanizing) in combination with a barrier coat of paint provide significantly better corrosion resistance than coating systems that rely on a barrier coat of paint alone. Typical high-performance coatings include epoxy primer with an aliphatic urethane finish. Steel frame corners can be welded watertight, producing superior durability against frame corner leakage compared to aluminum and wood windows.

Wood frames are prone to separation of frame joints from moisture, thermal, structural, and transportation movements. Wood that is not pressure-treated or not naturally rot resistant is prone to rot from prolonged contact with moisture. Wood coatings deteriorate rapidly. Exposed wood is subject to UV deterioration. Wood that is clad with aluminum or vinyl often deteriorates more rapidly than painted wood. This is primarily due to the propensity for aluminum and vinyl claddings to leak at joints resulting in water penetration into the wood frame and deterioration due to limited drying potential of the wood when encased in aluminum and vinyl.

Maintainability and Repairability

Windows and perimeter sealants require proper substrate preparation and maintenance to maximize their service lives. Perimeter sealants, properly designed and installed with high quality material, have a typical service life of 10 to 15 years although some breaches are likely from day one. Perimeter sealants require meticulous surface preparation to minimize breaches and maximize surface bond. All specified substrates should be verified for compatibility with specified sealants and adhesives.

Wood frames and black iron frames require frequent inspection and maintenance of coatings. Steel frames that have galvanic protection under the paint can generally tolerate longer intervals between paint maintenance than those without galvanic protection.

Aluminum frames are painted or anodized. Factory applied fluoropolymer thermoset coatings have good resistance to environmental degradation and require only periodic cleaning. Recoating with an air-dry fluoropolymer coating is possible but requires special surface preparation and is not as durable as the baked-on original coating.

Anodized aluminum frames cannot be "re-anodized" in place, but can be cleaned and protected by proprietary clear coatings.

Weatherstrips should be replaced as they become worn, which under normal conditions occurs in approximately 5 - 10 years.

The hardware should be inspected and adjusted to assure proper positioning of the sash, low operating force, and prevent user-inflicted damage that may be caused by maladjustment. Operable windows may require replacement of hardware after many operating cycles.

Glass should be regularly washed to comply with the manufacturers' requirements.

Sustainability

Energy-efficient window technologies and design for maximum service life of the installation play an essential part in ensuring the sustainability of windows.

The best strategy for durability of windows is to employ good design practices to ensure the maximum service life of the installation.

Energy efficiency can be maximized by using low-E coatings, low-conductance frames and other technologies that improve the thermal efficiency of the window system. Windows built by window manufacturers (as opposed to site-built windows) can be designated as qualifying for the ENERGY STAR®, a government-backed program aimed to protect the environment by promoting energy efficiency.

The use of durable tropical hardwoods (teak, cypress, mahogany) is controversial due to declining populations of native timber and unsustainable harvesting techniques and some public agencies prohibit the use of tropical hardwoods on their projects. The Forest Stewardship Council (an independent, membership-based organization that promotes responsible management of the world's forests through developing standards, a certification system, and trademark recognition) certifies sources for sustainable harvesting, but may include some controversial tree plantations. Domestic hardwoods such as white oak are an acceptable, but less durable, option, but there are a limited number of manufacturers who use these hardwoods.

Steel frames, if not substantially weakened by corrosion, can be removed, refinished and reinstalled.

Aluminum and steel frames are typically recycled at the end of their service life. Salvage and demolition contractors generally require a minimum of 1,000 sq ft or more of window/curtain wall to make material recycling economical (smaller amounts are generally disposed as general trash). Recycling is less economical if the aluminum is contaminated with sealants, fractured glazing, etc., as salvage companies pay considerably less for the material. There is a limited market for salvaged steel and wood frames.

Applications

Establish System Track Record

Select a window with a demonstrated track record in similar applications and exposures. Verifying track records may require significant research by the designer. ASTM E1825 provides guidance.

Review laboratory test results of window systems or similar custom systems for air, water, and structural resistance, heat transmission, condensation resistance, sound transmission, and operability. Verify that tests pertain to the window under consideration and not a version of the window with the same product name but of different construction.

For third-party-verified information on thermal performance, specify windows with energy ratings certified by the National Fenestration Rating Council (NFRC). The American Architectural Manufacturers Association (AAMA) sets standards for the certification of windows tested to meet air and water infiltration, structural integrity, and forced entry resistance criteria. Certification of the durability of the insulating glass unit edge seal system is overseen by organizations such as the Insulating Glass Manufacturers Association (IGMA).

Structural Considerations

The most basic of criteria is the wind speed and resulting structural loading. After determining the design wind pressure, the next step is to determine the test pressure for air and water penetration. AAMA recommends 20 percent of the wind load for "most parts of the country under the normally prevailing weather conditions". However, AAMA goes on to warn that the higher loads may be necessary in areas of sustained high winds and rain. The performance criteria should be adjusted to suit the particular project situation.

Designing for Waterproofing Performance

Window design should prevent leaks by using sound, proven technologies (mitered corners stiffened by spline inserts, molded corner gaskets, vulcanized perimeter gaskets, and shop-applied perimeter collars, as opposed to sealants). Otherwise, when designing interfaces for an off-the shelf window, you should start with the assumption that window frame corners, glazing seals, and perimeter sealant joints will leak at some point during normal service life. Provide frame sills with weeped glazing pockets, sloped to the exterior, to collect water that penetrates the glazing and drain it to the exterior. Do not use vertical mullions as drain leaders. For windows set into the plane of the stud wall, provide a sill flashing with an upturned back leg and end dams to collect and drain frame corner leakage; provide jamb flashings to direct perimeter leakage down to the sill flashing. The details provided at the end of this section of the WBDG show the windows in the plane of the insulation in the cavity behind the brick.

An effective strategy to reduce the exposure of windows to weather is to recess windows from the exterior face of the wall. Projecting horizontal features (e.g. roof overhangs) also help shield windows from the weather.

Critical window perimeter details are discussed below; also see reference Details provided at the end of this section of the WBDG.

Sill Flashing: Use durable metal flashings (e.g. zinc-tin coated-copper or stainless steel) where sill flashings will be exposed. Slope sill flashings to the exterior; provide an out-turned drip edge over face of wall cladding. Provide an upturned leg (1 inch minimum, greater for high wind exposures) at the interior, and end dams soldered water tight. Do not penetrate the horizontal portion of flashing with fasteners. To fasten the sill frame, provide an attachment angle inboard of the window sill and fasten through the upturned leg of the sill flashing into the inboard leg of the sill frame. Membrane flashings may be appropriate where the sill flashings are concealed and drain down into the wall cavity behind the cladding or onto sloped precast concrete or stone sills, but are less durable than metal.

Jamb Flashing: Jamb flashings may be metal but more typically are a flexible membrane. Where jamb flashings are part of the air barrier system, they must be metal or membrane continuously supported by a substrate capable of withstanding the design air pressures. Membrane flashings that bridge gaps must be continuous for the full height of the window, i.e. no lap seams, because there is no support at the gap against which to roll the lap seam watertight. Jamb flashings and wall waterproofing must be lapped and fully adhered at their intersection. Jamb flashings must be fully sealed to the window frame. Mechanical attachment of the jamb flashing to the window is generally required because there is insufficient surface area on which to adhere the flashing and rely on adhesion alone. The jamb flashings must be shingled over the sill flashing end dams and back leg to direct water that runs down the jambs into the sill pan.

Head Flashing: Use durable metal flashings (zinc-tin coated-copper or stainless steel). Slope window head flashings to the exterior; provide an out-turned drip edge over top of window frame. Extend head flashings several inches beyond the window frame. Provide end dams soldered watertight. Seal head flashings to the inner face of the windows and to the jamb flashings. Provide minimum 4-inch upturned leg and counter flash with wall waterproofing membrane adhered to the vertical leg of the metal flashing. For punched windows in openings that do not allow extension of the head flashing beyond the opening (e.g. concrete openings) use dual sealant joints in lieu of head flashing to capture water and direct it to the jamb flashings.

Critical glazing pocket details specific to frame materials are discussed below.

Aluminum frames: Slope the glazing pocket to promote drainage (reduces water exposure of insulating glass unit seals and frame corner seals). Receptor systems are not recommended as it is difficult to accomplish a reliable air seal at the receptor / jamb interface.

Steel frames: Fully weld all frame corners for watertight construction. Because steel frames typically have narrow profile and therefore shallow glazing pocket, weep covers and foam baffles are critical to air infiltration and water penetration performance.

Wood frames: Many manufacturers do not provide weep holes in their typical windows, but wept glazing systems are required to obtain glazing warranties from insulating glass (I.G.) unit manufacturers unless the glass manufacturer makes a specific accommodation to the window manufacturer. A separate recessed drainage channel in the glazing pocket allows drainage to the weep holes. Muntins (e.g. true divided lites) are rarely wept, and rely on glazing seals to prevent water infiltration into the glazing pocket. All I.G. units in true divided lites must be set on setting blocks. Use fixed interior glazing stops where possible to provide maximum water penetration resistance over time.

All: Coordinate placement of setting blocks with weep holes to avoid blocking drainage paths.

Coordinate attachment details with flashing details to avoid penetrating the flashings.

Designing for Condensation Resistance

AAMA's Window Selection Guide provides guidance on window selection for condensation resistance. Establish the required Condensation Resistance Factor (CRF) based on anticipated interior humidity and local climate data and select a window with an appropriate CRF. Designers should be aware that the CRF is a weighted average for a window assembly. The CRF does not give information about window cold spots that could result in local condensation. Projects for which condensation control is a critical concern, such as high interior humidity buildings, require project-specific thermal modeling. Careful analysis and modeling of interior conditions is required to accurately predict condensation on the glass and frame. Windows that are set well outboard of perimeter heating elements will have air temperatures along their interior surface that are significantly lower than the design wintertime interior temperatures. Thermal modeling of the building interior using Computational Fluid Dynamics (CFD) software can help establish a reasonable estimate for air temperatures at the inside surfaces of the glass and frame. These interior air temperatures are inputs for window thermal modeling software such as THERM.

Use thermally broken aluminum or wood or plastic frames for the best condensation resistance. The thermal break must be properly positioned with respect to the wall system to avoid exposing the aluminum frame inboard of the thermal break to cold air "short circuiting" the thermal break; see the details at the end of this section of the WBDG.

Consider frame geometry in humid conditions for thermally conductive frame materials (aluminum, steel). For projects with condensation risks, metal frames should be oriented with as much of their thermal mass on the warm (and humid) side of the wall as possible.

Refer to AAMA 1503 for descriptions of test method, parameters and equipment for determining U values and condensation resistance values for window products.

Designing for Finish Durability

Aluminum: Class I anodic coatings (AAMA 611) and high performance factory applied fluoropolymer thermoset coatings (AAMA 2604) have good resistance to environmental degradation.

Wood: Since wood readily absorbs moisture, wood finishes will have a limited service life if exterior protective coatings are not properly maintained. Slope all exposed wood surfaces to promote drainage. Seal and paint all surfaces of mahogany window frames before glazing because bleed-out of tannins will interfere with sealant adhesion.

Steel: Steel frames that have galvanic protection under the paint can generally tolerate longer intervals between paint maintenance.

All frame materials: Shielding windows from the weather by recessing them back from the exterior face of the wall and/or providing roof overhangs or projecting head flashings is an effective strategy for maximizing the service life of window finishes.

Hardware

AAMA's Window Selection Guide includes a useful overview of the operating hardware options for the various types of operating windows. Important design considerations for window hardware include life cycle serviceability, ability of operating hardware to resist wind loads when the window is in the open position, and resistance to forced entry for windows that are readily accessible from the exterior.

AAMA 910, Voluntary "Life Cycle" Specifications and Test Methods for Architectural Grade Windows and Sliding Glass Doors, sets forth means for testing that simulates the normal wear that can be expected during the life of a typical architectural grade product. The testing is required for all windows seeking Architectural Class designation.

An important design consideration for operable windows is resistance to wind loads in the open position. Unfortunately, the industry provides little guidance on this issue. Sliding seal windows are always supported on two sides whether open or closed. Projecting and side-hung windows rely on operating hardware for support against wind loads. The operating hardware for projecting or side-hung windows may not be adequate for severe exposures.

AAMA 1302.5, "Voluntary Specifications for Forced-Entry Resistant Aluminum Prime Windows", sets guidelines for construction and testing of aluminum windows to reduce vulnerability to forced entry.

Other AAMA Hardware Standards are listed at the end of this WBDG section.

Logistical and Construction Administration Issues

The service life of even the most durable window is likely to be shorter than that of the surrounding exterior wall construction. Therefore, the design of the window and perimeter construction should permit window removal and replacement without removing adjacent wall components that will remain.

The service life expectancy of components that are mated with the window into an assembly should match the service life expectancy of the window itself. Require durable flashing materials, non-corroding attachment hardware and fasteners, and moisture resistant materials in regions subject to wetting.

Laboratory testing: For projects with custom windows, require laboratory testing for structural, air infiltration, and water leakage of a mock-up window prior to production of windows for the project. Have a window specialist present to document mock-up window construction.

Field Mock-up: For all windows, stock or custom, require construction and testing of a field mock-up representative of the wall/window for air infiltration and water leakage. Do not allow any reduction in pressure from the laboratory test.

Testing of production windows: Require the field testing of production windows for quality assurance of window fabrication and installation. Require multiple tests early in the construction phase to catch problems early. Require additional testing if initial tests fail. Do not allow any reduction in pressure from the laboratory test.

Shop drawing coordination: Require window installation shop drawings showing all adjacent construction and related work, including flashings, window attachments, interior finishes, and indicating sequencing of the work. Shop drawings should show isometric or axonometric details of corner assemblies.

Details

Note that details a manufacturer presents will generally meet only the lowest level of performance. For higher performance the Architect will want to incorporate concepts covered in this section.

Due to the complexities and challenges of making receptor systems work, details of receptor systems have not been included in this section. It is recommended that a consultant be engaged to advise if receptor systems are to be employed.

PDF icon

The following details can be downloaded or viewed online in Adobe Acrobat PDF format. Download Adobe Reader.

The details associated with this section of the BEDG on the WBDG were developed by committee and are intended solely as a means to illustrate general design and construction concepts only. Appropriate use and application of the concepts illustrated in these details will vary based on performance considerations and environmental conditions unique to each project and, therefore, do not represent the final opinion or recommendation of the author of each section or the committee members responsible for the development of the WBDG.

Note: All details courtesy of The Facade Group

Window Head Detail in Cavity Wall (without Nailing Flange) (Detail 1/A) | PDF  Master Keynote List | PDF

This detail shows head construction at an aluminum window set in a masonry cavity wall under a steel lintel.

  • A through-wall metal flashing at the base of the brick cladding above the window protects the window head from leakage through the wall above-see Exterior Wall Claddings.
  • This detail will not accommodate differential movement between the window frame and the back-up wall.
  • At the head, jambs, and sill, the perimeter of the window frame includes substantial return legs that provide adequate bonding surfaces for a properly configured sealant joint between the brick and the window perimeter.

Window Jamb Detail in Cavity Wall (without Nailing Flange) (Detail 1/B) | PDF

This detail shows jamb construction at an aluminum window set in a masonry cavity wall.

  • Like the head and the sill, the wall membrane is wrapped into the rough opening and then elastomeric flashing is overlapped onto the membrane in the rough opening and then connected to the inside face of the window, where it is held in place by a continuous backer rod.
  • Like the head and the sill, the first few inches of the wall cavity is filled with spray polyurethane foam to assist in connecting the window to the wall insulation and (at the head and the jamb) to help keep moisture in the wall cavity from getting into the window.
  • The outside of the window frame is also filled with spray polyurethane foam to complete the continuity of the thermal barrier.

Window Sill Detail in Cavity Wall (without Nailing Flange) (Detail 1/C) | PDF

This detail shows sill construction at an aluminum window set in a masonry cavity wall.

  • The window has weep holes in the face of the frame, with weep covers to reduce water penetration from wind driven rain.

Window Deflection Head Detail in Cavity Wall (without Nailing Flange) (Detail 1/D) | PDF

This detail shows head construction at an aluminum window set in a masonry cavity wall under a steel relieving angle.

  • A through-wall metal flashing at the base of the brick cladding above the window protects the window head from leakage through the wall above-see Exterior Wall Claddings.
  • This detail will accommodate differential movement between the window frame and the back-up wall.

Window Head Detail in Cavity Wall (with Nailing Flange) (Detail 1/E) | PDF

This detail shows head construction at an aluminum window set in a masonry cavity wall under a steel lintel.

  • This detail will not accommodate differential movement between the window frame and the back-up wall.
  • At the head, jambs, and sill, the perimeter of the window frame includes flanges (fins) that provide bonding surfaces for the wall membrane to join to the window.

Window Jamb Detail in Cavity Wall (with Nailing Flange) (Detail 1/F) | PDF

This detail shows jamb construction at an aluminum window set in a masonry cavity wall.

  • Like the head and the sill, the first few inches of the wall cavity is filled with spray polyurethane foam to assist in connecting the window to the wall insulation and (at the head and the jamb) to help keep moisture in the wall cavity from getting into the window.
  • The outside of the window frame is also filled with spray polyurethane foam to complete the continuity of the thermal barrier.

Window Sill Detail in Cavity Wall (with Nailing Flange) (Detail 1/G) | PDF

This detail shows sill construction at an aluminum window set in a masonry cavity wall.

  • The window has weep holes in the face of the frame, with weep covers to reduce water penetration from wind driven rain.

Emerging Issues

See Glazing for discussion of emerging issues related to glazing.

Dynamic windows provide on-demand control of visible light and solar heat transmittance by using e.g. photochromic or electrochromic coatings.

Windows with three glass panes or suspended low-E films between glass panes provide superior insulation values. Research is being conducted on the development of vacuum insulating glass units, which will improve insulation values by limiting conductive and convective heat loss compared to conventional insulating glass units.

Air-flow windows incorporate a separate interior lite of glass and use either supply or exhaust air to modulate the surface temperature of the insulating glass unit.

ENERGY STAR, is a federal initiative to help consumers identify energy efficient products, currently includes residential windows only, but the program is may be expanding to commercial.

Relevant Codes and Standards

Note that these are standardized test procedures and specifications but do not establish the performance values required for a project. The architect must determine the performance values suitable for the project. Of particular concern is that the manufacturer-advertised performance might be based on a window much smaller than that required for the project and therefore, may not be representative of actual project performance. Other variables such as the glass type, anchorage, and stiffness of the wall in which the window is mounted can also result in a difference in performance between the tested samples versus the actual installed window.

Window Design and Selection

Window Installation

Thermal Performance

Air Infiltration

Water Penetration Resistance

Condensation Resistance

Acoustical Performance

Anodized Coatings

High Performance Organic Coatings

Durability

Hardware

Additional Resources

WBDG

Design Objectives

Functional / Operational—Ensure Appropriate Product/Systems Integration

Products and Systems

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

NOTE: Photographs, figures, and drawings were provided by the original author unless otherwise noted.