Building Envelope Design Guide - Windows
Last updated: 03-14-2006
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 and steel-framed windows comprise most of the remaining 35% of the market.
Description
The following describes commonly used window and frame components:
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.
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.
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:
- Single, double and triple hung windows, either with balance hardware or springs to retain sash in open position and assist lifting
- Horizontal sliding windows
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.
Commonly used window frame materials include aluminum, steel and wood. 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. 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. Wood frames are widely used in the residential market, often with aluminum or vinyl cladding to reduce maintenance, and are growing in commercial use. There are a limited number of manufacturers who produce wood windows using naturally durable woods such as teak, mahogany, cypress or domestic hardwoods such as white oak. Many wood window manufacturers use fast-growth soft woods that are not rot-resistant. Cladding of wood frames with aluminum or vinyl can accelerate wood rot if the cladding joints are not made permanently watertight.
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 page on Exterior Walls) generally includes a water resistant barrier behind the wall cladding, an air barrier and sometimes a vapor retarder. The "punched" window openings in the wall system threaten to create holes in the water/air/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.
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.
Window frame conductance is a function of the frame material, geometry and fabrication (e.g. thermal breaks in metal frames). Wood has low thermal conductivity and provides good thermal performance inherently. Steel has higher thermal conductivity than wood. 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. The overall U-value of steel frames compares favorably with aluminum frames, but less favorably with wood frames. For projects with humid conditions and condensation risks, the steel and aluminum frame should be oriented with as much of its thermal mass on the warm (and humid) side of the wall as possible.
Aluminum has the highest thermal conductivity of the three materials. It is common practice to incorporate thermal breaks of low thermal conductivity materials, traditionally polyurethane and more recently nylon, for improved thermal performance. Disadvantages of thermal breaks include inability to continuously weld frames and 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") for poured and de-bridged thermal breaks.
Proper placement of insulation in the voids at the window perimeter and maintaining continuity of the air barrier reduces drafts and energy loss around windows.
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 in. diameter) weep holes, three 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.
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 corners. To collect this leakage and drain it to the exterior, sill flashings with a panned up interior leg and end dams are required. 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.
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 primarily a function of the glass and glazing (see the discussion in Glazing). Sound insulation of windows can be improved by increasing the mass of the frames (albeit typically with a negative effect on thermal performance), improving the airtightness of the perimeter construction, placing sound absorptive materials at the perimeter of the windows, increasing the I.G. unit airspace, using laminated glass, and using I.G. units of uneven glass thicknesses. Providing sound isolators (such as rubber shims) at window attachments is a measure generally reserved for applications such as sound studios.
Safety
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.
Maintenance Access
- Window washing tiebacks are often provided to stabilize access equipment used by maintenance/cleaning crew.
Health and Indoor Air Quality
Water leakage through or around windows frequently contributes to IAQ problems by supplying moisture for mold growth. This leakage can often remain concealed within the wall system and not become evident until concealed wall components experience significant deterioration and mold growth requiring costly repairs.
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 many environments if anodized and properly sealed or painted with baked-on fluoropolymer paint. 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. 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.
Operating hardware on operable windows wears from normal operation, and can be damaged by wind loads when the window is left in the open position. Side-hung windows are generally a poor choice for applications with high wind loads.
Maintainability and Repairability
Windows and perimeter sealants require 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.
Wood frames and ungalvanized steel 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.
Operable windows may require replacement of hardware after many operating cycles.
Sustainability
The best strategy for sustainability of windows is to employ good design practices to ensure the durability (maximum service life) of the installation.
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.
Energy efficiency is also important when designing for sustainability. Windows with an Energy Star rating, a government-backed program aimed to protect the environment by promoting energy efficiency, will help to improve thermal efficiency of the window system and reduce energy consumption.
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.
Designing for Waterproofing Performance
Window design 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. 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.
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 reference Details 3.2-1 through 3.2-3:
Sill Flashing: Use durable metal flashings (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 (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 water tight. Provide minimum 4-inch upturned leg and counterflash 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).
Steel frames: Fully weld all frame corners for watertight construction. Stock rolled shapes generally do not have sloped glazing pockets. 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 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 frames or wood for 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 Detail 3.2-2.
Consider frame geometry for thermally conductive frame materials (aluminum, steel). Minimize the proportion of framing exposed to the exterior to improve condensation resistance.
Refer to AAMA 1503 for descriptions of test method, parameters and equipment for determining U values and CRF's for window products.
Designing for Finish Durability
Aluminum: Class I anodic coatings (AAMA 611, supercedes AAMA 606, 607 and 608) and high performance factory applied fluoropolymer thermoset coatings (AAMA 2604, supercedes AAMA 605) have good resistance to environmental degradation.
Wood: Since wood readily absorbs moisture, wood finishes will have a limited service life if 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 include
- AAMA 901—Voluntary Specification for Rotary Operators in Window Applications
- AAMA 902—Voluntary Specification for Sash Balances
- AAMA 904—Voluntary Specification for Multi-Bar Hinges in Window Applications
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 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 assembly for assembly testing and verification.
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.
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 axiometric details of corner assemblies.
Details
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.
Window Head Detail in Cavity Wall (Detail 3.2-1) DWG | DWF | PDF
This detail shows head construction at an aluminum window set in a masonry cavity wall.
- 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.
- The attachment at the window head may require slotted holes to accommodate differential movement between the window frame and the back-up wall.
Window Sill Detail in Cavity Wall (Detail 3.2-2) DWG | DWF | PDF
This detail shows sill construction at an aluminum window set in a masonry cavity wall.
- A metal sill flashing extends from the back-up wall across the cavity and over the top of the brick cladding, and turns down the face of the brick cladding. The flashing is sloped to the exterior to promote drainage, and is supported on sloped blocking and a sloped mortar bed. A non-curing glazing tape between the upturned leg of the flashing and the window serves as an air seal.
- A secondary sill flashing, a flexible membrane, is installed under the metal flashing as a second line of defense. The flexible membrane shingles over the wall waterproofing in the cavity below the window.
- The window has weep holes in the face of the frame, with weep covers to reduce water penetration from wind driven rain.
- The thermal break of the window frame is positioned to align with the wall insulation, to prevent exposing the warm side of the frame to the cold and creating a thermal "short circuit."
- See discussion of Schematic of Better Glazing Detail in Glazing for additional commentary on window attachment.
Window Jamb Detail in Cavity Wall (Detail 3.2-3) DWG | DWF | PDF
This detail shows jamb construction at an aluminum window set in a masonry cavity wall.
- A Z-shaped jamb flashing extends from the back-up wall to the interior face of the window. The outboard leg of the flashing is integrated with the wall waterproofing membrane. The inboard leg of the flashing is sealed to the window frame for an air seal. The jamb flashing is shingled into the sill flashing at the window sill.
Emerging Issues
See Glazing for discussion of emerging issues related to glazing.
Smart windows control visible light transmittance by using e.g. photochromic or electrochromic coatings. Some high R-value glazing includes the use of evacuated insulating glass units, which limit conductive and convective heat loss compared to conventional i.g. units. Air-flow windows incorporate a separate interior glass of lite and uses either supply or exhaust air to modulate the surface temperature of the i.g. unit. Energy Star is a federal initiative to help consumers identify energy efficient products, including windows. Available energy star ratings include residential windows only, but the program is expanding to commercial buildings.
Relevant Codes and Standards
Window Design and Selection
- AAMA Window Selection Guide
- ANSI/AAMA/NWWDA 101/I.S.2 Voluntary Specifications for Aluminum, Vinyl (PVC) and Wood Windows and Glass Doors
Thermal Performance
- AAMA 1503 Voluntary Test Method for Thermal Transmittance and Condensation Resistance of Windows, Doors and Glazed Wall Sections
- AAMA 1504 Voluntary Standard for Thermal Performance of Windows, Doors and Glazed Wall Sections
- AAMA 1505 Voluntary Standards for Thermal Transmittance and Performance
Air Infiltration
Water Penetration Resistance
- ASTM E331 (laboratory)
- ASTM E547 (laboratory)
- ASTM E1105 (field)
Acoustical Performance
Anodized Coatings
- AAMA 609 Cleaning and Maintenance Guide
- AAMA 610 Cleaning and Maintenance Guide
- AAMA 611 Voluntary Specification for Anodized Architectural Aluminum
High Performance Organic Coatings
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®
- American Architectural Manufacturers Association, information on aluminum, wood and vinyl windows
- Steel Window Institute, information on steel window manufacturers
- National Research Council Canada, information on good design practice, research, and emerging issues related to the building envelope
