ebuild Guide to Window Specifications

Window selection has wide-ranging impact on many areas in home-building and remodeling projects, including the home's overall appearance, energy performance, comfort, maintenance requirements, life cycle and budget. Because windows are complex assemblies with many options, there are many choices to make even within a single series or model made by a given manufacturer.

Certain window manufacturers, notably Andersen Corporation, Marvin Windows and Doors, and Pella Corporation, have national reputations and certain brand equity with builders and owners. But there are many other manufacturers of high-quality windows with diverse offerings. Specifying by manufacturer or brand name alone cannot assure that a given window is best for a given project.

Key specifications

• Material, which captures what the frame and sash are made from, e.g., wood, fiberglass, vinyl, etc.
• Shape, which essentially describes the outline of a window seen as a polygon, e.g., rectangle, half-round, cloverleaf, etc. Shape may also refer to special modeling at the top of a window, e.g., radius-top, elliptical top, Gothic arch, etc.
• Operating format, which captures whether windows open and close, and if they do, the mechanical design for their operation, e.g., fixed, awning, casement, double-hung, etc.
• Venting configuration also indicates whether a window has sash that are designed to open (venting) or not (stationary), and is a complement to operating format. The venting configuration spec is important because some manufacturers offer units with stationary sash that fall within their lines of casements and other windows with operable sash.
• Size type, which captures whether a particular model is available in stock sizes or only in custom sizes.
• Nominal width, which is sometimes called call-out width, captures the approximate overall measurement of a window unit from side to side, rounded to the nearest inch. Remodelers and replacement window contractors may want also to look at specifications for actual width and rough-opening width, which capture measurements critical for determining whether a window can fit into an existing rough opening.
• Nominal height, which is sometimes called the call-out height, captures the approximate measurement of the window unit from top to bottom, rounded to the nearest full inch. Remodelers and replacement window contractors may want also to look at specifications for actual height and rough-opening height, which capture measurements critical for determining whether a window can fit into an existing rough opening.
• Jamb depth captures the width of the window frame from front edge to back edge, which has implications for how a window will fit into a wall of a given finished thickness and the requirements for trimming out the window with interior sill and casings. Most manufacturers offer optional jamb extensions that can be used to fill space where needed.
• Glazing captures the type and design of glass or other material to be set in the sash. The glazing specification has impact on the energy performance, safety, security and sound-attenuation characteristics of a window.

Additional specifications may be critical for windows of certain materials and configurations. For example:

• Exterior color is important when specifying clad-wood, vinyl and aluminum windows.
• Projection, which captures the measurement from the finished exterior wall surface to the outermost plane of a window assembly, applies only to bay, bow and garden windows.
• Species may be critical for wood windows specified for certain home designs.
• Frame corner joint and sash corner joint, which indicate whether members are welded or joined with mechanical fasteners, apply only to vinyl windows.

Window Frame and Sash Materials

There are fundamental differences among windows made from different materials. Choices include solid wood, clad wood, fiberglass, vinyl and aluminum. The material specification has implications for price, as well as key performance and appearance criteria.

Wood Windows

Nearly half of the windows sold in the United States for housing construction have solid wood frames and sashes. Wood windows are the traditional favorite for a good reason: They combine most of the best appearance and performance characteristics that accrue to good value. That said, wood windows are relatively expensive.

Wood, because of its inherent strength, pleasing natural grain patterns and the variety of finishes that may be applied, lends itself to making windows with great visual appeal--both from inside and out. Frame elements can be relatively narrow, producing expansive sightlines and graceful forms. Wood sashes with divided lights can have refined muntins because of the material's strength; even units with insulating-glass can have traditional true divided lites instead of snap-on or between-the-panes grilles.

Most wood windows for the builder/remodeler market are made from common softwoods such as pine and fir, but many manufacturers offer other species, including cedar, redwood, mahogany and exotic hardwoods. Consider specifying windows made from less common species when there's a need to match or coordinate with similar wall finish materials and trim. Also consider the superior natural decay and insect resistance of cedar and redwood when a decorating scheme calls for clear or natural finishes.

Wood Window Finish Types

Wood windows require painting or other water-repellent treatment, which can add time and costs for builders. Ongoing maintenance requirements may also make wood windows less attractive to buyers.

Most manufacturers of wood windows offer at least three finish types: unfinished, primed and pre-finished. Each additional degree of finish carries an up-charge.

When selecting unfinished windows, check manufacturer specs for assurance of preservative and water-repellent treatment, which protect windows both before and after installation prior to finishing. Priming further protects windows on the job site and cuts finishing time. Pre-finished wood windows are available in a limited range of colors from many manufacturers. Factory-applied paint finishes generally retain their adhesion and color two to three times longer than site-applied paints.

Clad-wood Windows

Wood windows clad with vinyl, fiberglass or aluminum offer all of the advantages of conventional wood windows, including superior energy performance and traditional appearance inside and out--plus a bonus: low-maintenance. They're more expensive than wood but represent greater value.

Fiberglass cladding is said to be even more resistant to weathering than vinyl and aluminum. Clad windows are available in a variety of exterior colors. Interior surfaces are wood.

Fiberglass Windows

Generally less pricy than wood and clad-wood windows, fiberglass windows offer great structural strength, low thermal conductivity, high structural strength, stability under extreme temperature and moisture conditions, and require little maintenance. Fiberglass window frames can be painted with dark colors without risking loss of dimensional stability or compromising the factory-applied coating.

Vinyl Windows

Low cost and virtually maintenance-free, vinyl windows also offer good energy performance because of the material's low conductivity. Welded rather than mechanically fastened corners of vinyl frames and sash members indicate higher quality. Vinyl extrusions with more internal cells or chambers offer greater stiffness and are less prone to distortion than simpler designs. Most vinyl windows are made to order rather than in stock sizes. They are generally available in light exterior colors such as white, beige and gray.

Aluminum Windows

Strength and durability are aluminum's strong suits. Aluminum windows are valued for their narrow lines and minimal glass deflection under wind loads. Color choices in aluminum window frames are more varied than those for other materials, and dark frames are not as susceptible to distortion at high temperature as other low-cost frames.

Because aluminum has relatively high thermal conductivity compared to other window frame materials, the total energy performance of aluminum windows compares unfavorably with windows made from other materials with comparable glazing and weatherstripping. Some aluminum window designs incorporate a thermal break, which separates interior and exterior frame assemblies with a non-conductive material to improve energy performance.

Glazing

Most manufacturers offer standard glazing specific to each window model. Standard glazing may be single-pane clear glass, double-pane clear insulating glass, or double-glazing with low-emissivity (low-E) coatings, which admit light but reflect heat energy. Some low-E units are designed for cold-weather climates and reflect heat back into the interior space to reduce heating requirements; others are designed for hot-weather climates and reflect heat to the outdoors to reduce cooling loads. Other options include low-E double-glazing with gas-filled airspace, tinted glass, obscure glass, tempered glass and laminated glass.

The glazing spec can have great impact on a window's energy performance. Glazing performance is usually rated by R-value; the higher the R-value, the greater the resistance to the flow of heat energy. For the sake of comparison refer to the table below.

As the table shows, the addition of a glazing pane at least doubles a window's R-value. For further improvement over double-glazing, many manufacturers offer triple-glazing with the addition of an optional storm sash. The potential benefits of tinted glazing, which include reduction of heat gain and glare are indicated by a rating known as SHGC (solar heat gain coefficient). The SHGC of windows can also be enhanced with the addition of optional screens.

Laminated glass, in which two or more layers of glass may be bonded with interlayers of plastic or resin, may be specified for noise reduction, safety and security. Because it doesn't shatter under impact in the same way as standard sheet glass, laminated glass is often specified for hurricane- and tornado-prone locales, as well as skylights, sloped-glass applications and floor-to-ceiling windows. When specifying windows for storm-prone areas, also consider the design pressure rating of the unit, which indicates performance under high winds and wind-driven rain.

Noise-reduction characteristics of glazing and other materials are expressed in STC (sound transmission coefficient) or STL (sound transmission loss) ratings. Laminated glass is also said to offer enhanced solar-energy and UV control.

Electrochromic windows employ a glazing technology that changes from transparent to tinted or obscure with the application of electrical current. Application benefits include flexible daylighting control, glare control, solar heat control, reduced fading of furnishing through control of UV rays and enhanced privacy without the use of window treatments.

Total Energy Performance

While the glazing component of windows has a significant impact on energy performance, expressed through R-value ratings and SHGC values, overall energy efficiency results from the combination of window frame material, glazing and weatherstripping seals. Total energy performance of a window is most succinctly characterized by its U-value rating, another expression of resistance to the flow of thermal energy. With an inverse relationship to R-value measurements, the lower a window's U-value, the greater its resistance to energy flow.

An Energy Star rating also signifies that a window meets or exceeds energy-performance standards for its size and type. The Energy Star program is administered by the United States Environmental Protection Agency.

Building Code Considerations

Local building codes may set standards for windows that may be specified within their jurisdiction. It's important to consult the codes during the specification process.

Egress Window Standards

The International Residential Code (IRC)--and virtually all building codes applied to all regions--require that all bedrooms have at least one egress windows, defined as a window large enough to allow occupants to escape in the event of a fire, or to allow a fully equipped firefighter to enter from the outside. The standard applies to windows installed in finished basements as well as bedroom windows.The IRC specifies the minimum width for an egress window at 24 inches; the minimum height is 20 inches, and the net clear opening requirement is 5.7 square feet (5.0 square feet for ground-floor windows. To comply with the code, an egress window must be set with a maximum sill height of 44 inches.

Impact Resistance Standards

Increasingly special building code standards requiring impact-resistant window designs are being implemented in Eastern coastal regions and other storm-prone areas. Requirements generally include laminated or tempered glazing, a minimum design pressure rating and specs for acceptable installation. The basis for most standards is the Miami-Dade County, Florida, Building Code, and many manufacturers list acceptance by Miami-Dade among the specs for models that comply.

Additional Information

American Architectural Manufacturers Association
www.amanet.com

Efficient Windows Collaborative
www.efficientwindows.org

National Fenestration Rating Council
www.nfrc.org

National Wood Window and Door Association
www.wdma.com

Partnership for Advancing Technology in Housing (PATH)
www.pathnet.org

U.S. EPA Energy Star
www.energystar.com

Vinyl Window and Door Institute
www.vinylbydesign.com