Windows comprise 10-25% of the exterior wall area of new homes. According to recent studies windows:

• account for up to 25% of a typical house's heating load in heating-dominated climates

• account for up to 50% of a typical house's cooling load in cooling-dominated climates

Windows can improve the thermal performance of homes by minimizing heat loss in heating-dominated climates and by minimizing solar heat gain in cooling-dominated climates. Thus windows with lower U-factors or higher R-values perform better in heating-dominated climates and windows with lower solar heat gain coefficients (SHGC) perform better in cooling-dominated climates). 

Many technological advances have been made that significantly enhance the thermal performance of windows

Properties that make up a high-performance window

New Framing Materials and Designs

• Composite: Most people are familiar with composite wood products, such as particle board and laminated strand lumber, in which wood particles and resins are compressed to form a strong composite material. The wood window industry has now taken this a step further by creating a new generation of wood/polymer composites that are extruded into a series of lineal shapes for window frame and sash members. These composites are very stable, and have the same or better structural and thermal properties as conventional wood, with better moisture resistance and more decay resistance. They can be textured and stained or painted much like wood. They were initially used in critical elements, such as window sills and thresholds in sliding patio doors, but are now being used for entire window units. This approach has the added environmental advantage of reusing a volume of sawdust and wood scrap that would otherwise be discarded

• Insulated vinyl: Insulated vinyl frames are identical in most of their characteristics to standard vinyl frames. Vinyl window frames do not require painting and have good moisture resistance. Because the color goes all the way through, there is no finish coat that can be damaged or deteriorate over time -- the surface is therefore maintenance-free. Recent advances have improved dimensional stability and resistance to degradation from sunlight and temperature extremes. The major difference between insulated vinyl and standard vinyl frames is improved thermal performance. In insulated vinyl frames, the hollow cavities of the frame are filled with insulation making them thermally superior to standard vinyl and wood frames. Usually these high performance frames are used with high performance glazings.

• Fiberglass: Window frames can be made of glass-fiber-reinforced polyester, or fiberglass, which is extruded into lineal forms and then assembled into windows. These frames are dimensionally stable and have air cavities (similar to vinyl). When the cavities are filled with insulation, fiberglass frames have thermal performance superior to wood or vinyl (similar to insulated vinyl frames). Because the material is stronger than vinyl, it can have smaller cross-sectional shapes and thus less area. Another polymer-based approach is to use extruded engineered thermoplastics, another family of plastics used extensively in automobiles and appliances. Like fiberglass, they have some structural and other advantages over vinyl.

Low-emissivity and/or solar control coatings

Low-emittance (Low-E) coating are microscopically thin, virtually invisible, metal or metallic oxide layers deposited on a window or skylight glazing surface primarily to reduce the U-factor by suppressing radiative heat flow. The principal mechanism of heat transfer in multi-layer glazing is thermal radiation from a warm pane of glass to a cooler pane. Coating a glass surface with a low-emittance material and facing that coating into the gap between the glass layers blocks a significant amount of this radiant heat transfer, thus lowering the total heat flow through the window. Low-E coatings are transparent to visible light. Different types of Low-E coatings have been designed to allow for high solar gain, moderate solar gain, or low solar gain.

Low conductance gas fills

An improvement that can be made to the thermal performance of insulating glazing units is to reduce the conductance of the air space between the layers. Originally, the space was filled with air or flushed with dry nitrogen just prior to sealing. In a sealed glass insulating unit, air currents between the two panes of glazing carry heat to the top of the unit and settle into cold pools at the bottom. Filling the space with a less conductive, more viscous, or slow-moving gas minimizes the convection currents within the space, conduction through the gas is reduced, and the overall transfer of heat between the inside and outside is reduced.

Manufacturers have introduced the use of argon and krypton gas fills, with measurable improvement in thermal performance. Argon is inexpensive, nontoxic, nonreactive, clear, and odorless. The optimal spacing for an argon-filled unit is the same as for air, about 1/2 inch (11-13 mm). Krypton has better thermal performance, but is more expensive to produce. Krypton is particularly useful when the space between glazings must be thinner than normally desired, for example, 1/4 inch (6 mm). The optimum gap width for krypton is 3/8" (9mm). A mixture of krypton and argon gases is also used as a compromise between thermal performance and cost.

Insulating spacer between glazings

The layers of glazing in an insulating unit must be held apart at the appropriate distance by spacers. Window manufacturers have developed a series of innovative edge systems, including solutions that depend on material substitutions as well as radically new designs. One approach to reducing heat loss has been to replace the aluminum spacer with a metal that is less conductive, e.g. stainless steel, and change the cross-sectional shape of the spacer. These designs are widely used in windows today.

Another approach is to replace the metal with a design that uses materials that are better insulating. The most commonly used design incorporates spacer, sealer, and desiccant in a thermoplastic compound that contains a blend of desiccant materials and incorporates a thin, fluted metal shim of aluminum or stainless steel. Another approach uses an insulating silicone foam spacer that incorporates a desiccant and has a high-strength adhesive at its edges to bond to glass. The foam is backed with a secondary sealant. Both extruded vinyl and fiberglass spacers have also been used in place of metal designs.

There are several hybrid designs that incorporate thermal breaks in metal spacers or use one or more of the elements described above. Some of these are specifically designed to accommodate three-and four-layer glazings or IGUs incorporating stretched plastic films. All are designed to interrupt the heat transfer pathway at the glazing edge between two or more glazing layers.

Warm edge spacers have become increasingly important as manufacturers switch from conventional double glazing to higher-performance glazing. For purposes of determining the overall window U-factor, the edge spacer has an effect that extends beyond the physical size of the spacer to a band about 2-1/2 inches (64 mm) wide. The contribution of this 2-1/2-inch-wide "glass edge" to the total window U-factor depends on the size of the window. Glass edge effects are more important for smaller windows, which have a proportionately larger glass edge area. For a typical residential-size window (3 by 4 feet/0.8 by 1.2 meters), changing from a standard aluminum edge to a good-quality warm edge will reduce the overall window U-factor by approximately .02 Btu/hr-sq ft-°F.

A more significant benefit may be the rise in interior surface temperature at the bottom edge of the window, which is most subject to condensation. With an outside temperature of 0°F, a thermally improved spacer could result in temperature increases of 6-8°F (3-4°C) at the window sightline--or 4-6°F (2-4°C) at a point one inch in from the sightline, which is an important improvement. As new highly insulating multiple layer windows are developed, the improved edge spacer becomes an even more important element.

Solar heat gain coefficient (SHGC). The fraction of solar radiation admitted through a window or skylight, both directly transmitted, and absorbed and subsequently released inward. The solar heat gain coefficient has replaced the shading coefficient as the standard indicator of a window's shading ability. It is expressed as a number between 0 and 1. The lower a window's solar heat gain coefficient, the less solar heat it transmits, and the greater its shading ability. SHGC can be expressed in terms of the glass alone or can refer to the entire window assembly.

U-factor (U-value). A measure of the rate of non-solar heat loss or gain through a material or assembly. It is expressed in units of Btu/hr-sq ft-°F (W/sq m-°C). Values are normally given for NFRC/ASHRAE winter conditions of 0° F (18° C) outdoor temperature, 70° F (21° C) indoor temperature, 15 mph wind, and no solar load. The U-factor may be expressed for the glass alone or the entire window, which includes the effect of the frame and the spacer materials. The lower the U-factor, the greater a window's resistance to heat flow and the better its insulating value

R-value. A measure of the resistance of a glazing material or fenestration assembly to heat flow. It is the inverse of the U-factor (R = 1/U) and is expressed in units of hr-sq ft-°F/Btu. A high-R-value window has a greater resistance to heat flow and a higher insulating value than one with a low R-value