Manufacturers usually represent the energy efficiency of windows in terms of their U-values (conductance of heat) or their R-values (resistance to heat flow). If a window’s R-value is high, it will lose less heat than one with a lower R-value. Conversely, if a window’s U-value is low, it will lose less heat than one with a higher U-value. In other words, U-values are the reciprocals of R-values (U-value = 1/R-value). Most window manufacturers use R-values in rating their windows.
Usually, window R-values range from 0.9 to 3.0 (U-values range from 1.1 to 0.3), but some highly energy-efficient exceptions also exist. When comparing different windows, you should ensure that all U or R-values listed by manufacturers: (1) are based on current standards set by the American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE), (2) are calculated for the entire window, including the frame, and not just for the center of the glass, and (3) represent the same size and style of window.
The following five factors affect the R-value of a window:
Traditionally, clear glass has been the primary material available for window panes in homes. However, in recent years, the market for glazing—or cutting and fitting window panes into frames— has changed significantly. Now several types of special glazings are available that can help control heat loss and condensation.
Low-emissivity (low-e) glass has a special surface coating to reduce heat transfer back through the window. These coatings reflect from 40% to 70% of the heat that is normally transmitted through clear glass, while allowing the full amount of light to pass through.
Heat-absorbing glass contains special tints that allow it to absorb as much as 45% of the incoming solar energy, reducing heat gain. Some of the absorbed heat, however, passes through the window by conduction and re-radiation.
Reflective glass has been coated with a reflective film and is useful in controlling solar heat gain during the summer. It also reduces the passage of light all year long, and, like heat-absorbing glass, it reduces solar transmittance.
Plastic glazing materials—acrylic, polycarbonate, polyester, polyvinyl fluoride, and polyethylene—are also widely available. Plastics can be stronger, lighter, cheaper, and easier to cut than glass. Some plastics also have higher solar transmittance than glass. However, plastics tend to be less durable and more susceptible to the effects of weather than is glass.
Storm windows can increase the efficiency of single-pane windows, the least energy-efficient type of glazing. The simplest type of storm window is a plastic film taped to the inside of the window frame. These films are usually available in prepackaged kits. Although plastic films are easily installed and removed, they are easily damaged and may reduce visibility. Rigid or semirigid plastic sheets such as plexiglass, acrylic, polycarbonate, or fiber-reinforced polyester can be fastened directly to the window frame or mounted in channels around the frame—usually on the outside of the building. These more durable materials are also available in kits.
Standard single-pane glass has very little insulating value (approximately R-1). It provides only a thin barrier to the outside and can account for considerable heat loss and gain. Traditionally, the approach to improve a window’s energy efficiency has been to increase the number of glass panes in the unit, because multiple layers of glass increase the window’s ability to resist heat flow.
Double- or triple-pane windows have insulating air- or gas-filled spaces between each pane. Each layer of glass and the air spaces resist heat flow. The width of the air spaces between the panes is important, because air spaces that are too wide (more than 5/8 inch or 1.6 centimeters) or too narrow (less than 1/2 inch or 1.3 centimeters) have lower R-values (i.e., they allow too much heat transfer). Advanced, multi-pane windows are now manufactured with inert gases (argon or krypton) in the spaces between the panes because these gases transfer less heat than does air.
Multi-pane windows are considerably more expensive than single-pane windows and limit framing options because of their increased weight.
Window frames are available in a variety of materials including aluminum, wood, vinyl, and fiberglass. Frames may be primarily composed of one material, or they may be a combination of different materials such as wood clad with vinyl or aluminum-clad wood. Each frame material has its advantages and disadvantages.
Though ideal for strength and customized window design, aluminum frames conduct heat and therefore lose heat faster and are prone to condensation. Through anodizing or coating, the corrosion and electro-galvanic deterioration of aluminum frames can be avoided. Additionally, the thermal resistance of aluminum frames can be significantly improved by placing continuous insulating plastic strips between the interior and exterior of the frame.
Wood frames have higher R-values, are not affected by temperature extremes, and do not generally promote condensation. Wood frames do require considerable maintenance in the form of periodic painting or staining. If not properly protected, wood frames can swell, which leads to rot, warping, and sticking.
Vinyl window frames, which are made primarily from polyvinyl chloride (PVC), offer many advantages. Available in a wide range of styles and shapes, vinyl frames have moderate to high R-values, are easily customized, are competitively priced, and require very low maintenance. While vinyl frames do not possess the inherent strength of metal or wood, larger-sized windows are often strengthened with aluminum or steel reinforcing bars.
Fiberglass frames are relatively new and are not yet widely available. With some of the highest R-values, fiberglass frames are excellent for insulating and will not warp, shrink, swell, rot, or corrode. Unprotected fiberglass does not hold up to the weather and therefore is always painted. Some fiberglass frames are hollow; while others are filled with fiberglass insulation.
Spacers are used to separate multiple panes of glass within the windows. Although metal (usually aluminum) spacers are commonly installed to separate glass in multi-pane windows, they conduct heat. During cold weather, the thermal resistance around the edge of a window is lower than that in the center; thus, heat can escape, and condensation can occur along the edges. To alleviate these problems, one manufacturer has developed a multi-pane window using a 1/8-inch-wide (0.32 centimeters-wide) PVC foam separator placed along the edges of the frame. Like other multi-pane windows, these use metal spacers for support, but because the foam separator is secured on top of the spacer between the panes, heat loss and condensation are reduced. Several window manufacturers now sandwich foam separators, nylon spacers, and insulation materials such as polystyrene and rock-wool between the glass inside their windows.