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Reducing Heat Loss and Condensation in windows

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 replacement 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.

• The type of glazing material (e.g., glass, plastic, treated glass)

   

• The number of layers of glass

   

• The size of the air space between the layers of glass

   

• The thermal resistance or conductance of the frame and spacer materials

   

• The "tightness" of the installation (i.e., air leaks—:see previous discussion).

Types of Glazing Materials

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 reradiation.

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.

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