Savings and Strength
As recently as 20 years ago, the windows in most homes held a single pane of glass. But for an average-size house, those same windows today can result in significant losses: Single-glazed windows in a cold-climate house would result in a yearly energy bill of about $800, and in a warm-climate house, the energy bill would be about $850, according to Residential Windows: A Guide to New Technologies and Energy Performance , by John Carmody, Stephen Selkowitz and Lisa Heschong. The book, published in 1996, reveals that today's high-tech double glazing would reduce annual energy costs in these same houses to about $450 and $500, respectively.

Most of those savings come from two techniques manufacturers use to make double glazings more efficient: low-e coatings and inert gases that fill the spaces between the panes. Low-e coatings were first developed to stop heat loss; newer coatings also stop heat gain where cooling is the key consideration. These coatings usually add 10 to 15 percent to the cost of a window.

A rating system developed by the National Fenestration Rating Council (NFRC) makes gauging the efficiency of a window quick and easy. A label on each window includes its U-value; its solar heat gain coefficient (SHGC), which tells how much heat it lets in; and its visible light transmittance. If you want to cut energy loss during the heating season, look for windows with low U-values. For optimal cooling, go with units that have a low SHGC but let in plenty of visible light.

You can avoid these ratings comparisons altogether by shopping for windows that carry the Energy Star label. Developed by the U.S. Department of Energy and the Environmental Protection Agency, the program tags the most energy-efficient windows. It also targets requirements to specific climates. Windows that meet these energy requirements are 15 to 40 percent more efficient than those that meet typical building codes.

In cooling-dominated areas, U-values of 0.75 and an SHGC of 0.40 or below are required. In heating-dominated areas, U-values of 0.35 or below are required while there is no SHGC requirement. And in areas with mixed heating and cooling needs, U-values of 0.40 or below and SHGCs of 0.55 or below are required. (The determination for a heating-dominated climate means at least 70 percent of heating and cooling costs go to heating. In a cooling-dominated climate, at least 70 percent of heating and cooling costs go to cooling. In a mixed climate, at least 30 percent of heating and cooling costs go to each.)

Another new program helps ensure that wood, vinyl and aluminum windows meet minimum structural standards. Developed by the American Architectural Manufacturers Association in conjunction with the National Wood Window and Door Association, this nonmandatory program puts windows through their paces with a series of performance tests. Windows that pass the tests are granted a certification sticker.

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