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Choosing a Well-Insulated Window
Windows that have good insulating values will make your home more comfortable, particularly in winter. This is partly because they allow less heat to pass through, but there's another reason: the inside surface of a better-insulated window will be warmer. When you stand or sit by it, your body won't lose as much heat to it as it would to a less-insulated window. Sometimes the draftiness that people feel from windows isn't due to air movement at all, but rather to the fact that we radiate body heat to the cold window surface.
In addition to improving comfort, windows with high insulating values are less likely to have problems with condensation. Condensation occurs when warm, moist indoor air comes in contact with a cold surface, such as a poorly insulated window.
Better insulating values are most beneficial in cold climates. The bigger the difference in temperature between outside and inside, the faster heat will move through the window. You'll notice the difference if you replace an old window with a better-insulated one. For instance, when it is 0°F outside, the inside surface temperature of a single-pane glass window can be less than 15°F—easily cold enough for ice to form. A double-pane window will raise that temperature to about 45°F, and a high performance window will bring the temperature of the inside of the glass to around 60°F.
Windows with a good insulating value will help in the summer as well, particularly if you are using air conditioning. Again, the greater the temperature difference between inside and outside, the more important this is. (If you cool the house with window fans or cross ventilation, you'll be more interested in windows that provide large ventilation openings.)
Windows Ratings and Labels
It can be difficult to compare claims made by different manufacturers because they often use different measures to sell their products. For instance, some may use center-of-glass R-value and shading coefficient, while others use whole-window U-factor and solar heat gain coefficient. Fortunately there is now one place to look that has standardized ratings for windows. The National Fenestration Rating Council (NFRC) is a nonprofit coalition of manufacturers and window experts that has set standards for testing and labeling windows.
NFRC currently certifies whole-window U-factor, solar heat gain coefficient (SHGC), and visible transmittance (VT). Any or all of these may appear on an NFRC label on the window. If there is no label, ask the manufacturer for the window's NFRC ratings or look in the Certified Products Directory, which your window dealer may have. The directory can be obtained for $15 from NFRC (301-589-NFRC). If you get the NFRC ratings for two windows, you know that you're comparing the same characteristics.
Although the NFRC certification is voluntary, most major manufacturers now certify their products. In California, Washington, Oregon, Idaho, Minnesota, and Alaska, building energy codes require certification as evidence of code compliance. Several county building codes also accept NFRC certification, and more states are likely to adopt the requirement as they update their energy codes. These codes usually apply to additions as well as new construction.
Carmody, John, Stephen Selkowitz, and Lisa Heschong. "Residential Windows: A Guide to New Technologies and Energy Performance". New York: W.W. Norton & Co., 1996.
National Renewable Energy Laboratory, "Advances in Glazing Materials for Windows", and "Energy-Efficient Windows", Washington, DC: U.S. Department of Energy's Energy Efficiency and Renewable Energy Clearinghouse, 1994.
National Fenestration Rating Council (NFRC), 1300 Spring Street, Suite 120, Silver Spring, MD 20910. (301) 589-6372.
Excerpted with permission from No-Regrets Remodeling by Home Energy (1997)