The U-factor is a rating given to a window based on how much heat loss it allows. U-factors generally range from 0.2 (very little heat loss) to 1.2 (high heat loss). The U-factor is the inverse of the R-value of a window, which measures a window’s insulating value. Thus, a high R-value is the same as a low U-factor, and means that a window does not allow much heat to escape. A poorly-made window cannot get a low U-factor. Single-pane windows are about 1.0 and double-panes are about 0.4. If you live in a colder climate, or find that you are always heating your home, buying windows with a low U-factor is a good way to save energy and money. The National Fenestration Rating Council (NFRC) offers reliable U-factor ratings for windows that they have certified.
Read about other window ratings, Solar Heat Gain Coefficient (SHGC), Low-E, and Visible Transmittance, on ecomii.
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Learn more about energy efficient windows at ecomii's Green Building center.
See the NFRC’s website for more on window ratings.
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