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In an average home, about 20 percent of heat is lost through windows and doors. Use the touch test to see if yours are letting in cold air. If they are, caulk and weather-strip the edges (cheapest solution), or invest in glazed windows with the lowest U-factor (measure of heat transfer) you can find. The Department of Energy says that U.S. homeowners who replace single-pane windows with Energy Star–rated windows save an average of $126 to $465 a year.