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What to Look for in Energy-Efficient Windows by Alex Wilson Windows account for a huge percentage of heat loss in the typical New England home. Even though most houses now incorporate insulated (two-layer) glass, this still provides an R-value of only about 2 for window areas—hardly a match for the R-20 to R-26 going into walls. Further, the R-value tells only part of the story: the amount of heat lost by conduction. Windows also are major sources of infiltration (convective) heat loss. Most estimates suggest that windows account for 20 to 25 percent of the heat when other considerations rule out insulated glass. Historic-restoration projects, for example, may call for traditional 12-over-12 windows. Using applied grills over insulated glass is unacceptible