NAHB Research Center
This fact sheet is intended to provide you with information about windows -- the factors that affect their energy performance, available options, and potential savings and other benefits associated with better windows. In older homes, windows are often one of the largest sources of heat loss in winter due to their low insulating ability and high air leakage rates. Windows are also generally the major source of unwanted heat gain in the summer. As a result, windows are typically net energy losers, and can be responsible for 25 to 50 percent of the energy used to heat and cool homes.