Windows are the weakest link in a well-insulated home, with a square yard of conventional single-pane glass exposed to direct sun on a hot day generating as much heat as an electric space heater.  On a cold day, that same glass will lose more heat than the same area of insulated wall.  Double-glazed windows, using 2 sheets of glass with air or gas sealed between them, are up to twice as expensive, but also up to twice as efficient.