Windows are a very important aspect of any home. They let in natural light and fresh air as well as connecting the inside of the home with the outdoors. However windows are also a major source of unwanted heat gain in summer and significant heat loss in winter.
More heat loss occurs through windows than any other surface in the home. A single pane of glass can lose almost ten times as much heat as the same area of insulated wall. With heat being lost as fast as it is produced for warmth, windows are a very significant component of home heating costs.