Windows can be one of the single largest sources of unwanted heat gain and loss in the thermal envelope.
It is not unusual to find a glass area that comprises only 15 to 25 percent of the surface area of a building while contributing up to 75 percent of the heat loss.
The amount of energy that escapes through American windows every winter is the equivalent of all the oil that flows through the Alaska pipeline each year. Windows typically lose heat through conduction and air movement around the frames, and gain heat through solar radiation.
Windows in older homes can cause up to a third of the home’s total heat loss. In newer homes, windows typically account for 15 to 40 percent of total heat loss.
Solar Heat Gain – Windows can also account for as much as 75 percent of heat gained during the summer months, adding to air conditioning costs.