Window annual energy rating systems: What they tell us about residential window design and selection

Residential window annual energy rating systems have been developed in Canada and the US. These systems combine window properties of solar heat gain coefficient, U-factor, and air-infiltration into a single number representative of the energy performance for each of the heating season and the cooling season. These systems provide a simple means for designers to select the best energy performing window for low-rise residential buildings over the heating and cooling seasons. The two systems, which rank windows in the same order, give different information on optimum window design and selection than just a simple U-factor comparison. These systems show the importance of a high window SHGC in cold climates and a low SHGC in hot climates. The impact of window air infiltration is surprisingly small relative to the solar heat gain and heat conduction losses.