In Canada and the United States, ENERGY STAR is the certification system used to identify energy efficient products such as windows and doors. The ENERGY STAR symbol indicates that a product has met or exceeded certain standards.
U-factor, is a measurement of a product’s resistance to heat loss. Unlike Low E glass, windows with a lower U-factor provide better insulation, minimizes heat loss and delivers higher energy efficiency, Choosing windows with a low U-factor is especially beneficial during winter month.
Expressed as a percentage, visible transmittance refers to the amount of visible light allowed to pass through a window. A lower number means less light will enter the home or building. This is typically the result with Low-E glass windows, which use a special coating to reduce solar heat gain. But the highest quality Low-E windows can achieve a comfortable balance—letting in lots of natural light while minimizing unwanted heat gain.
Air leakage is a measurement of the amount of air that passes through cracks in a window. A higher rate of air leakage indicates the window is more susceptible to heat loss and gain, which reduces energy efficiency and interior comfort.
Air leakage can also result from improper installation.
This figure captures the overall energy-efficiency score of a window. It compares U-factor, air leakage and SHGC, giving consumers a more convenient way to examine products. The standards for energy ratings vary according to climate zone. To earn the ENERGY STAR label in Canada, a window must have a minimum energy rating of 25 in zone 1, 29 in zone 2 and 34 in zone 3.
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