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Getting To Know Window Terms

Solar Heat Gain Coefficient, U-Factor, Air Leakage, Visible Transmittance

Hello. This is Greg Zimmerman, executive editor of Building Operating Management magazine.

Today’s topic is understanding some of the terms used to measure window performance.

Probably the most commonly used terms are U-factor and Solar Heat Gain Coefficient. U-factor is a measure of a window assembly’s insulating value, or its ability to prevent heat gain or loss through the window. Ranging from 1.0 to 0, a lower U-factor is generally better, meaning that little heat is lost or gained through the window assembly.

Solar heat gain coefficient is a measure of a window assembly’s ability to control heat gain from direct or indirect solar radiation. It also sometimes called the shading coefficient. Represented as a number between 0 and 1, the lower a window’s solar heat gain coefficient, the less solar heat it transmits, so the lower the number, the better.

Two other measures facility executives might see are visible transmittance – the amount of light transmitted through the grass – and air leakage, which, as it indicates, is the amount of air that leaks out from or in through a window assembly.

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