Getting To Know Window Terms
Solar Heat Gain Coefficient, U-Factor, Air Leakage, Visible Transmittance August 28, 2008
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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