225 Oxmoor Circle,
Homewood, AL 35209
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What does all this mean?
U-Factor measures the amount of heat that
escapes through the product. The lower the
rating, the better the window is at preventing
heat loss (as imagined, this is more of a
concern in northern climates). NFRC certified
products require U-Factor ratings. In the Mid-
Atlantic, always look for the U-Factor and
select windows with a U-Factor at least as low
Solar Heat Gain Coefficient (SHGC) measures how well a product blocks heat caused by sunlight. The SHGC is the fraction of incident solar radiation admitted through a window, both directly transmitted, and absorbed and subsequently released inward. SHGC is expressed as a number between 0 and 1. A window with an SHGC rating of 0.70 will allow 70% of the solar heat to pass through it. The lower a window's solar heat gain coefficient, the less solar heat it transmits. A high solar gain window will let in solar heat and trap it, increasing cooling load in the summer. In the Mid-Atlantic, a low SHGC is the most important consideration for reduction of summer home cooling costs.
Visible Transmittance (VT) measures how much light comes through the window. The visible transmittance is an optical property that indicates the amount of visible light transmitted. VT is expressed as a number between 0 and 1. The higher the VT, the more visible light that is transmitted. Dark tinted coatings can significantly reduce visible transmittance, but it is possible to get windows with high VT and low SHGC. Good VT ranges are 0.70 for the glass or 0.50 for the whole window.
Air Leakage (AL) is indicated by an air leakage rating, expressed as the equivalent cubic feet of air passing through a square foot of window area. Heat loss and gain occur by infiltration through cracks in the window assembly. The lower the AL, the less air will pass through cracks in the window assembly.
Condensation Resistance (CR) measures the ability of a product to resist the formation of condensation on the interior surface of that product. While this rating cannot predict condensation, it can provide a credible method of comparing the potential of various products for condensation formation. CR is expressed as a number between 0 and 100. The higher the CR rating, the better that product is at resisting condensation formation.
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