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U-Factor: U-factor measures how well a product prevents heat from escaping. The rate of heat loss is indicated in terms of the U-factor (U-value) of a window assembly. U-factor ratings generally fall between 0.20 and 1.20. The lower the U-value, the greater a window's resistance to heat flow and the better its insulating value.
Visible Transmittance: Visible Transmittance (VT) measures how much light comes through a product. VT is expressed as a number between 0 and 1. The higher the VT, the more light is transmitted.
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R-Value: The R-value indicates a window's insulating value. If the R-value is high, it will lose less heat than one with a lower R-value. The higher the R-value, the greater the insulating value. Make sure the R-value is calculated for the entire window, including the frame, and not just for the center of the glass.
Air Leakage: Air Leakage (AL) is indicated by an air leakage rating. Heat loss and gain occur through cracks in the window assembly. The lower the AL, the less air will pass through cracks in the window assembly.
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Solar Heat Gain Coefficient: Solar Heat Gain Coefficient (SHGC) measures how well a product blocks heat caused by sunlight. SHGC is expressed as a number between 0 and 1. The lower a window's solar heat gain coefficient, the less solar heat it transmits.
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