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NFRC
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The National Fenestration Rating Council (NFRC) has developed a fenestration energy rating system based on whole product performance.

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Fenestration - any opening in a building's envelope including windows, doors, and skylights.

NFRC gives ratings for U-value, solar heat gain coefficient, and visible light transmittance. (Used to label as ENERGY STAR^®)

U-value is the measurement of heat loss through a window. The lower the number, the better the insulating value.

Solar heat gain coefficient measures how well a product blocks heat caused by sunlight. The lower a windows SHGC, the less solar heat it transmits. (Values 0 to 1)

Visible transmittance is how much light comes through a product. The higher the VT, the more light is transmitted. (Values 0 to 1)