The National Fenestration Rating Council (NFRC) is a non-profit organization created to provide accurate information for measuring and comparing the energy performance of window, door and skylight products. It is comprised of manufacturers, builders, architects and designers, code officials, utilities and government agencies.

Solar Heat Gain Coefficient (SHGC): Measures how well a window blocks heat caused by sunlight. The lower a window's SHGC, the less solar heat it transmits.

Visible Transmittance (VT): Determines how much light passes through a product. The higher the VT, the more light is transmitted.