Emissivity is a measure of a surface’s ability to emit long-wave infrared radiation or room temperature radiant heat energy. Emissivity varies from 0 (no emitted infrared) to 1 (100% emitted infrared). The lower the emissivity, the lower the resultant U-Value.
U-Value is the amount of heat transmitted by the window. The lower the number, the more efficient the window is in reducing winter heat loads and costs. Expressed in Btu/hr.ft2.F.
R-Value designates a window’s resistance to heat flow. This number is the inverse of the U-Value: the higher the number, the more efficient the window in reducing winter heating costs. Expressed in hr.ft2.F/Btu.
Solar Heat Gain Coefficent is the fraction of incident solar radiation entering a home through the windows. The lower the number, the better the window is at preventing solar gain – critical to reducing summer cooling costs.
