U-Value

A measure of the rate of non-solar heat gain or loss through a material or assembly. The lower the U-value, the slower the rate of heat flow and the better the insulating quality. In a window, the U-value may be expressed for the glass alone or the entire window, which includes the effect of the frame and the spacer materials.

U-values are the reciprocal of R-values. (e.g. a U-value of 0.20 is equal to R-value of 5).