Windows and doors (also referred to as fenestration) are rated for energy efficiency. These ratings come in the form of U-factors, which is a measure of the equivalent insulation level, and the SHGC (Solar Heat Gain Coefficient), which measures the reflective value of the glass. As opposed to R-values, which utilize a numbering system from lower to higher with higher being better, U-factors and SHGC are the opposite. The lower number represented by the U-factor and SHGC measures greater resistance to heat, cold and solar radiation.
