The National Fenestration Rating Council (NFRC) is an independent non-profit organization that sponsors window certification ratings. U-factor is the measure of a window’s ability to resist heat transfer. All-Energy Star-rated windows must display their U-factor rating. According to the EPA, windows range from a low of U .25 to a high of U 1.25. Unlike R-factor, the lower the number the greater the performance. Windows also may be rated for solar heat gain coefficient, air leakage, visible light transmission, and condensation resistance. It’s important to understand that window performance is a measure of the entire window unit, including the frame, sash, and glass -- not just the glass.