U-factor

U-factor rates the whole product: glass, frame and spacer together. It tells you how much heat leaks through when the inside and outside temperatures differ. Lower is better. It matters in July as much as in January, because heat moves in both directions.

Two things lower it most: the glass package and the frame. A low-E coating, a gas fill and a thermal break in a metal frame all help. Compare U-factor on the NFRC label, and read our guide to SHGC and U-factor for Texas before you choose. A lower number is better, and it is rated for the whole unit, not only the glass.