U-factor
How well a window resists heat loss — lower numbers insulate better.
U-factor measures how quickly a window loses heat: the lower the number (most residential windows fall between about 0.20 and 0.40), the better it insulates. It's the rating to watch in cold climates, where heat escaping through glass drives up your heating bill. Look for the U-factor on the National Fenestration Rating Council (NFRC) label and choose an ENERGY STAR window rated for your climate zone — a lower U-factor usually adds a little to the price but pays back in comfort and energy savings.