What is a U-factor?
U-factor is a measure of how much heat escapes through a window, with lower numbers indicating better insulation performance; the scale runs from 0.20 to 1.20, and it appears on the NFRC label.
The U-factor tells you how well a window keeps heat inside your home. It measures the rate of heat loss through the window assembly, including the frame, glass, and spacers. The lower the U-factor number, the better the window insulates. Connecticut installers and manufacturers rely on this metric because cold winters make insulation critical for both comfort and energy bills.
The U-factor scale ranges from 0.20 to 1.20, with lower numbers indicating superior performance. Most high-efficiency windows fall between 0.20 and 0.35. A U-factor of 0.30 means less heat escapes than a window rated 0.70, making the difference significant over a heating season. You'll find this number on the NFRC (National Fenestration Rating Council) label affixed to new windows, along with other ratings like R-value and solar heat gain.
Connecticut's climate makes U-factor especially relevant. Winters demand windows that resist heat loss, so selecting windows with lower U-factors reduces heating costs and improves comfort. When comparing replacement windows from local installers, checking the U-factor on the NFRC label gives you an objective basis for evaluating which window performs better for your climate.