What is U-factor?

U-factor, sometimes written as U-value, is the rate at which a window, door, or skylight transfers heat. It is measured in British thermal units per hour per square foot per degree Fahrenheit (Btu/hr·ft²·°F). The rating covers the whole assembly, not just the glass, so the glazing, frame, and curb all count toward the final number.

The scale runs in one direction: a lower U-factor means the skylight resists heat flow better. A low number tells you less heat moves through the unit when indoor and outdoor temperatures differ. A higher number means the assembly lets more heat pass through, which is a bigger issue when air conditioning is running most of the year.

U-factor matters most in hot, humid places. In Citrus County, cooling loads tend to be the main energy concern, so the rating helps show how much solar and outdoor heat a skylight may add to a room. Ratings are often listed alongside solar heat gain coefficient (SHGC), which measures how much solar energy comes through the glass. The two ratings answer different questions, and neither one replaces the other.

When comparing ratings, check that they come from the same testing standard, usually NFRC (National Fenestration Rating Council) certified values. Lab-tested and on-site estimates can differ. If you are looking for installers who handle these products in the area, you can browse skylight replacement providers in the directory.

"