U-Factor (Windows)

U-Factor is the measure of the rate of heat transfer through an entire window assembly, including the glazing, frame, sash, and spacer. It indicates how well the window insulates against heat loss. The U-Factor is expressed in BTU per hour per square foot per degree Fahrenheit (BTU/hr·ft²·°F).

A lower U-Factor means the window provides better thermal insulation, reducing energy loss in winter and limiting heat gain in summer. In the United States, U-Factor ratings are determined through standardized testing and simulation procedures set by the National Fenestration Rating Council (NFRC).

This value is a key performance indicator used in building codes, ENERGY STAR certification, and efficiency standards for residential and commercial buildings across the U.S.ieszczeniach.