Article #102 - What is a Window U-Factor?

Window U-factor. The rate of heat loss is indicated in terms of the U-factor of a window assembly.

The U-factor measures how well a product prevents heat from escaping. The rate of heat loss is indicated in terms of the U-factor of a window assembly. U-factor ratings generally fall between 0.20 and 1.20.

In the United States, the U-factor is usually expressed in Btu/h ft² F. The metric equivalent unit is W/m² K.

The insulating value is indicated by the R-value, which is the inverse of the U-factor. The lower the U-factor, the greater a window's resistance to heat flow and the better its insulating value. The U-factor is included in the energy performance rating (label) offered by the National Fenestration Rating Council (NFRC).