Window Energy Ratings

NFRC labels provide ratings for U-factor and Solar Heat Gain Coefficient, and Visible Transmittance, information is included on testing for Air Leakage and Condensation Resistance.

ENERGY STAR ® label to products


U-factor measures how well a product stops heat from escaping a home. U-factor ratings commonly fall between 0.15 and 1.20. The lower the U-factor, the better a product is at keeping heat inside the building. U-factor is particularly important during the winter heating season in colder climates. This label shows the U-factor in U.S. units.

What’s the Difference between U-factor and R-value?

The biggest difference between U-factor and R-value is that U-factor measures the rate of heat transmission (or loss) while R-value measures the resistance to heat loss. R-value is a measure of conductance and resistance. A product with high conductance will conduct heat quickly, like a hot pan on the stove. U-factor, on the other hand, takes into consideration more than conductance. It also is affected by the airflow around the window and the emissivity (radiated or reflected heat) of the glass.

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