Window U Factor measures the rate of heat transfer through a complete window assembly, expressed in Btu/(h·ft²·°F), with lower values indicating better thermal insulation. U Factor accounts for heat loss through glass, frames, spacers, and air films. NFRC (National Fenestration Rating Council) ratings provide standardized U Factor measurements for comparison between window products.
Standards and Ratings
ENERGY STAR certified windows typically have U factors of 0.30 or lower. Double-pane windows range from 0.30 to 0.60 U Factor, while triple-pane windows achieve U factors as low as 0.15 to 0.25. High-performance windows with low-emissivity coatings, argon gas fills, and insulated frames meet cold climate requirements. Building codes specify maximum U factors from 0.27 in IECC Zone 1 to 0.57 in warmer zones.
Performance and Selection
Lower U Factor windows reduce heating and cooling loads by 10-15 percent annually. Window selection impacts both energy costs and indoor comfort, particularly near large glazed areas. NFRC labels clearly display U Factor along with Solar Heat Gain Coefficient and Visible Transmittance for informed purchasing decisions.