U-factor measures the heat transfer through a window and tells you how well the product insulates. The lower the U-factor, the greater resistance to heat flow (in and out) and the better its insulation value. ENERGY STAR window U-factor values generally range from less than or equal to 0.27 up to 0.40 depending on the climate zone. The top performing ENERGY STAR Most Efficient windows have U-factors of less than or equal to 0.20.
Learn more about the ENERGY STAR Most Efficient window program at www.energystar.gov/mostefficient.