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What is a U-Factor in terms of window efficiency? Follow

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 ENERGY STAR certified residential windows, doors and skylights at www.energystar.gov/windows.


Learn more about the ENERGY STAR Most Efficient window program at www.energystar.gov/mostefficient.

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