Windows term
U-Factor
Definition: U-factor is the rate at which a window, door or skylight conducts non-solar heat, as rated by the National Fenestration Rating Council; the lower the U-factor, the better the unit insulates.
Why it matters
U-factor is the main number for comparing how well windows keep heat in during Maryland winters. It appears on the NFRC label of rated windows, and ENERGY STAR sets a maximum U-factor for each climate zone.
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