U-Factor

The rate of heat loss is indicated in terms of the U-factor (U-value) of a window assembly. The insulating value is indicated by the R-value which is the inverse of the U-value. The lower the U-value, the greater a window's resistance to heat flow and the better its insulating value.

Northern Climate: Select windows with a U-factor of 0.35 or less. If air conditioning loads are minimal, windows with U-factors as high as 0.40 are also energy-efficient if the Solar Heat Gain Coefficient is 0.50 or higher. Some double-glazed low-e products have U-factors below 0.30. Some three-layer products have U-factors as low as 0.15.

North/Central Climate: Select windows with a U-factor of 0.40 or less. The larger your heating bill, the more important a low U-factor becomes.

South/Central Climate: Select windows with a U-factor of 0.40 or less. The larger your heating bill, the more important a low U-factor becomes.

Southern Climate: A low U-factor is useful during cold days when heating is needed. A low U-factor is also helpful during hot days when it is important to keep the heat out, but it is less important than SHGC in warm climates. Select windows with a U-factor lower than 0.75 and preferably lower than 0.60.

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