U-Factor

U-factor (also written U-value) measures how quickly a window, door, or skylight loses or gains heat by conduction. The U.S. Department of Energy defines it as "the rate at which a window conducts non-solar heat flow." Residential fenestration U-factors usually run from about 0.20 to 1.20, and the lower the number, the slower heat moves through and the better the unit insulates. A window rated U-0.25 holds heat far better than one rated U-0.60.

U-factor is essentially the opposite of R-value. R-value measures resistance to heat flow, so higher is better and it is used for insulation. U-factor measures the rate of heat flow, so lower is better and it is used for windows and doors. They are mathematically inverse: a higher R means a lower U. The U-factor on a window covers the whole assembly, glass and frame together, and the National Fenestration Rating Council (NFRC) certifies the number on the label.

For example, a custom home in Cave Creek with single-pane or builder-grade windows might have a U-factor near 0.60. Those windows bleed cooled air on a 110-degree afternoon and bleed warmth on a cold January night. Upgrading to low-E double-pane windows around U-0.30 cuts that conducted heat roughly in half, so the home holds its conditioned air longer and the HVAC runs less in both seasons.

U-factor is not the only window number that matters. It rates conducted heat, while SHGC rates the sun's radiant heat coming through the glass. In Arizona you want both low, but U-factor matters most for keeping conditioned air in, and SHGC matters most for keeping the desert sun out. Energy code sets a maximum U-factor by climate zone, which is why it shows up on every fenestration schedule at plan review.