Solar Heat Gain Coefficient (SHGC)

The number on a window label that tells you how much of the sun's heat passes through the glass into your home is called Solar Heat Gain Coefficient, or SHGC. The U.S. Department of Energy defines it as a measure of the solar radiation admitted through a window. It runs from 0 to 1, where a window rated 0.25 lets in only a quarter of the solar heat hitting it, and a window rated 0.70 lets in most of it.

SHGC is set by the glass, any coatings, and the frame together, and the National Fenestration Rating Council (NFRC) certifies the rating you see on the label. The lower the SHGC, the more solar heat the window rejects. In a cold climate you might want a higher SHGC to capture free winter warmth. In a cooling-dominated climate like the Phoenix metro, you want it low, because the sun is the enemy of your AC bill for most of the year.

For example, two identical front windows facing west in Fountain Hills will feel completely different at 5 p.m. in July. A window at SHGC 0.60 dumps afternoon sun heat into the room and the AC chases it. A spectrally selective low-E window at SHGC 0.25 blocks most of that heat while still letting in daylight, so the room stays comfortable and the system cycles less.

SHGC is different from U-factor, which rates heat that conducts through the window when there is no sun. A desert home wants both numbers low. SHGC is the one that controls how hard the afternoon sun cooks your living room, so it carries the most weight on Arizona's south- and west-facing glass.