Deciphering the Sticker

Manufacturers typically submit their windows to tests that follow standards set by the nonprofit National Fenestration Rating Council (NFRC). Their findings, reported on a sticker affixed to most new windows, rate several factors, each expressed as a number. The sample below shows what all the figures actually mean:

(A) U-FACTOR measures how much heat escapes from a room through the window; the lower the number (usually between 0.20 and 1.20), the better the window's insulating ability. In colder climates, look for 0.35 or lower. Warmer climates can go as high as 0.75.

(B) SOLAR HEAT GAIN COEFFICIENT (SHGC) indicates how much heat from the sun the window allows to come into a room, expressed on a scale of 0 to 1. In warmer climates, look for an SHGC lower than 0.4, which means that less than 40 percent of the sun's heat enters a room.

(C) VISIBLE LIGHT TRANSMITTANCE (VT) measures how much light passes through the window. Anything higher than 0.6 (meaning 60 percent of the light is transmitted) will appear clear to the naked eye. Numbers below 0.5 can significantly darken the view.

(D) AIR LEAKAGE (optional) essentially says how drafty a window is, measured in cubic feet of air per square foot of window. A good number is between 0.1 and 0.3, and few manufacturers report this rating unless the window falls into that range.

(E) CONDENSATION RESISTANCE (optional) shows, through a number between 1 and 100, how well a window resists forming condensation (on the inside during cold months, on the outside during a humid summer). The higher the number, the better the window is at resisting condensation.

DESIGN PRESSURE (Not shown) (optional) is a structural rating that shows how well a window can withstand pressure from wind and rain in pounds per square foot. A large window should rate at least DP-30; 40 or 50 is even better. In hurricane regions they must also have impact glass, which, like a windshield, doesn't shatter. Check codes for local requirements.
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