How to Read a Window Label

The National Fenestration Rating Council (NFRC) certifies windows and provides energy performance labels that help you determine how well windows are constructed. The information on the label allows you to compare one window to another and make informed decisions about the windows you buy.

The current label, adopted in 2005, lists the manufacturer, describes the product, provides a source for additional information, and includes ratings for energy performance characteristics. Because the label can be confusing, this slideshow breaks it down and describes what each rating means. So the next time you buy windows, you can do it with confidence.

The U-Factor measures how well a window prevents heat from escaping. The lower the U-Factor, the greater the window's resistance to heat flow and the better its insulating value. U-Factor ratings generally range from .20 to 1.20.

Solar Heat Gain Coefficient (SHGC) measures how well a window blocks radiant heat caused by sunlight. The SHGC is indicated by a number between 0 and 1; the lower a window's SHGC, the less solar heat it transmits into your house.

Visible Transmittance (VT) measures how much visible light is allowed through the window. A higher VT means more light is transmitted. It's expressed as a number between 0 and 1.

This indicates how much heat loss and gain occur through cracks in the window assembly. The lower the Air Leakage rating, the less air will pass through. This rating is optional, so manufactures may choose not to include it.

Another optional rating, the Condensation Resistance (CR) measures the ability of a window to resist condensation forming on its interior surface. The higher the CR rating, the better the window is at resisting condensation. It's indicated by a number between 0 and 100.

C.E.C stands for the California Energy Commission, the state's primary energy policy and planning agency that promotes energy-efficiency through appliance and building standards. If a window meets or exceeds C.E.C. Air Infiltration Standards, the window construction is tight enough to comply with the commission's air leakage standards.