The National Fenestration Rating Council (NFRC) has developed an objective way for homeowners to determine the energy-saving properties of one brand or model of windows against another.

The NFRC is a nonprofit, quasi-private organization created by the window, door, and skylight industry. Membership includes manufacturers, suppliers, researchers, architects, designers, code officials, utilities, and government agencies.

The window energy rating system is based on whole-product performance. It gives relative quantitative values to U-factor, Solar Heat Gain Coefficient (SHGF), and Visible Light Transmittance. Air Leakage (AL) and Condensation Resistance (AL) are optional ratings that are sometimes provided.

The NFRC label appears on all products certified to the NFRC standards as well as on all window, door, and skylight products that are part of the ENERGYSTAR® program.


U-factor is the rate at which the window assembly passes heat in either direction. The lower the number, the better the window’s insulating properties. High-performance double-pane windows can have U-factors of 0.30 or lower. Some triple-pane windows boast U-factors as low as 0.15.

Solar Heat Gain Coefficient (SHGC)

SHGC quantifies the amount of solar radiation allowed into a room by a window, some by direct passage through the window, and some that is initially absorbed by the window and slowly released into the room. SHGC numbers are between zero and one, and the lower the number the better the window.

Visible Light Transmittance (VT)

VT indicates the amount of visible light transmitted by the entire window assembly. This means that window assemblies with relatively more glass area will rate better than those with less glass and more frame. A high VT rating means more daylight. VT numbers are between zero and one, and most values among double- and triple-pane windows are between 0.30 and 0.70.

Air Leakage (AL)

AL is a measure of the heat loss or gain. The lower the AL number, the less air will pass through cracks in the window assembly. At this time, the AL is optional among NFRC ratings. AL numbers are between zero and one. Select windows with an AL of 0.30 or less.

Condensation Resistance (CR)

CR is an optional rating on the NFRC label. It doesn’t indicate whether a window will cause condensation at a specific relative humidity and outdoor temperature. It only provides a way to compare one window’s condensation resistance to another. It is expressed as a number between 1 and 100. The higher the number the better. 

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