The National Fenestration Ratings Council (NFRC) is a nonprofit, public/private organization created by the window, door, and skylight industry. In 2005, the NFRC developed a label to provide consumers with energy performance data on windows, doors, skylights and similar products. The NFRC label has become an industry standard in evaluation of energy and performance.

This sticker is similar to the familiar “miles-per-gallon” sticker seen on new cars. The NFRC sticker tagged to the glass tells how well the window works in cold and warm weather. Many states now require NFRC ratings in order to get a permit for window replacement of any kind. The label is also on all Energy Star qualified window products.

Two of the most important factors to look for on the label are the Solar Heat Gain Coefficient and the U Factor. The lower these numbers, the better.

During product testing, the NFRC measures heat flow. Windows that allow the least amount of heat to leave your house have low U-value numbers. A low U-factor helps keep the summer heat out in warmer climates, and the winter cold out in colder climates. The NFRC examines windows for this summer heat gain. The results of the test produce a number called the Solar Heat Gain Coefficient (SHGC). This number works just like U-values. Low SHGC numbers mean the window glass prevents the solar energy trying to enter your home during the summer season. The best solar heat gain coefficient (SHGC) rating can vary with a home's particular location and climate zone. In climates that are dominated by heating more than cooling needs, higher SHGC windows may be more appropriate.

Specifying efficient windows controls solar energy gains and helps reduce heating and cooling loads. Building America experts recommend that, nationwide, windows have a U-factor of 0.35 or lower and a SHGC of 0.35 or less. The tradeoff in using a smaller SHGC is that the sun is blocked in both winter, when the sun helps to heat the house, and summer, when the sun works against air conditioning. Note that ENERGY STAR qualification can be met with windows at less stringent ratings.[1]

Below are the label terms and ratings that you should know when comparing and choosing windows and doors:

SHGC: Solar Heat Gain Coefficient is the fraction of solar heat that actually penetrates a window and enters the living area of a home. The lower the number, the better the window is for reducing solar heat gain. In the southern climate zone, the SHGC should be 0.4 or less, which normally consists of an insulated window (two layers of glass with an air space between them) with a low-E coating inside one of the glazings.

VT: Visible Light Transmittance is the percentage of visible light that penetrates a window. The higher this number, the more daylight is transmitted.

U-factor: U-values measure the ability of the window to conduct heat—the inverse of R-value (a term used for insulation). The lower this number, the better the insulating ability (which combats winter heat loss).

AL: The rated Air Leakage measures air infiltration of the window unit (in cubic feet per minute, or cfm, per square foot of area). The lower this number is, the more airtight the unit.

CR: Condensation Resistance (CR) measures the ability of a product to resist the formation of condensation on the interior surface of that product. The higher the CR rating, the better that product is at resisting condensation formation. While this rating cannot predict condensation, it can provide a credible method of comparing the potential of various products for condensation formation. CR is expressed as a number between 0 and 100.

Keep in mind, small U-factor numbers equal BIG energy savings. Also, the NFRC is currently working on a more user-friendly label that will show two numbers—one for heating and one for cooling. Be sure to utilize this important NFRC certification label when shopping for your new windows.

[1] EERE, “Building Technologies Program,” n.d., < http://www1.eere.energy.gov/buildings/residential/windows.html> (07 August 2008)