New Labels On Windows Indicate Energy Efficiency

Fri., July 26, 1996

Amy Mickelson Washington State Energy Office

If you’ve looked at windows recently, you’ve probably noticed something new. Windows are now carrying labels on them from the National Fenestration Rating Council (NFRC). The labels indicate the relative energy efficiency of the window under standard test conditions. The NFRC is a non-profit association of window manufacturers and other interested parties established to provide a standardized national energy performance rating system.

Windows bearing the NFRC label are tested and certified by recognized independent laboratories. The test results are reported on the labels as a U-value or U-factor.

By comparing the labeled U-values, you can determine which window has the better energy performance. Smaller U-values mean better performance.

NFRC-tested values should always be compared to other NFRC tested values when comparing windows. Other tested U-values and manufacturers’ literature specifying U-values may be misleading. All U-values are not based on the same test standards, and you may be comparing apples to oranges. National promotion of the NFRC standard tries to avoid this problem.

U-values are only one measure of window performance. NFRC labeling has been expanded to include the solar heat gain coefficient, visible light transmittance, and air leakage. The solar heat gain coefficient indicates the amount of available heat energy from the sun that passes through the glass. The higher the value the more heat received from sunlight. Unfortunately, without proper shading, this solar heat gain may also add to cooling costs during the summer.

Windows, especially windows that open and close, can contribute to the air leakage of your building, which in turn can cause additional heat loss. The new NFRC labels now include an air leakage factor to use when comparing the leakage rates of different windows. A larger number indicates greater leakage.

Not all windows let in comparable amounts of light. The NFRC label shows a visible light transmittance rating, which measures the percentage of visible light passing through the window. A value of .70 means that 70 percent of the visible light passes through the window.

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