What do those numbers on the manufacturer's stickers in new windows mean?
Wednesday, October 17, 2018
They are energy performance ratings. Here’s a typical sticker you would see on a new window, with the five performance rating numbers. Letters in blue have been added so that you can reference each rating in sequence in the explanation that follows. Window ratings are certified by the National Fenestration Rating Council (NFRC). The first two performance ratings, U-Factor and Solar Heat Gain Coefficient, are the most important.
- The U-Factor is a measure of the insulating ability of the window: how well it prevents heat from entering or escaping through the window. U-Factor ratings generally fall between 0.20 and 1.20. The lower the U-Factor, the better the window is at insulating. To convert a U-Factor to an R-Value (the number used to rate the insulating ability of other building materials), divide 1 by the U-Factor. So, a window with a U-Factor of 0.25 would have an R-Value of 4 (1 divided by 0.25 = 4). A low U-Factor is more important in cold climates, where it is important to keep the heat from escaping .
- Solar Heat Gain Coefficient (SHGC) measures how well a window blocks the transmission of heat from the sun through the window. SHGC is expressed as a number between 0 and 1. The lower the SHGC, the better the product is at blocking unwanted heat gain, which is particularly important in hotter, Southern climates like Gainesville where the homes are air conditioned part of the year.
- Visible Transmittance (VT) measures how much light comes through the window, and is expressed as a number between 0 and 1. The higher the VT, the higher the percentage of sunlight that penetrates the window for better daylighting.
- Air Leakage (AL) measure how much outside air comes into a home through the window. AL rates usually fall between 0.1 and 0.3. The lower the AL, the better the window is at keeping air out. It is an optional rating, and not all manufacturers choose to include it on their labels.
- Condensation Resistance (CR) measures how well an window resists the formation of condensation. It is expressed as a number between 1 and 100, with higher numbers indicating better resistance to condensation on the window.
For more details about the rating system, visit the NFRC website at: http://www.nfrc.org/. Also, see our blog post What are the small slots at the bottom of the outside of my window?
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To learn more about doors and windows, see these other blog posts:
Sample label above from - National Fenestration Rating Council
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