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Replacement Window Ratings Explained

Posted on November 29, 2019

Aledo, IL Energy Efficient Window Replacement

If you've been searching for windows, you've probably heard the phrase "energy efficient windows" several times already. In fact, your reason for searching for new windows may be a need for more energy efficiency in your home. As window technology has advanced, more and more energy efficient windows are available, as well as ways to rate energy efficient windows.

Chances are you've seen an NFRC (National Fenestration Rating Council) sticker on a window or two, along with several ratings listed below. The NFRC was founded to rate the energy efficiency of any windows on the market. Typically, you'll find four categories: U-Factor, Solar Heat Gain Coefficient, Visible Transmittance, and Air Leakage. What do these ratings mean for your window? Let's break them down.


U-Factor measures the window's ability to keep heat in, so it comes especially handy during the cooler months of the year. U-Factor is rated on a scale between .20 and 1.20. The lower the number, the better the window's energy efficiency. While those in warmer climates might not worry so much about U-Factor, those in the Midwest such as Illinois can appreciate a window with a low U-Factor. A window is typically considered energy efficient when the U-Factor is .30 or lower. Double pane windows with non-toxic gas in between panes are often used to keep a low U-Factor

Solar Heat Gain Coefficient

Meanwhile, Solar Heat Gain Coefficient (SHGC) is used to rate the amount of heat that the window blocks. SHGC is rated on a scale between 0 and 1, with lower numbers again being the most desirable. The lower the SHGC rating, the better the window blocks solar heat gain. Typically, SHGC ratings are lower than .8. Some homeowners use low-E coatings to keep SHGC low, or again, double pane windows.

Visible Transmittance

Have you considered using your windows to "daylight" your home, so that you can rely less on electric lighting and instead use natural sunlight from your windows? Then you'll want to pay attention to your window's Visible Transmittance (VT) rating. This rating is a scale between 0 and 1, and in this case, higher numbers are better. The higher your VT rating, the more light the window lets through, allowing for better daylighting. While low-E window coatings can keep down SHGC ratings, the downside can be that less light is allowed through. However, some low-E coatings let in more sunlight while blocking harmful UV rays than others.

Air Leakage

Air Leakage has to do with how much air is allowed to pass through (or leak) from one side of your window to the other. This is rated on a scale between .1 and .3. This is an optional rating that might not even be included on your NFRC label, but it can be important to know. The higher your AL rating, the more air the window lets through, causing the feeling of drafts in your home. Argon or krypton gas in between multiple panes of glass can help to keep air leakage to a minimum. 

Want to learn more about what different energy efficiency criteria actually means for your windows? Contact Mainstream Home Improvements today for more information or a free estimate.

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