The Basics of Window Energy Ratings

by Bryn HuntpalmerSeptember 26, 2016

If you’ve never purchased a window for your home before, you may think that they’re all the same. But the energy efficiency of the glass, frame, insulation, and glazing can vastly affect the interior temperatures of your home. That’s why the window ratings system was designed.

But not every region has the same requirements. Climate plays a big part in dictating the minimums required to achieve energy efficiency. It makes sense when you think about it: the ideal windows for hotter areas have different design features than those that do well in colder spots, and vice versa. To categorize those differences, the National Fenestration Ratings Council created several measurements, including solar heat gain coefficient, U-factor, and air leakage. Understanding these ratings will help you determine just how energy efficient a window really is, and whether or not it’s right for your area.
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ENERGY STAR Certifications and NFRC Ratings

The Department of Energy created the ENERGY STAR Certification program to help consumers understand the efficiency of appliances, building materials, and other items that affect their home’s energy performance. Windows that are ENERGY STAR compliant bear that certification on their packaging and brochures.

However, the certification status relies on several ratings given to the window by the National Fenestration Ratings Council, or NFRC. The council tests various products and assigns them a rating based on several different factors. These include the amount of solar heat the window allows through the glass, the amount of heat the window allows to escape to the exterior, and the measure of how drafty the window is.

To achieve ENERGY STAR certification, the window must meet the minimum regional requirements – see if you meet the program requirements in your area by visiting the ENERGY STAR website.

Solar Heat Gain Coefficient

A window’s solar heat gain coefficient, also known as the SHGC, indicates how much solar heat the window glazing allows into your home’s interiors. That makes it a particularly important measurement for homes in southern climates, where high solar heat gain can drive up air conditioning expenses. The lower the SHGC, the more effective the window is at blocking solar heat. In fact, if you live in the southernmost regions of the US, you’ll want to seek out a window with an SHGC of 0.25 or less.

Don’t make the mistake of thinking that every home needs a very low SHGC, however. In colder climates, a higher measurement can raise interior temperatures, making those windows helpful to offset heating costs. Keep in mind that a window must still rate below 0.40 to achieve ENERGY STAR certification, even in these colder areas.

U-Factor

On the other hand, a window’s U-factor is the measure of how well the window insulates your home—more specifically, how much heat is transmitted through the glass and frame. That makes it a concern for homes across the US, but particularly those in the north. Here, lower U-factor ratings can be used to offset heating costs in homes. However, in this area, they should be combined with compatible SHGC ratings, so the home can achieve the qualifying equivalent energy performance. A table with those compatible ratings can also be found on the ENERGY STAR site.

The opposite is true for homes located in southern regions—that is, a higher U-factor can be used to create air flow in the home that can help reduce cooling costs. To maintain good efficiency, however, the window’s U-factor should measure no higher than 0.40.

Air Leakage

SHGC and U-factor have to do with heat loss and gain. However, there’s also a separate NFRC rating that indicates the amount of outside air a window allows into the home’s interior (think drafts). That measurement is known as the window’s air leakage, or AL. To achieve the ENERGY STAR rating, a window’s AL should rate no higher than 0.30 cubic feet per minute. That’s the same requirement across all regions of the country.

Understanding how windows affect the temperature of your interiors can make a real difference in your home energy costs—and help you select the best windows for your climate and savings goals.


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About The Author
Bryn Huntpalmer
Bryn Huntpalmer is a mother of two young children living in Austin, Texas where she currently works as an editor for Modernize. In addition to regularly contributing to Home Remodeling and Design websites around the web, her writing can be found on Lifehacker and About.com.

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