Comparing Energy Efficiency of Windows

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Oct 10, 2010 | Cris Carl

Looking to replace your old windows? Here's good news. Energy Star has instituted a higher standard of energy efficiency for windows. This means less heat transfer via your windows and increased energy savings in your home.

The old standard

The former standards of Energy Star called for windows to have at least a .35 “E rating.” An E rating is the quality of emissivity, or the efficiency with which a material radiates infrared energy. Robert Bushy, owner of Window World in Agawam, MA explains emissivity more simply. “Low E glass reflects wide-band heat waves back to the source, allowing mainly the narrow-band light, such as ultraviolet light, through.”

In addition, old Energy Star regulations required window glass to have a .35 or lower “solar-heat coefficient.” Bushy clarifies that the solar heat gain coefficient means how much heat is kept out, usually by the inclusion of silver or other metal particles in the glass composite. Preferably, you want a glass composite with silver embedded (also referred to as a “soft coat”). If you’ve ever noticed windows with a sheen that looks a bit like oil on water, what you are seeing is metallic content made up of tin oxide (or a “hard coat”).

The new standard

The lower the “E” number or solar heat gain coefficient number, the more energy efficient the glass is. Now, the standard is .30 or below for both emissivity and solar heat gain coefficient. However, in regard to lower solar heat gain coefficients, Bushy mentions, “It’s really better for southern climates. It does a better job of keeping heat out, which doesn’t matter so much here in New England.”

What to look for in terms of energy ratings:

While it can get pretty technical, the simplest way to have a good idea about what the energy rating of your new windows is to look for the NFRC label on the product. NFRC, stands for the National Fenestration Rating Council, an independent organization. (Fenestration comes from the Latin "fenestra" for window). There are several other things to consider when comparing energy efficiency in windows: the “U-factor” (a measure of how much heat escapes through the glass), as well as the air leakage, and condensation ratings. You always want to have the lowest numbers in all areas of consideration.

NFRC adopted new energy performance labels in 2005. The labels list manufacturer, product description, various performance ratings, and sources for additional information.

Balancing energy efficiency of windows with ventilation:

You might have the best windows in the world in terms of energy efficiency, but if you don’t have proper ventilation in your home, you may be creating a different problem – mold and mildew. “If you are going to tighten up your house, you need enough ventilation for the moisture to get out,” warns Bushy. “It’s crucial.” The solution? Hire a roofer to install  soffit and ridge vents, which will counteract the problem of too much moisture in the energy-efficient home. “There is always moisture being created in the home – showers, laundry, even breathing."

What about the materials that encase the window glass?

First, it's just common sense that the window materials need to be as tight as possible. Second, there is a range of quality for the vinyl used in replacement window casings. “You want premium quality material that won’t self-destruct,” according to Bushy.

While that also sounds like common sense, Bushy points out that some vinyl is a composite of recycled material mixed with “virgin” vinyl. In many cases, recycled products sound like a good thing. However, “vinyl retains molecular memory. Once it’s made into something it always tries to go back to its original shape.” That's why reputable window companies use all virgin vinyl so that it will have the longest possible life. Virgin vinyl also has better resistance to heat, cold, and moisture and will, with proper installation, create a tighter fit.

If you select good-quality, well installed windows, all you will have to do for years to come is wash them.

Updated July 23, 2018.

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