Comparing Energy Efficiency of Windows
One outcome of federal stimulus initiatives has been a higher standard of energy efficiency for windows. This is good news for consumers looking to replace older windows for improved energy efficiency in their homes. The new standards exceed earlier Energy Star ratings. Energy Star is an international standard for energy efficient products.
Prior to stimulus initiatives:
The 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 explained 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.” Window World is the country’s largest replacement window franchise, though each store is owned and operated individually.
In addition, Energy Star regulations required window glass to have a .35 or lower “solar-heat coefficient.” Bushy explained 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 imbedded (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 a metallic content that is made up of tin oxide (or a “hard coat”).
After the stimulus initiates:
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 regards to lower solar heat gain coefficients, Bushy said, “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. The NFRC, which is an independent organization, is the National Fenestration Rating Council. (Fenestration is Latin for window and generally means an opening in a building). There are a few other things to consider when comparing energy efficiency in windows. The “U-factor,” which is a rating of how much heat escapes through the glass, as well as the air leakage, and condensation rating. 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 regards to 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,” said Bushy. “It’s crucial.” Bushy suggested soffit and ridge vents to 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,” he said.
What about the materials that encase the window glass?
First of all, it is just common sense that the window materials need to be as tight as possible. Second, there is a range of quality regarding the type of vinyl used in replacement window casings. “You want premium quality material that won’t self-destruct,” said Bushy. While that also sounds like common sense, Bushy explained that some vinyl is a composite of recycled material mixed with “virgin” vinyl. In many cases recycled products sound like a good thing. However, Bushy said that “vinyl retains molecular memory. Once it’s made into something it always tries to go back to its original shape,” he said. Bushy said his company and other reputable companies use all virgin vinyl so that it has 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.