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What you need to know about energy ratings for windows

April 9, 2014

The windows in your home bring the beauty of your surroundings inside, but they might also be letting your energy dollars slip out. About 10% to 25% of your heating costs could be passing through your windows. In warm weather, your air conditioner is using more energy to chill the hot air streaming through your windows. Maybe you’ve looked into energy-efficient windows and wondered if the expense—yes, it’s not cheap—is going to show a return on investment. An energy-efficient window—one that qualifies as ENERGY STAR-qualified—can range from $120 for a 36” x 72” window to about $1,000, or more. Before you decide to commit to energy-efficient windows, here are some features and terms you should know:
  • Glazing is not a coating, but the number of layers of glass. Having more layers doesn’t necessarily mean more energy savings.
  • Low-E is an abbreviation for “low emissivity”, and refers to a coating that helps the window to absorb or reflect heat. This can be added later, applied as a film to existing windows.
  • Gas fill improves insulation by sandwiching argon or krypton gas between the layers (glazing), which adds to the insulation and heat transfer efficiency.
  • Spacers are separate sheets of glass that are used to increase a window’s insulation.
  • U-factor is the value of a window’s ability to conduct non-solar heat flow. A low U-factor means higher efficiency.
  • Air leakage measures how air moves around a window. A good, tight window will have a low air leakage rating.
When looking for energy-efficient windows, you need to pay attention to the frame materials as well as the glass inside. Vinyl is a popular choice because it’s a low-maintenance material that offers good insulation, but vinyl also reacts to temperature changes—contracting in cold and expanding in heat—which can allow leakage. Wood looks beautiful, but reacts much the same way as vinyl, and requires more maintenance. Fiberglass is more stable than vinyl and low-maintenance, but fiberglass windows will drive up your cost. Finally, aluminum is an option, but remember that is conducts heat, which could absorb energy savings. Talk to your builder about your choices, and take into consideration where your house is, because certain features will work better, depending on the climate. Also, check out your federal tax credit for installing ENERGY STAR-qualified windows.

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