Energy Efficient Windows
September, 2008
The energy efficiency of your windows can have a significant impact on your heating and cooling bills, as well as the environment. To find the right windows you must balance budget considerations and climate considerations with the energy performance of the windows.
What Makes a Window Energy Efficient
Manufacturers use a number of advanced technologies to make windows more energy efficient.
Improved Frame Materials. Frames are made of vinyl, wood composite and fiberglass to improve insulation and reduce heat transfer.
Low-E Glass. Many windows are coated to reflect infrared light. This helps keep heat in during the winter and out during the summer. These coatings also reflect damaging ultraviolet rays, keeping furniture, drapes and flooring from fading.
Multiple Panes and Gas Fills. Two or more panes of glass with air or gas between insulate much better than a single pane of glass. The space between panes is frequently filled with odorless, colorless, non-toxic gas that insulates better than air. Multiple panes also protect against impacts and provide sound insulation.
Warm Edge Spacers. Spacers keep the panes of glass the correct distance apart. Spacers are made of steel, foam, fiberglass or vinyl and work to reduce heat flow and prevent condensation.
Window Operation. Window design can affect the energy efficiency of your windows. Some operating types have lower air leakage rates than others, which will improve your home's energy efficiency.
Energy Performance Rating
Windows lose heat in three ways and each of these has an energy performance rating factor.
- Direct conduction of non-solar heat through the glass or frame. This rating is called the U-factor. When comparing products, make sure the U-factor is base on both the glass and framing, not just the glass. A lower U-factor is better.
- Radiation of heat into and out of the house, primarily from the sun. This is called the solar heat gain coefficient (SHGC). The lower the SHGC, the less solar heat the window transmits and the greater its shading ability. A window with a high SHGC rating is more effective at solar heat gain during the winter. A window with a low SHGC rating is more effective at blocking heat gained from the sun. Therefore, what SHGC you need for a window should be determined by such factors as your climate, orientation, and external shading.
- Air Leakage. A window with a low air leakage rating is tighter than a window with a high air leakage rating.
The National Fenestration Rating Council (NFRC) is a non-profit organization that operates a voluntary program that tests, certifies, and labels windows, doors, and skylights based on their energy performance ratings. You can use the NFRC label to determine a window's energy properties and to compare various windows.
Proper Installation
Make sure to hire a qualified contractor to insure the windows are properly installed. Improper installation can void your warranty and/or adversely affect the energy efficiency of your new windows. If scraping of older window sills is required you may want to consider hiring a certified lead abatement contractor to do the work. To learn more about proper window installation, visit the EERE web site.
ENERGY STAR® Rating
ENERGY STAR® is a joint program of the U.S. Environmental Protection Agency and the U.S. Department of Energy designed to help consumers identify energy efficient products. ENERGY STARŪ labeled windows are twice as energy efficient as the average window produced just ten years ago. If energy efficiency and environmental impact are important to you, look for windows that are ENERGY STAR® qualified.
More Information
For more information on selecting the right energy efficient windows, visit The Efficient Windows Collaborative or The Energy Efficiency and Renewable Energy web site.
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