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Purchasing Energy-Efficient Windows

October 13, 2008 - 11:29am

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Windows connect us with the "great outdoors." They let in the light and the rays of the sun and can make even the smallest room seem bright and spacious. Operable windows let fresh air in and stale air out. Windows that are properly selected, well designed and constructed, and properly installed can make a world of difference to a home, helping it to be warm and cozy in the winter, and cool and comfortable in the summer.

Yet windows have traditionally been the weak spot in the home's building envelope—that part of the house connected to the outdoors. They can be one of the leading sources of drafts, heat loss (or unwanted heat gain in summer), and condensation. Windows can account for 10%–30% of your home's heat loss in winter.

Some windows are best suited for cold climates, while others are more practical in warm, sunny climates. Orientation also may be a factor in window selection. The best window for the south-facing wall is not always the best window for the north-facing wall. East- and west-facing windows pose still different problems and may require still different windows. (Be forewarned, however, that different windows containing low-e coatings may have different appearances, so be careful in selecting different windows in view of each other.) Thus, it pays to spend some time to understand window technology and what's on the market, or choose a professional who does. This applies for whether you're selecting windows for a new home under design or for an addition to your home, or picking out replacement windows.

You're familiar with the ENERGY STAR® label. Hopefully, you insist on ENERGY STAR-qualified items when purchasing appliances and electronic equipment. Insist also on ENERGY STAR-qualified windows.

Since the energy efficiency performance of windows can vary by climate, windows are rated for four different climate zones: a mostly heating zone (Northern), two cooling and heating zones (North/Central and South/Central) and a mostly cooling zone (Southern).

The two most important ratings are U-factor and Solar Heat Gain.

U-factor, also known as thermal transmission, is a measure of the rate of heat loss through a product. Therefore, the lower the U-factor, the lower the amount of heat loss. U-factor is the reciprocal of R-value, the measure used for insulation products. (If the U-factor is 0.5, then the R-value is 2, since 1 divided by 0.5 = 2.)

The Solar Heat Gain Coefficient (SHGC) measures the rate of heat gain through a product. Therefore, the higher the SHGC, the higher the heat gain. Generally, a high SHGC is good for cold climates, while a low SHGC is good for hot climates.

Be sure to use the NFRC label when comparing window performance ratings. The National Fenestration Rating Council (NFRC) is a nonprofit organization that administers an independent, uniform rating and labeling system for windows, doors, and skylights. When you compare different windows using the ratings on the NFRC label, you can be sure you're comparing apples with apples. You can also compare windows using the NFRC's online Certified Products Directory.

ENERGY STAR buyers may also want to consider a number of other factors when choosing windows, such as:

  • Visible Light Transmittance
  • Condensation Resistance
  • Air leakage
  • Ultra Violet (UV) light transmittance

Windows are available with many other characteristics as well, including burglar resistance, bullet resistance, and wind-borne debris resistance, a valuable feature in areas prone to hurricanes or tornados.

Selecting the best windows for your home invariably requires tradeoffs between different energy performance features, and with other non-energy features. These performance ratings are described in publications by Lawrence Berkeley National Laboratory (PDF 544 KB), NFRC (PDF 223 KB), and Kansas State University (PDF 101 KB), developed to help homeowners select energy-efficient windows. Download Adobe Reader.

In addition, consult the Efficient Windows Collaborative fact sheet for the location near you. Efficient Windows Collaborative fact sheets provide a step-by-step guide to selecting energy-efficient windows and are available for one or more cities within your state and for Washington, D.C.

Manufacturers use an array of advanced technologies to make ENERGY STAR-qualified windows, including multiple panes of glass or plastic, special coatings consisting of thin metallic films applied to one or more panes, gas filling, warm edge spacer materials separating the panes, and improved frame materials.

Installing high performance windows can result in significant energy and cost savings. Use the Window Selection Tool to compare costs for a typical house by selecting a condition, window type, and a city. The annual energy use from computer simulations for a typical house in your city can be compared for different window options. Some manufacturers even guarantee a certain percentage savings on your electricity or heating bills or they'll pay the bill.

Compare warranties and select windows that come with a good warranty. Then select a qualified installer.

Check out the overview of utility and State programs (PDF 145 KB) that can help you as a resident, building owner, or builder to finance improvements in window energy efficiency. Since programs change constantly, contact your state energy office and local utility for the latest information on any incentives they may offer.

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