High-Performance Windows

Modern choices and high-performance windows can save you money on heating and cooling your home.

| December 2005/January 2006

Most people choose new windows based on appearance, style, convenience and price. Energy efficiency might not be a consideration, but this important factor actually should take top priority. Although high-performance windows initially cost more, they provide greater comfort, increase the value of your home and save you money in the long run. Heating and cooling expenses account for 44 percent of an average home’s energy bill. According to the U.S. Department of Energy (DOE), high-efficiency windows could reduce that bill by up to 25 percent. High-performance windows can even lower the cost of a new home because you may be able to install a smaller, less expensive heating and cooling system.

High-performance windows are available in such a wide array that the average homeowner might not know where to begin. “At last count, there were some 4,000 different window manufacturers in this country, so you have to look carefully,” says Bill Prindle, deputy director of the American Council for an Energy Efficient Economy. Unlike some household products, windows are relatively expensive and fairly permanent, so taking the time to become educated about the many choices will be time well spent.

There are several window rating systems. “They basically give you a benchmark of high efficiency for a given climate,” Prindle says. Energy Star is a government-backed program that distinguishes products that meet strict efficiency guidelines set by the Environmental Protection Agency and the DOE; the Energy Star label will help you find the best high-performance windows. The nonprofit National Fenestration Rating Council (NFRC) provides information for windows and skylights that are similar to fuel economy ratings on new cars. NFRC labels detail independent verification of product performance and allow homeowners, architects and builders to compare windows’ energy ratings. An NFRC rating is required in order to receive an Energy Star certification.

Another excellent source of objective information is the Efficient Windows Collaborative, which provides comprehensive information on the many features to consider when you buy windows. It will help you understand the basics of window energy transfer and how to interpret the manufacturer’s specifications. “It’s fairly nontechnical, and it can be a good resource for people who want to go into more depth than the Energy Star label,” Prindle says. 

Explaining Efficiency

The rating systems help you evaluate three main factors that impact the energy efficiency of windows. The first is heat flow through the glass and frame, which is generated by the temperature difference between the inside of your home and the outside environment. This is often thought of as a winter issue, but it can be just as important in the summer — especially in hot climates. Heat transferred through a window is expressed with U-values; the lower the U-value, the better. The resistance to that heat flow is expressed as an R-value (the mathematical inverse of U). In general, the higher the R-value, the better the insulation.

Multiple panes of glass are the best way to increase R-values because they increase the number of barriers between the outdoors and the interior of your home. More than two panes are rarely necessary except in extreme climates. Microscopically thin layers of clear metal oxides applied to the glass during production, called “low-emissivity” or “low-e” coatings, further help to reduce radiant heat transfer between panes of glass by reflecting some or all of the infrared radiation and heat flow exchanged between glazing layers. These coatings stop the heat, but let most of the sunshine through. Conversely, low-e coatings also keep your home’s warmth inside during the winter.

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