A Clear View On Windows
July 23rd, 2010
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By Pam Blair Windows provide ventilation, light and warmth, but also can drive up your electric bill. Efficient windows can reduce heating, cooling and even lighting costs, while improving overall comfort. According to the American Council for an Energy-Efficient Economy, the best window glazings today insulate almost four times as well as the best commonly available windows 20 years ago. In climates with a significant heating season, windows can be a major source of unwanted heat loss, discomfort and condensation. In climates that mainly require cooling, windows are a major source of unwanted heat gain. High-performance windows help reduce peak heating and cooling loads. That determines the size of the furnace, heat pump, air conditioner and fans that must be installed. Smaller units can cost less. Reducing the peak load also benefits your electric utility, which must have enough power to meet customers’ maximum requirements. According to the Efficient Windows Collaborative, several houses built in Las Vegas with energy-efficient improvements, including windows, allowed the total size of the air conditioning system to be reduced by 30 percent. Measuring Performance Key window energy performance measures include: U-value or U-factor, which measures how well a window prevents heat from escaping. It is similar to the R-value for insulation. The lower the U-value, the better the overall insulating value of the window. Ratings usually range from 0.20 to 1.20. Windows with lower U-factors result in a higher interior window temperature in winter and greater comfort. Proper installation with weatherstripping also reduces cold air leakage. An older, less-efficient window with a lower glass temperature feels colder because more heat is radiated from a person’s body to the window. Cold glass also can create uncomfortable drafts as air next to the window is cooled and drops to the floor. Solar Heat Gain Coefficient (SHGC), which measures how well a window blocks heat. The lower the SHGC, the less heat gain. Values range from 0 to 1. In summer, strong direct sunlight strikes people and interior surfaces, creating overheating and discomfort. Windows with a low SHGC reduce the heat coming through the glass. Air infiltration or air leakage, which is listed in cubic feet of air per minute per foot of window edge. A product with a low air leakage rating is tighter than one with a high air leakage rating. A Buyers' Guide When shopping for new windows, look for the National Fenestration Rating Council (NFRC) label and compare ratings on Energy Star-labeled windows for your climate. Look for windows with double panes; low-emissivity coatings; low-conductivity gas-fill between panes; and wood, vinyl or fiberglass frames. Windows manufactured with low-e coatings reduce energy loss by as much as 50 percent, according to the U.S. Department of Energy. They also reduce the ultraviolet rays that filter through the window by up to 75 percent. That can reduce fading of materials such as carpet, fabrics, paper, artwork and wood. A special type of low-emissivity coating is spectrally selective, filtering out up to 70 percent of the heat normally transmitted through insulated window glass or glazing. Both allow the full amount of light to be transmitted. Windows with warm edge technology and insulating frames have such a warm interior glass surface that condensation on interior surfaces is significantly reduced. |
To maximize energy performance, choose windows with larger unbroken glazing areas instead of multi-pane or divided-light windows. If you want to simulate the multi-pane look, use applied grills. They do not reduce energy efficiency. Choose windows with good warranties against the loss of the air seal. If the glazing seal is lost, window fogging will occur and any low-conductivity gas between the layers of glass will be lost. To ensure your new windows perform as well as they should, hire a skilled contractor to install them. n For information about the benefits of energy-efficient windows, descriptions of how they work, selection recommendations and links to rebate information, visit the Efficient Windows Collaborative at www.efficientwindows.org.
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