Many modern windows offer features like double or triple-glazing, air pockets and special seals to maximize energy efficiency. According to Energy Savers, these windows can reduce the average homeowner's annual heating and cooling costs by 10 to 25 percent. In addition to lowering utility bills, energy efficient windows reduce cold drafts and improve the overall comfort of residents.
Buyers can compare the overall energy efficiency of windows using a rating called the "U-factor." U-factor is a measure of the amount of heat that moves through the window every hour, and the lower the rating, the more insulation the window offers. According to Energy Savers, homeowners who live in Northern climate zones should look for a U-factor of 0.3 or lower, while those in the Northern Central region should choose a U-factor of 0.32 or lower. Buyers in the South Central climate zones require a U-factor of 0.35 or lower, while those in the warmest Southern regions should choose windows with a U-factor of 0.6 or lower.
Some window ratings are given in terms of both U-factor and R-value. Many consumers are more familiar with R-value, as it is used to measure the efficiency and insulating properties of walls, flooring and other building materials. R-values are essentially the opposite of U-factors, so the higher the R-factor, the more energy efficient the window will be. According to Consumer Reports, windows generally have a maximum R-value between two or three. Homeowners in colder climates should choose the highest possible R-value to maximize efficiency.
Solar Heat Gain Coefficient
Solar heat gain coefficient, or SHGC measures the amount of solar energy that passes through the window. The higher the SHGC, the more heat can enter the home through the window. According to the American Council for an Energy Efficient Economy, or ACEEE, buyers in cold climate zones should look for windows with a SHGC rating of 0.7 or higher. Homeowners in sunny, hot climate regions should choose windows with the lowest possible SHGC rating to help keep excess solar energy out of the home.
Air Leakage Rating
Air leakage rating measures the amount of air that leaks out of the home around the edges of a window. To maximize energy efficiency, look for windows with the lowest possible air leakage rating. According to ACEEE, the best-rated windows have an air leakage rating of 0.01 to 0.06 cfm/ft.
The National Fenestration Rating Council performs independent testing and certification of windows to check for energy efficiency and performance. Consumers can consult the Council's website to find current brands and models of windows with the highest possible energy efficiency ratings.
Energy Star provides a similar rating and certification process. All windows must first undergo testing and certification from the National Fenestration Rating Council before earning the Energy Star seal.
Products that display labels or seals from either of these two organizations can be expected to offer high performance and some of the best energy efficiency ratings in the industry.