Three labels can help you evaluate which window to buy.
1. The National Fenestration Rating Council (NFRC) label isn’t required on all new windows, but it’s valuable for determining which ones are best for your conditions. Through independent testing, the council rates windows for the factors below. Results are available at http://CPD.NFRC.org/pubsearch/psMain.asp.
• U-Factor: Measures how well the window prevents heat from escaping. The lower the number, the better the window insulates.
• Solar Heat Gain Coefficient: Measures how well the window blocks the sun. The lower the number, the less heat.
• Visible Transmittance: Measures how much light comes through. The higher the number, the more light.
• Air Leakage*: Indicated by the cubic feet of air passing through a square foot of window (cfm/sq ft). The lower the number, the less air will pass through cracks in the assembly.
• Condensation Resistance*: Measures resistance to condensation formation on the interior. The higher the number, the better the resistance.
* Not always included on NFRC label.
2. Energy Star, a U.S. government-sponsored energy-efficiency initiative, places its logo on NFRC-rated windows and doors that have optimal U-Factors and Solar Heat Gain Coefficients for each climate zone.
3. The Window Selection Tool at www.EfficientWindows.org can help you compare energy costs and savings between window types. It takes into consideration the regional location of the house, average public utility rates, window frame, Energy Star certification, and whether installation is in new construction or retrofitting. It also provides state-by- state fact sheets with recommended NFRC ratings for each region.