Only the most efficient windows, doors, and skylights on the market will qualify, and it's been quite a shake-up to the industry, says Jim Benney, the executive director of the National Fenestration Rating Council, which rates and labels windows and doors for efficiency. "The industry was surprised by the bill," says Benney. "The public needs to make sure they are getting products that actually meet these requirements."
Windows and doors must have a so-called U-factor below .30 and a Solar Heat Gain Coefficient (SHGC) less than .30 to meet the requirements for a tax credit. The U-factor measures the rate at which heat leaks from a window--the lower, the better. The SHGC measures how well the window blocks heat caused by sunlight.
Benney said the standard product lines of most manufacturers, even many with an Energy Star rating, don't meet this stringent standard. But there are qualified products out there. Andersen Windows is highlighting a few dozen on its Web site.
For even more choices, the NFRC has compiled an extensive list of qualifying windows, which you can download here. You should also check the U-factor and SHGC on the NFRC label on Energy Star--qualified windows and doors you're considering buying.