Energy-efficient windows got a big boost when President Obama signed the American Recovery and Reinvestment Act of 2009, the sweeping stimulus legislation giving homeowners a 30 percent tax credit for qualified energy-efficient home improvement projects such as replacement windows with a .30/.30 U-factor and solar heat gain coefficiency (SHGC).

But these performance numbers pale in comparison to what window manufacturers are actually capable of doing. Enter the ultra high-efficiency window.

Depending on the region of the country, an Energy Star window must have a U-factor of .30 to .60 and a SHGC of .27 to .40. Such windows are made with dual-pane glass and low-E coatings. But manufacturers are now producing windows that go way beyond those numbers. Earlier this year, for example, Ply Gem Windows in Cary, N.C., unveiled a new R-5 Series of windows for new construction and replacement projects.

“Windows with an R-value of 5 are some of the most energy-efficient windows commercially available in the U.S.,” the company said at the launch. “As a comparison, Energy Star–rated windows typically hold an R-value of 3. By increasing the R-value from 3 to 5, average heat loss through the window is reduced by 30 percent to 40 percent.” The new windows, in some cases, have U-factors as low as .15, the company said.

Ply Gem isn’t the only company producing high-efficiency windows. Bayport, Minn.–based Andersen Windows and Doors, Klamath Falls, Ore.–based JELD-WEN, Gorell Enterprises in Indiana, Pa., MI Windows and Doors in Gratz, Pa., and Sunnyvale, Calif.–based Serious Energy are just some of the other manufacturers that offer them and participate in the Department of Energy’s (DOE) High Performance Windows Volume Purchase Program for new and retrofit construction in residential and commercial buildings. To qualify, manufacturers’ windows must achieve a maximum .22 U-value for operable units and .20 U-factor for fixed openings.

But for some manufacturers, even R-5 windows are just the beginning. German brand Unilux Windows and Doors specializes in high-end, triple-glazed windows with tremendously high R-values. And Intus Windows, whose North American headquarters is in Washington, manufactures Passive House Institute–certified windows that achieve R-values close to 10.

One of the most high-profile U.S. companies offering ultra high-efficiency production windows is Serious Energy. The company offers windows that reach up to R-11 (for some styles). Company president and CEO Kevin Surace has taken the industry to task for not making more advancements in window energy-efficiency performance. “Dual-pane windows were invented back in 1865. So in 1870, they were truly best of class,” Surace has said publicly. “But I am thinking, ‘140 years later, and we call that energy efficient?’” Surace says all windows should perform no lower than an R-5 and believes an R-7 standard will soon be the base level in the marketplace.

The road toward ultra high-efficiency windows, however, is paved with obstacles—namely, high costs. The benefits are huge, but because high-efficiency windows require more panes, coatings, and gas inserts, the products are more expensive to produce. This is one reason the DOE launched the Volume Purchase Program.

“The goal of the program is to expand the market for highly insulating windows and low-E storm windows by reducing market barriers and offering window products at attractive prices, thus making them more affordable [for everyone],” the agency says.

But only time will tell if this effort is effective.