Traditionally, windows have been the weakest energy efficiency link in a building envelope, and early single-pane openings were the most egregious offenders. According to www.efficientwindows.org, single-glazed windows with clear glass allow “the highest transfer of energy (i.e., heat loss or heat gain depending on local climate conditions) while permitting the highest daylight transmission.” No wonder such windows are practically extinct in residential architecture.
The standard today is a double-glazed low-E window with insulation between the panes. A vast improvement over a single pane, insulated windows are better at preventing heat loss and heat gain, keeping the internal temperature of a house relatively stable. Depending on your region of the country, such a window—if it’s Energy Star rated—has a U-factor (the rate of heat transfer and an indication of how well the window insulates) of 0.30 to 0.60 and a solar heat gain coefficient (which indicates how well the window blocks heat caused by sunlight) of 0.27 to 0.40. Still, the best double-pane window is inadequate compared to the exterior wall.
In recent years, a new breed of windows—the so-called ultra high-efficiency products—has been gaining traction and things have gotten quite interesting. The windows are usually twice as efficient as double-pane units and some narrow the performance gap between the opening and the wall on which they’re installed.
Earlier this year, Ply Gem Windows in Cary, N.C., unveiled the R-5 Series, the company’s version of a high-efficiency window. “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 time of 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.” Designed for new construction and replacement projects, the windows can achieve a U-factor as low as 0.15.
Ply Gem isn’t the only company producing ultra high-efficiency products. Other manufacturers include Bayport, Minn.–based Andersen Windows and Doors; Klamath Falls, Ore.–based Jeld-Wen; Gorell Enterprises in Indiana, Pa.; MI Doors and Windows in Gratz, Pa.; Sunnyvale, Calif.–based Serious Energy; and Marvin Windows and Doors in Warroad, Minn., among many others.
Some critics say R-5 windows are still behind the times, especially because the technology is available to produce a window with a much higher performance rating. Serious Energy, for example, has taken the industry to task for not making more far-reaching advancements in energy performance and for being content with traditional low-E products. “Dual-pane windows were invented back in 1865. So in 1870, they were truly best of class,” Serious Energy’s president and CEO Kevin 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 R-5, but he believes R-7 should (and will) be the standard. The company, for its part, produces some of the highest-performing products on the market, including fiberglass and vinyl products that range anywhere from R-5.9 to R-11.1.
R-5 windows are the lowest-performing product that Intus Windows offers. Managing director Aurimas Sabulis says the problem with most window companies is that their products aren’t versatile enough to be effective throughout the United States. Intus, he says, specializes in high-performance windows that cater to any region of the country. “One solution does not fit all,” he notes. “The U.S. has seven different climate zones.”
Intus, a Lithuania-based manufacturer that recently set up a U.S. division in Washington, D.C., offers all-wood and aluminum-clad wood windows with R-values up to about 10. The company also manufactures Passive House–certified products, which is why students from Parsons The New School for Design; the Milano School of International Affairs, Management and Urban Policy at The New School; and the Stevens Institute of Technology selected the windows for their Empowerhouse project, an entry in the 2011 Solar Decathlon.
“The Intus windows give us an overall annual energy gain,” says Laura Briggs, faculty lead and chairwoman of sustainable architecture at Parsons. “Our team carefully analyzed the size and placement of the windows in order to take full advantage of solar gain and daylighting to improve comfort and meet Passive House standards.” Briggs adds that the right sizing of the windows also was an economic choice. “The Intus windows are beautiful wood frames that are meticulously designed, specifically to reduce thermal bridging by having few conductive breaks in the frame,” she explains. “They prevent air infiltration due to the fully gasketed sashes and the triple panel gives us the U-values we wanted to attain.”
The DOE states that there is no specific definition for “highly insulating” windows, but the agency says the term usually refers to windows with a U-factor of around 0.2 or less for fixed units (venting units must meet 0.22). “Typically these are triple-pane windows with advanced features such as gas fills, suspended films, advanced spacers, and low-E coatings,” according to the DOE. “A U-factor measures a window’s insulating abilities; the lower the U-factor, the less heat loss through the window.” The windows’ good U-value ratings make them ideal for cold climates when you want better insulation and resistance to heat flow, but some architects also use the products in warm climates.
Architect Eric Lewis, AIA, LEED AP, used the windows on a Baltimore row house when he wanted to maintain views with large glass openings and high performance. “Because of the orientation of [the north side of the home], we used triple-insulated windows so we would still get our R-value and plenty of views and openness,” says Lewis, a senior associate at Alexander Design Studio in Ellicott City, Md.
Despite the rave reviews from architects and recommendations from the DOE, ultra high-efficiency windows do have some drawbacks. Because many high-efficiency products use triple-pane construction, products can be thicker and heavier than traditional units so you’ll have to resolve your window details carefully. But cost might be the most significant obstacle: products are more expensive to produce and, as a result, costly to buy. Intus Windows’ Sabulis says his company, at least, has solved that problem.
“We have figured out how to do triple-pane windows for the lowest possible cost,” he says. Without giving away any secrets, Intus claims it offers products at 20 percent to 30 percent less than other manufacturers, which means a typical 4-foot-by-2-foot window will cost roughly $300 to $400.
Still, high-efficiency windows are expensive—costing anywhere from $70 per square foot and up to $150 per square foot for some brands—which is the main reason the DOE launched the High Performance Windows 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 windows products at attractive prices, thus making highly insulating windows more affordable,” the agency says.