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When you step into the University of Illinois' Solar Decathlon house on a chilly Washington, D.C., fall day, you are greeted by comfortably warm surroundings. Then, a student tells you the heat isn't even on; body heat from a steady stream of visitors keeps the house toasty.

The Gable Home, which has 516 square feet of conditioned space, is part of the biannual Solar Decathlon competition, which features 20 student-designed-and-built dwellings on display on the National Mall through Oct. 18.

The unit was built with thermally broken window frames, triple-pane glass, and a combination of rigid and spray-foam insulation that gives the walls a rating of R-50 and the ceiling a rating of R-70. The team performed a blower door test to make sure the house was sealed tight. Because of the efficient envelope, "all the mechanical systems used to condition the house can be compact and consolidated into a single unit," the team states in its brochure.

These characteristics allow the Illinois house to meet the strict standards of the Passive House Institute. It has a heating and cooling need of 15 kWh per square meter per year, 80% less than conventional new building standards, according to the institute.

"We haven't put heat in the house at all," says Mark Taylor, an assistant professor of architecture and a team advisor. "The daily activity of washing and cooking have kept the house above the required temperatures."

One particularly innovative technology works to cool the house in the summer and improves the performance of the water heater. Manufactured by AirGenerate, the AirTap heat pump system uses surrounding air to heat water and produce cold air.

"You can use any normal water tank," explains Mark Adams, a mechanical engineering graduate student. "You put down copper tubing into the water tank, and you put this unit on top. It pulls in the heat from the air and heats the water and blows out cold air."

Students ducted the system to blow into the living room in the summer, and exhaust out of the house in the winter.

In another clever twist, the Gable Home's frame is constructed from Lamboo, laminated bamboo.

"Bamboo can be harvested a lot quicker and used as a renewable resource," Taylor says. "As a structural frame system, we could have a thin profile to try to prevent thermal breaks." The product is 10 times stronger than traditional timber, so instead of a 1 ½-inch stud, ¾ inches of bamboo could be used, Taylor adds. The extra space was filled with insulation.

In addition, 40 thin-film photovoltaic SunPower 225 solar panels integrate with one side of the roof to provide 100% of the Gable Home's energy needs. In fact, the house is producing more energy than it uses, says Britta Monson, a senior architecture major who worked on the house.

However, sustainability was not the sole concern of the team. The Illinois students also wanted to make sure the house was marketable. To do that, they included technology and appliances that are readily available, and they designed the house with the traditional look of a Midwestern barn.

Reclaimed barn siding configured in a simple plank pattern graces the outside of the dwelling. A peaked, standing-seam metal roof completes the structure; solar panels do not require mounts that could distract from the roof's profile.

"If you can present the house in the aesthetic people are comfortable with, and introduce the technology, there's more of a chance of people taking up that technology," says Taylor.

Victoria Markovitz is associate editor for Custom Home magazine.

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