Passive, super-sealed home resists subzero temps

In December, Nancy Schultz's lake home in Isabella, Minn., lost heat for 10 cloudy days when outside temperatures dipped well below zero. How bad did it get inside? The thermostat held at 51 degrees Fahrenheit.

"It's like living in a thermos," says architect Schultz, who designed the super-insulated, airtight home in the middle of Superior National Forest, which has some of the coldest weather in North America.

"It's a glass thermos," adds her husband John Eckfeldt. He says the thick exterior walls and German windows keep the inside temperature so even that when he looks outside and sees it's snowing, he reminds himself: "It must be cold!"

The couple broke ground on their super-green project -- chosen as "This Week's Green House" -- in spring 2007 and completed most of the work last fall. In March, it earned the top or platinum rating from the private U.S. Green Building Council's LEED (Leadership in Energy and Environmental Design) program.

In October, the three-bedroom home further distinguished itself by receiving certification from the Passive House Institute, a green rating system begun in Germany that requires homes use up to 90% less energy than standard houses.

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The Illinois-based Institute has certified only about a dozen U.S. homes, although many more are in the works. (I'm looking into whether my house, designed to be LEED platinum, might qualify for certification.) Thousands of such homes have been built in Europe.

The key is the home's exterior or envelope, which has to be very well sealed and insulated. The institute requires air infiltration of less than 0.6 air changes per hour at 50 pascals, a standard about 10 times as rigorous as the one set by the U.S. government's Energy Star program.

To obtain that, Schultz used triple-paned Optiwin windows (insulating values between R11 and R14), a heavy timber-framed roof (R90) and walls (R50) of insulated concrete forms and staggered double wall stud framing. The home got an amazing HERS (home energy rating) score of 3, on a 0-100 scale, meaning it's 97% more efficient than a regular code-built home.

Schultz, 54, and Eckfeldt, 64, added rooftop solar panels that, combined with an experimental solar thermal storage system, provide all their home's energy. Because the home is so airtight, it has mechanical ventilation.

All this didn't come cheap. Schultz estimates they spent about $450 per square foot. The house has 2,134 square feet of interior conditioned (heated) space. It also has a garage.

"We could have made it a lot simpler," says Schultz, noting the curved windows added cost and made energy modeling more difficult. You can read more about the home on their "Isabella Eco Home" blog.

She and her husband describe themselves as serious environmentalists and say it was important for them to build a home with the lowest carbon footprint possible.

"It's part of our values system," says Schultz, who runs her sustainability management firm largely from their new home. Eckfeldt, a pathologist at the University of Minnesota, spends part of his time teaching in the Twin Cities, about 220 miles to the south.

They started going to Isabella for cross-country skiing and bought a condo there, which they used often while monitoring their home's construction on a nearby 4-acre lot. They have several neighbors, but the nearest grocery store is 25 miles away, and the Canadian border is 20 miles.

Schultz says, "This is definitely wilderness."