NDSU students show off super-efficient 'passive house' at State Fair

Passive house
The North Dakota State University student-constructed passive house at the Minnesota State Fair in Falcon Heights, Minn.
MPR Photo/Stephanie Hemphill

A new exhibit at the State Fair this year shows just how efficient modern energy designs can be.

Based on German designs, a "passive house" needs only the equivalent of nine light bulbs to heat it — even in a cold Minnesota winter.

Fourteen architecture students from North Dakota State University are putting the finishing touches on an 800-square-foot cabin. There's nothing space-age-y about the house; it's made from common building materials, but it's very efficiently designed and engineered.

Eventually it will be nestled into Itasca State Park, where faculty members in the University of Minnesota's biology department have research projects.

The cabin has a calm northwoods feel, even in the hectic busyness of the State Fair eco-experience building. The cedar siding is taken from an old granary donated by the farm family of one of the students.

Future architect Peter Atwood is excited to see his ideas turn into reality.

"If we want to start here, at the back, you can see we've exposed all the different layers," he said, showing off the house.

Behind the cedar siding, there are four inches of rigid foam insulation, a layer of plywood sheathing, a double-stud wall filled with a foot of cellulose insulation, and another layer of sheathing.

"On the interior here, behind this double-stud wall, is one more wall which houses all the mechanical, plumbing, electrical, so we don't have to put any of that in our insulation wall," Atwood said.

The big idea is to block any connection between the weather outside and the climate inside. Even the windows are super-insulated — which means three panes of glass, and insulation in the framing designed to separate the inside from the outside.

Most windows are on the south side of the house, with just a few on the other sides.

"All the windows are optimized so the house doesn't overheat but it gets enough heat so it doesn't have to rely on active sources of heating," Atwood said.

That optimization means matching the building design with climate data, in this case climate data recorded at the Bemidji airport.

A free-standing structure in front of the south windows behaves a lot like a well-planted tree. Atwood said it's an important component of the passive heating system.

"They're all designed in specific orientations so that sunlight can come through in the winter and get shaded in the summer," he said.

Inside, there's a living space with a high ceiling, two sleeping lofts, a kitchen and bath.

Even the heating system in this house is mostly passive; it operates using a device called a energy-recovery ventilator.

"It takes all the incoming air that you're bringing into the house, the new fresh air, and it passes it through the air that you're exhausting out of the house, your warm air," he said.

Atwood said it's like a bunch of straws meshed together. The students' teacher, Malini Srivastava, explains that in the winter, the air leaving the house through one set of straws heats up the incoming air in the other set. And in the summer, the heat exchange goes the other way. "These things are so efficient that it's recovered almost 90 percent to 95 percent from the heat of that outgoing air," Srivastava said. "This is what makes it work."

And Srivastava said these straws tie in with another heat exchanger below ground. A couple of coils of plastic tubing under the foundation are filled with glycol, and they bring the constant 40-to-50-degree temperature of the earth up into the house. In the winter, that warms incoming air; in the summer it cools it down.

"That's why we have such a low energy demand, because when you heat that up, you just need to heat it up just a little bit more because it's already passively been tempered," Srivastava said.

Srivastava said in Minnesota it costs about 15 percent more to build a passive house than a standard house. But she said it's designed to last a hundred years.

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