House in a house

“A house inside a house” — that’s how Don and Rose Schwennesen describe their “envelope house” situated a few miles south of Woods Bay.

They bought the old farmhouse and orchard in 1980 because “it was cheap,” Don said. But they decided to make the house over to be their energy-efficient dream home.

“We were excited about retrofitting this thing, and we wanted a greenhouse,” Don said.

The couple built their greenhouse onto the south side of the farmhouse. They installed extra insulation in the walls, and for even more insulating power, they had an interior shell built just inside the external walls. The airspace between the outer and inner shells acts as additional insulation.

“You wrap the whole house in air, like an envelope,” Don said.

The east and west-facing walls are “super-insulated” to protect against the winds that blow up from the lake and down from the mountains, Rose explained.

But insulation is only one piece of what makes the envelope house energy-efficient.

The greenhouse’s large, south-facing windows let in loads of sunshine on fair days, making the space a comfortable 70 degrees or so, even on a below-zero winter day, Don said.

“It’s wonderful,” he said. “It’s gorgeous.”

The roof overhang shades out the sun in the summertime but allows the rays in during the cold months, when the sun rides low in the south. The solar-generated heat rises up to the second floor, which is open to the greenhouse, and circulates through the “plenum,” the airspace between the interior walls and the outer walls and roof. The warm air continues flowing around the north side of the house and down into the basement, driving cool basement air up into the greenhouse to be warmed by the sun.

“It has really changed the dynamics of the house a lot,” Don said.

The circulating warm air keeps the house reasonably warm, but the small wood-burning stove in the living room-kitchen area takes the chill off at night and on cloudy days. The Schwennesens burn only two to two-and-a-half cords of wood a year. They have electric baseboard heaters for back-up, but they don’t need to use it much, Don said.

Even during the hot summer months, the house doesn’t need air conditioning. A contractor who helped them build and insulate advised against putting any windows on the east and west sides of the house, Rose said. “I said, ‘Yeah, but I live here and there will be windows on the ends,’” she said, laughing.

A bay window to the west allows a grand view of Flathead Lake, and the east and west windows turned out to be an essential part of the house’s ventilation system during summer.

“We get upslope breezes in the daytime, but at night that reverses,” Don said. They simply open the windows to take advantage of those breezes.

“That’s been our cooling system forever,” Rose said.

Cool air circulating up from the basement also helps with relieving the summer heat, Don said.

“It’s not something that’s very prevalent, but it worked out well for us,” Rose said of envelope-house technology. “It’s still an old farmhouse, but it’s an efficient one. I think it’s a good way to build.”

“The added cost [of building secondary interior walls] isn’t that much, really,” Don added.

The Schwennesens’ son Daniel framed the first floor ceiling and the stairway, which climbs through the greenhouse to the second-floor bedrooms. Steve Loken, now an internationally known authority on “green” building technology and materials, helped design and build the envelope house, Don said.

With their children grown and on their own, Don and Rose have closed off the kids’ bedrooms to conserve heat in the parts of the house they use most.

“You shouldn’t need to heat areas you don’t use all the time,” Don said. “If you aren’t using a bedroom, why heat it?”

The house is still a work in progress. The couple is considering adding more insulation and maybe a radiant heating system to the greenhouse floor. Radiant heat uses coiled hot-water tubes embedded in the floor to heat rooms.

Don is thinking about adding a heat pump, which would draw heat from the ground into the house and would also function as a cooling system during summer. He wants to make the house capable of being energy-self-sufficient.

I think nationally we really should have an objective to make every home as self-sufficient as possible,” he said. “Public utilities should take the lead.”

A natural disaster such as a severe storms or earthquake could take down power in large areas, but if most homes could get by on their own power systems for a few days, fewer people would need to leave home to find emergency shelter and the stresses of power outages would be greatly reduced, he said.

“And we’d be so much less vulnerable to terrorist attacks,” he said. “That could take out a big part of the [energy] grid. For national security we really should be pushing for energy independence at the individual homeowner level.”