A blizzard can rage outside in -40C weather but inside the airtight Passive Home it may as well be June. “If it gets too hot you have to train yourself to open a window in winter,” says Dr. John Eckfeldt of his home in Isabella, Minn. “After all, you aren’t paying for the heat.”
In fact, Dr. Eckfeldt and his wife, architect Nancy Schultz, need to check the outdoor thermometer to dress for the weather. “It feels like you are in a thermos and outside elements don’t have an impact,” he says.
Isabella is nestled in the Superior National Forest north of Lake Superior near the Iron Range, where iron ore has been mined since 1884. It is one of the coldest regions in the lower 48 states. The Canadian border is 40 kilometres north over the pristine Boundary Waters Canoe Area.
The Isabella Eco Home, as it is known, is a certified Passive House. The Passivhaus concept was pioneered in 1991 by physicist Wolfgang Feist in Darmstadt, Germany and today represents the highest energy standard for commercial and residential buildings. To be certified you must slash heating energy consumption by about 90 per cent – an achievement precisely modelled and measured by the U.S. Passive House Institute.
Innovations of the Passive House include ultra-thick insulation, doors and windows engineered to withstand cold and walls that encase the home in an airtight shell to retain heat. The Isabella Eco Home has 12-inch staggered double wall stud framing, 30-inch heavy timber framing on the roof and a continuous air barrier to minimize heat loss.
Another critical component is the lungs of the home – a mechanical heat recovery ventilation system. “In most buildings you lose much of your heat by warm air moving out and cold air moving in – so the control of air movement is key,” Dr. Eckfeldt says. According to the rules, the Passive House air-tightness pressurization test result must be 0.5 air changes per hour – a standard about 10 times more stringent than in the U.S. Energy Star program.
The Isabella Eco Home heat recovery system reminds Ms. Schultz, owner of Compass Rose – an architecture firm that specializes in sustainability planning – of a 20- by 20-foot waffle iron with corrugated baffles. “The warm air going out passes side by side with clean, cold air coming in and heat is exchanged efficiently,” Ms. Schultz says. “You need a big surface area for a counter-current exchange.”
Passive House is a deceptively gentle description for an aggressive project kicked off by Dr. Eckfeldt and Ms. Schultz, passionate environmentalists, in 2007. “We took on net-zero energy [the home’s annual net use of energy is zero] in a cold climate to challenge ourselves and see if we could push the envelope,” says Dr. Eckfeldt, vice-chair for clinical affairs in the Laboratory Medicine and Pathology Department at the University of Minnesota.
Passive House standards, like old-fashioned passive solar homes, use non-mechanical means, including south-facing site placement, to get optimal solar collection from their triple-paned windows. “This is your first line of defence,” Ms. Schultz says. “Then your active systems can be smaller.”
The Isabella Eco House collects energy through 92 solar heat collector vacuum tubes and a photovoltaic system on the garage roof.
One of their most acclaimed features is their experimental solar thermal storage system of 9,000 cubic feet of sand and taconite beneath the home’s insulated slab. Dr. Eckfeldt calculated heat values in various metals and found taconite pellets, which are almost pure iron oxide, refined from a local low-grade ore, had three times the heat storage capacity of sand or gravel. They bought nearly 20 truckloads from a nearby plant.
