We wrap up in wool sweaters to keep warm in winter. Why not extend the same courtesy to our drafty, energy-gobbling homes?
The “super-insulated” house got its start in Canada in 1977. The Germans followed up in 1991 with an improved version by eliminating the furnace altogether. They called it a “passive house,” which quickly caught on in their chilly climate.
This revolutionary concept has only recently spread to the United States, where it is gaining devoted followers among green building enthusiasts.
“The irony is that we knew about this (Canadian) house in the 1980s,” says Mike O’Brien, residential green building specialist for the city of Portland. “Everybody at the time thought it was overkill. But now our energy bills have caught up with us and we’re ready to hear about it.”
Portland offers the perfect climate — both politically and environmentally — for a boom in passive houses, he says.
The passive house concept — which can cut the amount of energy homes consume by up to 90 percent — has the power to dramatically reduce our energy bills and our global carbon dioxide emissions.
What’s more, it has the potential to be quite affordable. The cost to build a passive house in Germany, for example, where standard features like triple-pane windows with foam seals and multiple latches are readily available, is only about 7 percent more than the cost of building a standard home.
The trade-off is that you put money you would have spent on a state-of-the-art heating system into the home’s exterior shell, or “structural envelope.”
The Pacific Northwest’s temperate climate and mild, Mediterranean summers also offer the optimal environment for passive home design — much more so than frigid Frankfurt.
Several local builders and architects have already embraced the movement’s strict code of construction that allows extremely little air leakage — less than one air change per hour. (By comparison, an Energy Star-rated home allows up to seven air changes per hour).
Energy “retrofits,” using the passive house model, are also an option, though meeting the herculean standards for air sealing is near-impossible in existing structures.
Nonetheless, you can still tighten your home’s envelope using the same principles of design, says Jonathan Cohen, founder of Imagine Energy, an energy consulting and contracting firm. Essentially, this is what is required to construct a passive home:
Super-insulate and make it airtight. Buildings are encased in ultra-thick insulation that acts as an airtight shell. This prevents warm air from escaping and cold air from entering the home, thanks to 10-inch-thick walls, triple-pane, argon-gas-filled windows and elaborate airtight seals around each window and door opening.
High-performance windows and doors. In addition to triple-pane, foam-sealed windows, there are also prefabricated walls that are insulated to passive house standards. These products are much more affordable in Germany, where they can be purchased off-the-shelf.
Eliminate thermal bridges. Homes are designed so that wood framing does not provide a conduit for cold or heat from the outside walls to be transmitted to the home’s interior.
Optimize passive solar and internal heat gains. Passive homes also take into account the amount of “free” heat that residents and household electrical appliances create. Careful siting also enlists other passive solar energy sources, such as south-facing windows and the thermal mass of the building itself.
Heat recovery ventilation. The key to indoor comfort in passive homes is a central ventilation system that is widely used in Europe but still relatively unknown in the United States. As warm, stagnant air is expelled through the heat recovery ventilator, it passes by incoming streams of fresh, cooler air, allowing the heat to transfer without mixing the two streams of air. As a result, says local passive house advocate Tad Everhart, “a passive house loses very little heat and then recycles it.”
Everhart hopes enthusiasm for passive house design will grow enough here that products and technologies will soon become readily available — and therefore more affordable — in the United States.
“My vision of the future is that all of these products will become available here in five years,” says Everhart. “This gets back to the fundamentals of how we build our buildings. If we build our houses right, we won’t have to strap on this expensive heat source.”
– Ruth Mullen
Oct. 11, 2009