By George Harvey
Some months ago, architect Robert Scarano of Bright ‘n Green told us something in an interview that really made us sit up and exclaim. “You can get 90% of the way to the passive standard for the same cost as conventional construction,” he said. He then explained, “The extra cost of insulation and air sealing is offset by the cost saving of not having to install a furnace and chimney.”
We brought the idea up with Bob Irving, of R. H. Irving Homebuilders. He says following the passive standard strictly can be expensive, so many builders are adapting elements of the passive standard, finding ways to get superior performance at reduced cost. He builds to about 75% reduction in energy consumption. “From there, it is usually not hard to get to a net-zero energy home with photovoltaic panels,” he said.
Irving said the most important consideration is air movement through the house. A typical existing, conventionally built home has extensive air leaks between the sill and the foundation. “You might not see them, but that does not mean they are not there.” Hollow interior walls act like chimneys, if there is any way for air to get into them from the basement and escape from them into the attic. Wiring, plumbing, and other penetrations provide a perfect pathways for the air to get through. This can be avoided by finding and sealing the leaks. It is best to get an energy audit, including a blower door test.
One of the most important things to learn from the passive standard, Irving says, is the idea that everything to be heated should be be within the sealed and insulated building envelope. If the attic is to be considered part of the living area and heated, it should be within the envelope; otherwise, it should be outside. Everything, including basement walls and attic access ways should be insulated and air-sealed.
There should be no thermal bridges. A thermal bridge conducts heat through insulation. For example, in conventional construction, if the exterior wall sheathing and interior plaster are attached to the same studs, the stud is a thermal bridges, giving heat an opportunity to bypass the insulation between the studs. One way of preventing this is to have the exterior and interior walls on different studs; they are not connected, and there is insulation between them, making a double wall. In Irving’s building, the studs for outside wall are load-bearing, and the studs for the inside wall are not.
Ventilation is of utmost importance. Irving’s buildings are equipped with heat exchangers so incoming fresh air should be raised close to room temperature by heat exchangers capturing heat from air venting out. Scarano’s buildings have incoming air that passes through underground pipes to pick up geothermal heat, bringing it close to room temperature before getting to the buildings’ interiors.
Unsurprisingly, insulation and high-performance windows are important issues. What is surprising, however, is that when a house is really well insulated, the heat distributes through the house far more easily than it gets outside. Thus, a small number of heating distribution points can supply very even heat throughout a house, even to rooms that do not have their own heat sources. A well-built house should be both inexpensive to heat and comfortable.