[Published in AIA East Bay's ARCHnews July 2008]
Imagine on a cold winter day, a house can be heated solely by people, lights, equipment, and sunshine. A Passive House is a building with enough insulation and air-tightness to eliminate the need for conventional space heating. After the 1970s energy crisis, architects in the Northeastern U.S. developed “superinsulation” (e.g. double-stud walls) and fresh-air ventilation. In the last fifteen years, European architects have perfected these innovations to create the Passive House Standard. This architectural formula will become part of the European Union’s building code by 2012. It could also be the most elegant and inexpensive means to reduce building energy use in the Bay Area.
How does it work? As we increase the insulation and air-tightness of a building envelope, the building becomes more comfortable and saves more energy, but it also becomes more expensive to build. However, there is a threshold of insulation and air-tightness at which the heating system becomes superfluous, and the total cost falls to the price-point of conventional construction. This is the key to designing a Passive House: simplify the mechanical system to pay for envelope upgrades.
It gets better. Because insulation blocks sound, the interior of a Passive building is quieter than a conventional building. Being inside engenders a feeling of serenity to know that the building is perfectly balanced between internal and external heat. Considering the building metaphorically, comfort is no longer attained with fire, but with breath. To cool off, one opens the windows. To warm up, one closes them.
Passive buildings are also healthier than conventional buildings, because they are flooded with filtered outdoor air. By using a fresh-air ventilator with heat recovery (an HRV or ERV), a Passive House loses less heat than a conventional building that is fully buttoned-up. Furthermore, the ductwork can be smaller and simpler, since it only provides fresh air.
So how do we design Passive buildings for the Bay Area? We turn our typical design sequence on its head by starting with a piece of mechanical equipment (the HRV). Next, we “size” the architectural shell to meet the heating loads, the way an engineer would have sized a boiler in the past. The calculations can be done on a napkin, but planning software is available. See the resources listed at the end of this article.
East Bay architect Nabih Tahan used the “Passiv Haus Planning Package” (PHPP) software to design a Passive House in Berkeley. He raised and remodeled a typical early-1900s bungalow. When I paid him a visit recently, I expected to see thick walls and high-tech windows. Surprisingly, Tahan used 2x6 studs at 24” centers, 2” of rigid insulation over the original 2x4 walls, and conventional windows. He paid careful attention to airsealing by caulking and foaming gaps in the plywood sheathing (continuous through the attic), and gasketing under sill plates.
Ironically, Tahan had trouble passing the Title 24 Energy Code. California’s current Alternative Compliance software cannot understand a house without a heater, so in order to get his certificate, he had to install $35 electric baseboards. When a basic gas furnace costs $8,000 and radiant floors cost upwards of $20,000, the upfront cost advantage of Passive buildings is clear. As if we needed more incentive, forthcoming revisions to the Title 24 Energy Code will mandate mechanical ventilation for houses—the Passive House essential. All that’s left to do is seal the gaps and get rid of the heater.
The following resources offer more information, including the PHPP software:
The Passivhaus Institut in Darmstadt, Germany: http://www.passiv.de/
The Passive House Institute, US: http://www.passivehouse.us
Passive House, Wikipedia: http://en.wikipedia.org/wiki/Passive_house
Sill Plate Gasket: Owens Corning Foam SEAL-R