Passive House in North America
Our April EBN feature article--"Passive House Arrives in North America: Could it Revolutionize the Way We Build?"--went online today. This was a fun article to research and write, because it put me in touch with my low-energy building roots. Until digging into the history of Wolfgang Feist's German Passivhaus standard, I hadn't realized that this building system really had its origins in North America--with the passive solar energy and superinsulation movements of the late 1970s (back when I got involved in this field while working in New Mexico).
Feist combined passive solar and extraordinarily well-insulated building envelopes to create buildings (both residential and commercial) so energy efficient that they can be heated using ventilation systems with small, 1,000-watt, in-duct electric heaters. Feist credits Amory Lovins of the Rocky Mountain Institute with this idea--that by investing in the building envelope, heating and cooling systems can be downsized dramatically. Energy loads are so small that buildings can be made net-zero-energy by installing rooftop PV.
Germans are known to value precision, so it is little surprise that the Passivhaus standard is highly quantitative, rigid, and performance-based. In bringing Passivhaus across the Atlantic and creating the Passive House Institute – U.S. (PHIUS), German-trained architect Katrin Klingenberg adopted the German standard exactly, retaining the 15 kWh/m2/year (4,755 Btu/ft2/yr) standard for heating, the same standard for cooling, a total primary energy consumption (including lighting, appliances, and plug loads) of 120 kWh/m2/yr (38,000 Btu/ft2/yr), and an airtightness standard of 0.6 air changes per hour at 50 pascals of pressure difference across the envelope (0.6 ACH50).
What I like about Passive House is how clear it is. The energy consumption and airtightness targets are spelled out precisely, leaving very little room for ambiguity. And those standards are really rigorous. If a house meets the Passive House standard it will be one of the most energy-efficient buildings in the country--period. As noted above, taking such buildings to the next step--making them net-zero-energy--is relatively easy.
The problem with Passive House is that same rigidity. The Passive House requirements could be tweaked, I believe, to make it work better in North America and for existing buildings. Background on these ideas is covered in the EBN article, but let me get right to a handful of specific recommendations. I'll look forward to comments about why these suggestions do (or don't) make sense, what I'm missing, what else could improve Passive House, and any other comments you might have.
- In very cold or very hot climates, relax the heating and cooling requirements while maintaining the total primary energy consumption limit. In climates with very high heating or cooling loads, the Passive House standard right now may be too difficult to achieve. As long as the total primary energy use standard (120 kWh/m2/yr) is met, why not let more energy be used for either heating or cooling? In very cold climates, when little or none of the 15 kWh/m2/yr for cooling will be needed, why not allow some of that energy to be used for heating? And vice-versa for hot climates where little or no space heating is needed, but more energy may be needed for cooling.
- Eliminate or minimize the bias against small houses. Because Passive House standards are based on floor area, larger houses can use more energy and meet the standard, and it's harder to certify small houses. The same holds true with the airtightness requirement, which is based on air changes per hour instead of cfm of air leakage per unit area of envelope. There is a bias in favor of large houses, even though a large house meeting the Passive House standard may use significantly more energy than a really compact house that doesn't achieve the Passive House standards. So why not tweak the standard to give a break to small houses--for example, allowing an additional 2 kWh/m2/yr for heating or cooling if the house is under a certain size, say 100 m2 (1,076 ft2)? Alternately, the standard could be pegged to the number of bedrooms rather than floor area (but that would be a more fundamental--and difficult--change).
- Relax the Passive House standard for existing buildings. Solving our climate crisis will require a huge focus on existing buildings, and a strong standard like Passive House could be a tremendously important tool in getting there. But it's just too hard to achieve right now. I'd like to see a panel of leading energy experts who are familiar with the North American housing stock--but also committed to dramatic reductions in building energy use--put their heads together and come up with a more reasonable Passive House Retrofit Standard for North America. I'm guessing that such a group would come up with something like doubling the energy consumption limits (to 30 kWh/m2/yr for heating and the same for cooling) and raising the airtightness standard from 0.6 to to 1.5 ACH50.
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What do you think? Are these three recommendations reasonable? If not, what would you suggest? Post comments below.
Photo: Dan Whitmore of Blackbird Builders used a "Larsen truss" detail in this Passive House he is building in Seattle for his family. The 14" wall cavity will be insulated with dense-pack fiberglass to achieve approximately R-55. Photo: Dan Whitmore.
(2010, March 31). Passive House in North America. Retrieved from https://www.buildinggreen.com/blog/passive-house-north-america