PASSIVE RESISTANCE WORKS
The Super Shell 03
What makes a building shell “super” efficient? That’s a moving target, but German-inspired passive house (or passivhaus) concepts are a good baseline: an airtight building enclosure, extreme insulating values, high-performance fenestration, passive solar heating and natural ventilation. To hit those marks, however, you have very little room for error.
BY BARBARA HORWITZ-BENNETT
Although Germany is light-years ahead in building passive houses where the home’s air infiltration rate is practically airtight, the U.S. is finally starting to catch on. While no match for Europe’s some 20,000 passive homes, a couple dozen construction wonders have been built on U.S. shores, with lots more on the boards.
In simplified terms, the passive house standard states that air infiltration cannot exceed 0.6 air changes per hour at 50 pascals, and annual heating and cooling use cannot exceed 4,755 Btu per square foot. By contrast, Energy Star requires less than 5 or 6 air changes per hour, depending on climate zone, and an average home built to current code standards typically consumes more than 10 times as many Btu.
In other words, a passive house can actually operate on an astonishing 90% less energy. Although this sounds pie-in-the-sky, it’s actually not. Granted, a genuine passive house requires an airtight envelope, a significant amount of insulation, high performance fenestration, passive heating and energy-recovery ventilation—but it can be done.
Addressing each of these key points, passive house and green building experts share some best practices for achieving this lofty goal with its associated energy savings.
Insulation Insights
Generally speaking, in order to build the super shell, significant amounts of insulation must be used—sometimes as much as 16” of rigid foam at the foundation. However, the exact thickness is dependent on climate, so energy modeling can be a good way to determine this.
At the same time, placement can be a tricky issue as the direction of moisture dissipation, which creates condensation, also varies based on climate.
“The insulation should be located on the exterior of the structure, or in-line with it, but the approach we found that works the best in the widest range of conditions is to have vapor permeable wall construction both inside and out,” explains Miloš Jovanović, co-founder of Root Design Build in Portland, Oregon.
Starting with the below-grade walls, an insulation level of R-20 is a good rule of thumb, and can easily be achieved with products like rigid foam insulation on the exterior wall. This type of insulation can also be effective in protecting the concrete foundation walls from water and the effects of freeze/thaw.
For above-grade walls, Mark Wahl, co-owner, Cobblestone Homes, Saginaw, Mich., says, “we typically go with 19.2” or 24“ on-center 2’ x 4’ and 2’ x 6’ stud framing combined with closed-cell spray foam cavity insulation and insulated sheathing to provide the necessary thermal breaks. As an example, the whole-wall R-value of our last zero [energy] home was about R-34.”
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