How Passive House Can Help Keep the Smoke Out

It combines a tight seal with controlled (and filtered) ventilation

smoke over san francisco
Smoke over San Francisco.

Justin Sullivan/ Getty Images 

The air over parts of California has been full of smoke recently, and will likely be so again. A few years ago, we described how Chie Kawahara's Midori Haus, designed to the Passive House standard, handled fires in 2018. She wrote:

"The tightly sealed enclosure, about 10 times tighter than conventionally built houses, keeps random air from coming in from random places. The heat recovery ventilator provides us with continuous filtered fresh air. Only during these extended bad air quality days do we need to pay special attention to our ventilation system to keep our indoor air clean."

Chie's reporting was anecdotal, but now there is research from Australia that gives us real data. Cameron Munro and Joel Seagren describe in Renew Magazine how houses designed to the Passive House standard had significantly lower levels of Particulate Matter less than 2.5 microns in diameter (PM2.5) during the recent Australian bush fires.

We have been discussing PM2.5 on Treehugger forever, noting that the effects were worse than we knew, and damaging every organ in the body. They are not tightly regulated, and many consider that the WHO standard of 25 micrograms (µg) per m3 averaged over 24 hours is way too high and that there is no level without health effects. Munro and Seagren note that the particles from fires are even finer, usually under 0.5 microns. "These very small particles can travel far from the fire zone and readily penetrate into homes and buildings." They are so small that they can go straight through your lungs into the bloodstream.

Figure 2
Figure 2.  Via Renew Magazine
Figure 2 shows two homes adjacent to one another during a recent smoke event in Melbourne. [The blue line is outdoor air.] The conventional leaky building without mechanical ventilation [red line] reached PM2.5 concentrations of just under 500 µg/m3 when the outdoor levels were close to 600 µg/m3. By comparison the airtight home [yellow line] reached peaks of 320 to 380 µg/m3. In other words, the airtight home achieved smoke concentrations about 30% lower than the leaky home.

Even in the airtight home, the levels of PM2.5 are way above acceptable standards. However, when the filters were swapped out for HEPA filter, it stayed within "healthy" levels, if there is such a thing. But this is not without its own problems.

"HEPA filters can capture over 90% of smoke-sized particles ... However, this performance comes at a cost. Increased pressure drop across the filters will increase fan power consumption and can increase noise as the fans have to work at a higher speed to compensate. Additional filter replacement costs are also the other obvious change, with finer grade filters typically being more expensive, and requiring more frequent replacement."

Munro and Seagren recommend only swapping filters when you really need to. They also conclude that while fires are a significant problem, we have to worry about PM2.5 all the time, because there is no known threshold level, notwithstanding the EPA's continued ignoring of the issue because, in the USA, they come from coal-fired power plants and from fossil-fueled cars and trucks. Munro and Seagren's prescriptions are: no wood or gas fires indoors. "Shifting to using electric vehicles or, better, riding or walking, can reduce particulate levels as transport is a major contributor to human-induced particulates in built-up areas."

But one thing we should be doing is building safer, tighter houses with controlled and filtered air like you get with the Passive House standard. As I noted in the earlier post, "It may well be time to make Passive House level airtightness requirements part of the building code; these forest fires will not be the last."