Mechanical ventilation, building pressure, and air quality
I recently picked up an indoor air quality monitor (a foobot). I quickly discovered that while particulate levels in our home were excellent, VOC’s climbed quite high during the day.
With the windows open at night, VOC levels were excellent. With the windows closed, they would slowly creep up. But then when I turned on the ERV, they would climb rapidly.
Over several days I ruled almost everything out (e.g. the fresh air intake AQI was great). Then I realized, our ERV had been set up to generate negative pressure to minimize condensation risks.
I adjusted the balance to generate positive pressure. Lo, the VOC levels are now excellent & remain excellent.
This of course leads me to conclude that air drawn in through leaks were picking up significant VOC’s. It’s a conundrum- on the one hand, negative pressure mitigates condensation. On the other hand, positive pressure improves the AQI. I can only conclude the best thing to do is probably to balance slightly positive but very close to neutral, and check the balance often. Fortunately the iris dampers I used should make this very easy.
Anyway, just wondering if other people have been through similar trials & discoveries, and what they might have done about it.
For the record, we have an EV90. The house is fifteen years old and sealed to somewhere around 1.5ACH, so while I could always revisit the attached garage interface, there’s not a whole lot of sealing opportunity left. I suspect the VOC’s are probably being picked up in building cavities from the large quantities of OSB.
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Replies
I think you're probably right about the source of VOC's. I think a slightly positive pressure should be OK for you and your house - but it may help to know your general location and climate zone before anyone can make effective recommendations.
Also, I was under the impression that it is often (depending on climate) beneficial to have the house ventilation under positive pressure during the summer, to help keep outside hot, humid air from potentially condensing on the interior of cooler air conditioned walls. However, if your summer night air is dry enough to leave the windows open all night, this might not be a significant problem for you.
Negative pressure during the winter could help keep warm humid house air from condensing on the inside of the exterior walls. However, if your house is that well sealed, you may not have a lot of issue with air leakage causing any real problems inside the walls of your home.
As Debra says, the ideal for moisture is negative when there is a condensation risk from ex-filtration (ie, certain Winter conditions). That risk might be outweighed by VOC concerns.
I expect that some wind conditions would also produce infiltration and high VOCs.
Fortunately this is Colorado, Zone 5, so winters are quite dry, lots of opportunities for things to dry out. You're right, I should have provided those details up front.
Pshyvers,
Your anecdote is interesting. I've always been in favor of balanced ventilation rather than adjusting an HRV to pressurize or depressurize a building, and your anecdote adds another argument in favor of a balanced approach.
As I've written before, there is no evidence that the small airflow rates required for mechanical ventilation can cause condensation problems in building envelopes, so the problem you were trying to avoid isn't really a problem.