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Community and Q&A

Ensuring good air quality in garage apartment

user-7523133 | Posted in General Questions on

We are building a garage with an upstairs apartment in southern Minnesota. Our house is from the 1890’s, so we have lots of experience with drafts, but little experience with a tight, well insulated space. The garage is a simple design (24×24) and the apartment is equally simple (bathroom, and an open living space). We are fully insulating the garage and apartment (3” exterior insulation extending all the way down the footing, 2” sub-slab insulation, 2×6 cavity fill, and r-50 roof). There will also be insulation in the floor, and everything will be fully sealed and detailed to prevent airflow between apartment and the garage. Only penetrations are electrical and water/waste lines, which will also be sealed. At this point, we don’t plan to condition the garage, but would have that option in the future. We’ll use a small ductless ERV in the apartment, and there will be a bathroom and kitchen fan. There is a passive radon system (required for ADU in our town). We could install an exhaust fan in the garage if needed. The question is, is the garage exhaust needed to be sure the garage air isn’t making it into the apartment? Or only if we start conditioning the garage? Would adding a active radon fan help with pressure somehow?   Are there other aspects to consider that aren’t on the list above?  What have I missed?

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  1. Expert Member
    PETER G ENGLE PE | | #1

    With a good airtight seal between the garage and living space and an ERV in the apartment, your risk is low. Be aware that the kitchen and bath exhaust fans depressurize the apartment and will tend to suck garage air into the apartment through any gaps that do exist in the air barrier. Operating balanced ventilation (in&out) in the garage for 10-15 minutes after leaving will help to reduce the CO levels quicker. With construction that tight, CO will linger for quite a while, increasing risk of spreading to apt. Better yet, switch to EVs - no CO production at all. Of course you still have to deal with solvents and other nasty stuff that typically gets stored in garages.

    1. user-7523133 | | #3

      That’s a good idea to run the ventilation on a limited basis, as needed

  2. plumb_bob | | #2

    There will need to be an effective air barrier between the garage and the living space. For a floor over a garage, a good solution could be detailing the T&G subfloor as an air barrier by caulking the joints. You could also have closed cell spray foam applied to the underside of the subfloor as both insulation and air barrier. Penetrations for plumbing and other services should be detailed carefully.

    You could have an exhaust fan in the garage wired to a CO sensor. For a setup like this I would have mechanical louvers wired into the same system so when the exhaust fan is activated, the louvers open and fresh air is supplied as the poor air is exhausted. This will also ensure the space does not have negative pressure.

    1. user-7523133 | | #4

      I suppose a thin layer of spray foam would do the trick for sealing. I like the idea of louvers, as a simple way to supply fresh air while a fan is running. Good food for thought, thanks!

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