Barrier Between Basement Garage and First Floor Living Space
I am in the PNW CZ 4C where we have long wet shoulder seasons (although I’m not so sure how that might play here). I have a 1 car garage that is in the basement and is under the kitchen. We are wanting to make certain that the garage is air sealed as much as possible from the living space, so besides the excessive caulking where we can, we are thinking of putting up an air-barrier. The question is, would a plain Tyvek like wrap work or do we need to worry about vapor drive and thus need a smart membrane (although the recent article by Ben Boogie has me really questioning that). I will add three additional considerations. First, although I plan to have make-up air, the kitchen can be negative when the vent hood is in use. I have hydronic heat in the garage floor designed for keeping it at least 45 degrees on the 1% coldest design days, so the delta-T will not be extreme. Lastly, expect a very wet Subaru to be parked down there during the rainy winter/shoulder season.
I have talked to the support line for a couple of US makers of smart vapor systems and they seem perplexed by this use case, but have not talked to the European ones yet. Would love to hear from the expert community on their thoughts for this.
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First: the air barrier between the garage and the living space should be ruthlessly airtight, not just for energy efficiency but also for fire safety and to prevent the possibility of fumes entering the living space. Drywall can be that barrier, but it has to be carefully sealed.
Is the entire basement the garage? If not, the wall between the garage and the rest of the basement also needs to be sealed.
Vapor sealing is a separate matter from air sealing, as moisture can pass through drywall, wood and many other common building materials. What's tricky in this situation is trying to predict which way the vapor drive is going to be, vapor moves from wet to dry and from warm to cool; normally the warmer side also has more moisture but that may not be true in this case.
Generally vapor barriers are necessary when you have a temperature gradient, and the humidity on the warm side is above the dew point of the cold side, so there's a potential for condensation within the assembly. If the garage side is at 45F that might be a problem, 70F air at 40% RH has a dewpoint of 45F. So to the extent you want a vapor barrier you'd want it to be on the warm side. I don't see any possibility of the inhabited side ever being below the dew point of the garage air.
The smart vapor barriers are for situations when either side could have a temperature below the dew point of the other side. You would see this in an air-conditioned building in a climate that has hot, humid summers but cold winters. Their smartness is they act like a vapor barrier in the winter but are vapor open in the summer so the assembly can dry to the interior. That's not what you need.