Air sealing common wall between garage and house
Adam Emter - Zone 7a
| Posted in Energy Efficiency and Durability on
Hi,
I’m starting to frame my house, which is going to share the 12’x28′ 2×6 wall of my existing garage. Garage wall is taped sheetrock, poly sheeting, r-19 fiberglass, and OSB. The garage will not be air conditioned (may use dehumidifier in the summer) and will be heated to at least 50 all winter. I live in zone 7a. My question is how meticulous should I get with air sealing between the garage and house and what would be the easiest and most effective methods? Would air sealing create a problem with the poly on the other side of the 2×6 wall? Any help would be greatly appreciated!
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Replies
Air sealing between the house and the garage is critical, not for energy use, but to avoid noxious fumes from vehicles getting into the house. (Engines emit fumes even when they are off.)
If I understand right, the vapor barrier is on the garage side of the insulation, opposite where you'd normally put it. That's generally bad because moisture from the house can condense on cold poly when the poly is on the cold side of the insulation. If you truly keep the garage at 50 F or above, then you'd want the dew point temperature in the house to stay at 50 F or lower. That's 50% humidity at 70 F. That's possible with good ventilation. You might want to aim for 40% to give some margin. But you'd be better off without that poly.
You might consider putting the walls close but not actually sharing them. The construction cost savings from sharing a wall can be negated by the extra figuring and jiggering needed to fit everything together. The energy savings from having less heat loss there are what would lead me toward sharing a wall, if you insist on keeping the garage at 50 all winter. A possible compromise might be to space the walls close together, but have a semi-conditioned space between them that would have enough ventilation to stay at the outdoor dew point, but not so much ventilation that the temperature gets all the way down to the outdoor temperature. The semi-conditioned space between is just an idea I'm tossing out, not a proven concept, so I hope others will tell me what's wrong with it.
Adam,
The first issue to be settled is to clear up the ambiguity in your post. You never told us which side of the wall has the polyethylene. It's crucial to know that fact.
I agree with Charlie: in most cases, polyethylene in a wall assembly is not a good idea. It is an especially bad idea if it is on the cold side of the wall.
I don't really agree with Charlie's suggested solution, however. One thing is for sure: you need to pay attention to airtightness when detailing this wall.
Martin,
From the interior of the garage: taped drywall, poly, fiberglass, then OSB. My dad helped me build the garage and his building methods are still leftover from the 70's when he started building, hence the poly.
Adam,
If you keep the garage heated, the risk of leaving the polyethylene in place is reduced. But if rising fuel prices in the future lead you to leave your garage unheated -- after all, most garages in North America are unheated -- the poly will become a liability and a risk. It would be safest to remove it.
Another solution would be to install R-15 rigid foam on the garage side of your wall, over the existing drywall, followed by a new layer of 5/8" drywall. There are three benefits to this approach: (1) It removes the risk from the existing polyethylene; (2) It raises the R-value of your wall; and (3) it reduces (or may reduce) air leakage through the wall.
Martin's rigid foam suggestion sounds like a better idea than my crazy scheme. You could probably get away with 2" of rigid foam, because even if the garage is unheated, it will "float" at a higher temperature than outside.
Adam,
You are going to have to remove at least some of the OSB on the shared wall to run wiring. Why not remove the batts and cut away the poly a the same time? Probably an hours work.
Thank you all for the suggestions. Martin, I have three daughters, so I will probably spend more time in my garage than the house, even in the winter. Seriously though, I do like the suggestion of rigid insulation on the garage side. That joining wall is where I have most of my mechanicals, so it wouldn't be as easy as if the wall were empty, but it would still be fairly straightforward.
Malcolm, I only have one electrical outlet planned on that wall (it's mostly stairwell) so I won't be running electrical or any other utilities parallel through the wall.