Attached garage wall assembly
CZ 3A/4A (N GA area) – 2001 gut reno
For context, the OSB seams on the exterior side of our attached unconditioned garage walls were not taped like the rest of the house was.
For the 3 walls that aren’t shared with living spaces:
My contractor mentioned insulating with fiberglass and then putting in 5/8″ drywall on the garage’s interior wall.
Without an exterior air barrier, would closing in this space with drywall be a recipe for a moisture mess? Is it better to just leave the garage walls open stud bays (or any other solutions)?
For the wall that’s shared with a living space:
We’re trying to decide on the best route for air sealing this wall effectively. The interior will be drywall with latex paint and rockwool.
We thought about putting OSB on the garage side and taping the seams – essentially treating it the same as the exterior of the rest of the home. Is that a good option, or does someone have another suggestion? Alternatively, would just adding 5/8″ drywall be sufficient?
If the taped OSB is the better route, can we add drywall directly on top of it (on the garage side)?
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Replies
Pretty sure you need 5/8 drywall between living space and garage.
Ok, so I could put OSB with taped seams behind it, or will just drywall suffice? If I need to just do drywall, what steps can we take to ensure that shared wall is air tight?
Are the 3 walls unshared by living space worth insulating if the exterior isn't air sealed in those areas?
tjones,
Much as with enclosed porches, insulating garages is basically future-proofing them, so if down the road you decide to fully condition the space you don't need to backtrack.
I'm partly responding because I hope others more knowledgeable than me (and they are legion) will contribute. So ... bump!
Some thoughts about the 3 exterior walls - If the garage is unconditioned, then what is the purpose in insulating it?
If the garage interior temperature/humidity only varies moderately from the exterior (for example heated to 55 degrees in winter when it's 30 degrees outside) there should be a fairly low likelihood of condensation issues. (Note I haven't done the math on those specific temperatures.) That assumes, of course, similar humidity inside the garage and outside.
If the wall has the ability to dry out both ways, which would be the case if you used rockwool or fiberglass and no vapor barriers anywhere, then it should be a pretty resilient setup.
I normally prefer taping exterior sheathing as an air barrier for new construction. But if yours is already built then with some modest care you should be able to use the gypsum board of your walls AND ceiling as an air barrier. Try to reduce the number of penetrations.
You didn't mention whether or not there is conditioned space above the garage or whether or not you are conditioning the garage. If no to both of those, you don't have to use any insulation in the garage. If you want some tempering of the garage without heating it, I would install FG or mineral wool batt in the walls and just painted gypsum inside. Those walls can dry to both sides and would be safe in your climate.
For the wall to conditioned space, your local codes people may require upgrading to current standards. IRC/2024 requires R20 cavity insulation or R13 plus R5 continuous. High density batts would give you the R13 of cavity insulation, and a layer of rigid foam on the garage side would give you the R5. Tape the seams for airtightness, then install 5/8" gypsum, taped and painted. With R5 foam on the garage side, I wouldn't be terribly picky about what type of foam to use. You could also strap out the wall enough to get your R20 in the cavities. If you used cross strapping (rather than up and down), you would get pretty close to the same effective R-value as full continuous insulation, so 1.5" strapping should suffice with either batt or foam insulation. Or just use dense packed cellulose for the whole wall if that's available inyour area.