Moisture control for retrofit double studded wall
edited this as I am exploring a double studded wall. My question here is about moisture concerns.
We’re designing a 2 car garage conversion to an apartment. Our primary design goal is to use what’s there as much as possible to keep costs low while still making it acceptably (if less than pretty good) energy efficient.
If we leave a painted OSB layer in between the studded walls is that helpful or more concerning? In the attached detail(from building science corp) there’s a vapor retarder on the exterior side of the interior wall. Could the osb be painted and taped/sealed to act as this barrier? Should we take that osb off instead? Then build the interior wall, wire, then Intello interior and fill entire cavity with cellulose.
Existing Structure
Western Massachusetts (5a)
2×4 frame on top of frost walls with concrete slab (typical 2 car garage in the area)
the interior walls are currently painted OSB. No insulation and I doubt there’s a membrane in the current assembly. Painted wood clapboards that are in great shape.
Does the painted OSB mitigate concerns of excess moisture inside wall assembly from condensation? Does it make it worse because it interferes with drying?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
I'd definitely take the OSB off rather than the siding. Even if you end up not being able to reuse it, OSB is $15 a sheet at my local big box store, probably less than $300 for the whole interior.
A 2x4 wall with fiberglass insulation isn't terrible. If you want to go up for there, my intuition is the cheapest way to do that would be to rip pieces of 2x6 or 2x8 to make the wall 2" thicker, so it's effectively a 2x6, and use batt insulation. Regular fiberglass gets you R19, high density or rock wool gets you R21. Another alternative would be keeping the 2x4 and batt insulation, and doing a layer of continuous foam on the interior, then 1x3 strapping and drywall.
The vapor drive in this assembly is going to be from interior to exterior and the siding is able to dry to the exterior. If you used faced batts or continuous foam that should be enough vapor resistance to avoid problems.
Before doing too much I'd like to know what you're doing with the ceiling and floor, money might be better spent there.
one reason I like a second 2x4 interior wall and then dense pack the entire cavity is we can frame the wall inside of the frost wall that comes up past slab and sticks outside the building beyond the structural wall. This way I can rigid foam the stem wall on the inside and then have a solid wall for drywall instead of screwing into the concrete or hanging notched straps off the structural studs.
For the floor thinking 3” eps taped then two layers glued 1/2 osb. Stem walls will get rigid too.
Ceiling 2x10 joists, I don’t think ~r35 is enough to keep the heating bill low so im thinking either dense pack ceiling or batts, then add another joisted floor (2x6 or 8) on top of attic floor and fill that too. Homeowner wants to continue to be able to store stuff up there.
Sounds good. Make sure the roof is vented and the ceiling is well air-sealed to the interior. Often the hatch to the attic is the weak spot, the best thing is just to put a door in the gable and access it from the outside with a ladder.