Any benefit to WRB on both sides of sheathing?
Hello,
In the scenario of a thick insulated wall in a cold climate (such as a 2×8, or a staggered stud assembly), without any exterior insulation, is there a benefit to having a WRB on both sides of the exterior sheathing in order to help protect the OSB from condensation?
From other GBA articles I understand that since the OSB stays very cold with these types of walls, there is a heightened risk of condensation on the inside of the OSB from any interior air leaks, and the wall will struggle to dry outward due to lack of heat flux coming from inside the building.
In addition to the standard vapor-permeable external WRB, I’m wondering if there would be benefit to installing an air barrier/WRB on the inner side of the sheathing. My thinking is that the cold interior WRB would become the first condensing surface instead of the cold OSB, and thereby keep the inner side of the OSB drier and prevent deterioration. Meanwhile, the other components of the wall cavity (dimensional lumber studs) are more resistant to moisture and will have better drying potential (staying warmer than the sheathing due to being a thermal bridge to the interior). Meanwhile, use of a smart vapor barrier on the inside of the drywall would enable drying to the interior.
Is this a correct understanding, and would this type of “WRB sheathing sandwich” wall assembly make any sense? If so, would it be better to use a vapor-open WRB on the inside of the sheathing, or a vapor-retarding WRB?
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Replies
Kyle,
Cold sheathing doesn’t take on moisture from the interior by condensation, but by adsorption. WRBs are specifically designed to stop bulk water but allow vapour to move through them, so unfortunately a second WRB won’t help.
Thanks Malcolm - that makes sense, so a vapor-open WRB like Tyvek wouldn't work for this purpose.
What about a vapor-retarder just inside the OSB? I'd have some reservations about using a full vapor barrier like 6mil poly for this purpose, but maybe a 1-3 perm vapor retarder could work to catch outward vapor drive before it hits the cold OSB, and meanwhile using a smart vapor barrier behind the drywall to enable the rest of the assembly to dry to the inside.
Perhaps one of the thinner versions of Zip-R could work similarly, with the thin layer of vapor-retarding rigid foam just inside of the outer OSB.
Kyle,
There is a bit of a misconception around vapour movement in walls. In heat dominated climates for most of the year the vapour-drive is to the outside. Putting low-perm materials part way the wall doesn’t change that. It doesn’t just reverse direction and decide to dry to the inside.
You don’t want any throttle on vapour movement near the exterior of the wall. The place to slow the outward vapour-drive is close enough to the interior that the moisture is below the dew point. Building codes acknowledge that by requiring vapour-retarders to be located at or near the warm side of the wall. If you include an interior warm side variable-perm membrane, and detail either that or your drywall as a secondary air-barrier, there won’t be much moisture to accumulate in the sheathing - and what does can dry through the exterior WRB into the rain-screen cavity.
Thanks - I see there's a good argument that it would be most beneficial to spend the time and effort on a really good detailing of the standard air barrier / smart vapor retarder on the inside of the wall assembly, instead of trying to create a "last line of defence" vapor retarder just inside the OSB, which could still result in moisture inside the wall cavity that may have a hard time drying back to the inside.
Kyle,
For those of us not operating on the cutting edge of building science, I suggest two things:
- If you haven't seen something done, there is probably a good reason for that.
- Benefit from the efforts of those people who have tried innovative building assemblies, and use the ones they have found to work.