Airtight wall sheathing options
Having read on this site some questions about the airtightness of OSB, we asked our builder to price using Zip panels (uninsulated variety) instead of commodity OSB as our wall sheathing. Since we’re using EPS foam exterior to the sheathing, and then housewrap, the water barrier aspect of Zip panels would essentially be wasted. But we’d be getting the air barrier.
Builder says this would increase costs by about $2500, so we’re looking for alternatives, or thinking about just taking our chances with commodity OSB, since so many seem to have achieved high levels of airtightness using it.
Wondering if anyone can share any recent experiences with particular brands of OSB?
Would it be fair to assume that increasing thickness of OSB might give us a better chance of it being airtight?
Plywood seems like a decent alternative that would probably price somewhere between standard OSB and Zip panels. But how careful do you have to be about knots? One idea I had was to use a liquid applied air barrier such as StoGuard wherever we see knots in the stud bays. (We plan to caulk from the inside at framing/sheathing intersections around perimeter of stud bay,as wall as at other important wood to wood joints, and to use spray foam in the bands, so the only place where the sheathing would have to be airtight would be in the stud bays.) Seems like with a small amount of StoGuard, we could remove any doubts about the knots leaking air. Any thoughts? Would this be overkill?
We’ve considered the insulated R-6 Zip panels but think we prefer EPS to polyiso for the walls. Also like the idea of the foam being exterior to the sheathing, and keeping the sheathing warm and dry. (I’m thinking that insulated Zip panels are installed with the polyiso interior to the OSB layer, correct me if I’m wrong.)
Walls from inside out: gypsum drywall; 2×6 stud bays filled with cellulose, sheathing, 1.5 inch EPS foam, housewrap; furring strips; engineered wood lap siding. Location: Southwest Virginia. Not seeking any kind of certification, just trying to build the most airtight house we can.
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I've used ⅝" commodity OSB and taped all seams and joints with SIGA Wigluv tape (vapour permeable) and achieved excellent results (final blower door test was 0.25 ACH50). Though this route was likely as expensive an upgrade as the ZIP sheathing option, factoring in both the increased OSB cost and the SIGA vapour permeable tape. If the permeance of the tapes isn't critical to you (and likely is not if it's behind the EPS), then you could switch to 3M's "All Weather Flashing Tape" which offers terrific performance in terms of adhesion, durability and air sealing, but at less than half the cost of the SIGA permeable Wigluv product.
My house has almost exactly the wall stack-up that you are planning. We used normal OSB. The seems of the OSB were taped with 3M All Weather tape and the stud bays were caulked. Our blower door test came out at 0.82 ACH50. I suspect that this wall section is good enough that the walls themselves are negligible contributors to air leakage compared to transitions and penetrations. Your efforts to minimize air leakage should focus on avoiding penetrations and detailing the penetrations you cannot avoid.
Just an FYI..
The sheathing doesn't need to stay warm as long as it can dry. Using ZIP-R with a rainscreen keeps your sheathing dry because interior condensation is no longer an issue and the rainscreen allows any moisture that gets behind the cladding to dry out. The idea behind exterior foam isn't necessarily to keep your sheathing dry (added benefit), but to keep it warm enough to prevent condensation from forming on the interior face of the OSB..
If your builder is more comfortable using exterior rigid foam and the price is right then run with that.
John,
Your comment is a little baffling. You wrote, "The idea behind exterior foam isn't necessarily to keep your sheathing dry (added benefit), but to keep it warm enough to prevent condensation from forming on the interior face of the OSB."
In other words, to keep it dry. That's why we want to avoid condensation, after all -- to keep the sheathing dry.
-- Martin Holladay
@Martin
My understanding is that when using exterior foam the warm sheathing will still take on moisture from the interior vapor (winter time), but because it's warm the moisture won't condense out on the surface of the OSB until is gets really saturated from something such as a water leak.
Apologies for the misunderstanding.
John,
Warm wood is dry. Cold wood is wet. As long as the sheathing stays warm during the winter, it will stay dry. If it gets cold, it will begin absorbing moisture.
-- Martin Holladay