Air-Sealing at the Sheathing vs. Drywall
To achieve airtightness in my home I’m planning to use the air tight sheathing approach as my air barrier. We are using OSB for our sheathing and plan to tape all the seams. I’ll caulk the bottom of the sheathing to the sill plate and the top will be sealed to the roofing sheathing with closed cell spray foam. We’re doing an unvented room with the rafter cavities filled with 5.5″ of closed cell spray foam.
That’s my primary air barrier. Which seems much easier than doing the air tight drywall approach or am I missing something?
With the airtight drywall approach you have to seal each top plate and bottom plate and all intersecting walls. Plus you have to seal each piece of drywall to the top and bottom plates and all the electrical boxes, direct vent fireplaces, switches and light and ceiling fan penetrations. See:
It seems it is much harder to get good sealing at the drywall. I was half thinking of doing a belt and suspenders approach to air sealing and having my primary air barrier being the sheathing and the secondary air barrier being the drywall. However looking at how hard it is to get air tight drywall I might just make sure my primary air barrier is well done.
Why do people choose the airtight drywall approach over the airtight sheathing approach when there is such a difference in complexity?
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Imagine you had fairly free (eg, fiberglass batts) airflow from the moist warm interior, to some cold sheathing (ie, no foam), and then back into the interior. Clearly more moisture on the sheathing than if the interior side was air sealed. But perhaps not enough to be a problem.
Q. "Why do people choose the airtight drywall approach over the airtight sheathing approach when there is such a difference in complexity?"
A. In fact, far more builders these days establish their primary air barrier at the sheathing level (as you plan to do) than at the drywall level. The ADA approach has been losing ground for many years.
That said, paying attention to air sealing at the interior is always a good idea, even if your efforts fall short of a full-fledged implementation of the Airtight Drywall Approach.
In many really tight homes, there are both; a belt and suspenders approach.
In general, getting continuity of your air barrier is much easier on the exterior than the interior. And, exterior air barriers do a better job managing windwashing at outside corners.
If you are taping your OSB as your air barrier, make sure you have an agressive acrylic pressure-sensitive adhesive (PSA) tape. In tape testing that I have done, the best tape of rough side OSB was Huber ZIP.
I’m interested in this as well. My 1850’s house in Zone 5, central MA that needs to have a building envelope straight up *created* probably doesn’t need any sheathing structurally, but should I put zip or similar on anyway to get a water and air barrier? Is that vapor open enough if I want a dry-to-both-sides vapor open wall and roof? Very humid summers. But yeah, I wonder what an interior extra air barrier like Intello or really trying hard to do the ADA thing would yield too.