Air barrier transition from taped sheathing to ceiling drywall
Trying to plan out how to handle the air barrier transition from taped exterior sheathing to drywall on 2nd floor ceiling. Clearly we have to seal the sheathing to the top wall plate, and we assumed we’d use construction adhesive for this. (Also assuming that tape wouldn’t work in this location since roof trusses will have to be set subsequently.) From there we thought we’d just install ceiling drywall first, tight against the top plate, then apply some sealant along that seam before pushing the wall drywall up into it. Same thing for partition walls. With tape, mud, and paint, wouldn’t that be sufficient to maintain a continuous air barrier?
Reason I ask is because I came across this article, which recommends what seems a much more robust approach:
Seems like a great idea to have a plywood cap plate, but I’m wondering if it’s really necessary, or if the approach we originally had planned would fall into the “good enough” category. What’s the standard practice for this transition?
Also, any recommendations on the best sealant to use on drywall, both for seams between sheets as in our original, and also for penetrations such as for lighting fixtures, vent fans? Any problems with joint compound compatibility to watch out for?
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Replies
William,
Q. "What's the standard practice for this transition?"
A. The standard practice is to do nothing. That's why most homes built "the standard way" are leaky.
The trick described by Andrew Webster in "A Practical Air-Sealing Sequence" is a good one. And it's cheap.
Q. "Any recommendations on the best sealant to use on drywall, both for seams between sheets as in our original, and also for penetrations such as for lighting fixtures, vent fans?"
A. Seams between sheets of drywall are sealed, of course, with paper tape and drywall mud. This makes for a durable, airtight seal. If you need to seal the seam between drywall and an electrical box, you have two choices: either seal the seam with caulk, or specify airtight drywall boxes. To seal the seam between drywall and an exhaust fan, use caulk.
Q. "Any problems with joint compound compatibility to watch out for?"
A. I can't think of any.
So Martin, to cut to the chase, do you think the original plan we had would be sufficient? I’m having a hard time seeing where the air would get through as long as a good job is done sealing that joint between the ceiling drywall and wall drywall.
I certainly like the approach Andrew Webster suggests. But I guess the installation of the plywood cap plate and strapping all along the ceiling would raise our labor costs enough to ask the “good enough” question. All of this work will be done by hands other than our own, and we’re tight up against our budgetary limit at this point.
I gather that a lot of green builders use both the exterior sheathing and the ceiling drywall as part of their air barriers. So in asking what the standard practice is in handling this transition, I’m wondering about these builders in particular.
William,
Q. "We thought we’d just install ceiling drywall first, tight against the top plate, then apply some sealant along that seam before pushing the wall drywall up into it. Same thing for partition walls. With tape, mud, and paint, wouldn’t that be sufficient to maintain a continuous air barrier?"
A. Sealant (caulk) isn't as foolproof or long-lasting as high-quality tape. The problem with the drywall-to-top-plate seam is that interior air enters the stud cavities through electrical outlets and escapes into the attic through the crack between partition top plates and the drywall installed on the top plate.
One way to seal these cracks -- a method usually used for retrofit work in older buildings that weren't properly air sealed when they were built -- is to get up into the attic and to seal these cracks from above with canned spray foam.
I think that Andrew Webster's approach is better.
OK, Thank you Martin. Looks like we might be better off scrapping our original plan for the sake of reliability and longevity.
Andrew Webster's approach relies on construction adhesive and acoustical sealant to close the different cracks. Wish there were a less toxic but comparably performing alternative to the acoustical sealant ...
William,
You don't have to use acoustical sealant if you don't want to. You can use a high-quality European tape from Siga or Pro Clima.
For a much less toxic alternative to the typical smelly "black death" acoustical sealant, I like Contega HF, available for 475 building products. Its only VOC is ethanol, so its vapors are no more toxic than the air in a pub, it's easy to clean up, and it works well.
Yes, tapes would be preferable and we’re planning to use them for several applications. (Besides performance, something I really like about tape is that its use is usually verifiable and inspectable, unlike hidden beads of sealant/adhesive.)
In this application though, as a replacement for the acoustical sealant, it would have to be a double sided tape, no? Since we’re trying to seal the underside face of the cap plate to the back face of the ceiling drywall?
Another thing about Andrew Webster’s approach has me thinking, and it’s that the air seal of the top plate to the exterior sheathing, and of the cap plate to the top plate both rely on construction adhesive. I’ve read elsewhere on this site that construction adhesive can’t be relied on over the long run to provide an air seal, maybe just like your average caulk can’t be. Is there something different about this application that would make that advice inapplicable? I guess we could use tape here but then we’d have to cut the sheathing to stop at that level. I think the plan was to have it extend continuously up the raised heel of the trusses. And then what’s the chance that the part of the tape that would be applied to the upper face of the cap plate wouldn’t be damaged in the process of setting the trusses in place?
Anyway, we’ll have 16 plus inches of cellulose blown in there, so I’m hoping that will help mitigate effect of any air leaks we do end up with.
And thank you Charlie, for advice on the Contega HF. I had actually come across previous posts of yours suggesting the same. Seems like a great alternative to acoustical sealant, if pricey. Any more advice on where in the air barrier you like to use this product?