Air/vapor barrier between tongue-and-groove ceiling?
I am working on a project where the interior ceilings in a large portion of the house will be clad with tongue-and-groove Atlantic white cedar. It is a low slope truss roof sheathed with 5/8” plywood. There is no exterior insulation above the roof plane and the assembly is insulated with closed-cell spray foam.
I live in central North Carolina in climate zone 4A. I am curious about the necessity of an interior air and/or vapor barrier between the framing and the T&G as well as the riskiness of not installing any sort of barrier. By code, it is not required here, I believe, but the installation of cedar makes me concerned about moisture issues long-term because I live in a very humid climate.
Any suggestions would be super helpful. Thanks!
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Replies
In theory, with closed-cell spray foam at least 2-3" thick, you shouldn't need any additional air barrier or vapor retarder. But there are many stories about foam that shrinks or pulls away from the framing, or never cures properly, so there is some amount of risk to not including a separate membrane.
You're going to want an air barrier for a robust assembly. As Michael mentioned, spray foam IN THEORY will give you an air barrier here, so the space between the interior side of the spray foam and the T+G will be conditioned space. In practice though, I find that it's common for things to get missed by the spray foam crews, such as gaps between sistered rafters, gaps at the ends of framing members, etc. With drywall this doesn't matter so much, but with something as air-leaky as T+G, I think the extra insurance of an air barrier behind the T+G is a plus. I normally use 1/2" foil faced polyiso for this, with the seams taped, but in your case I think this runs the risk of creating a double vapor barrier, potentially allowing for moisture accumulation in the space between the spray foam and the T+G. I'd use something like house wrap instead here, to allow for vapor migration. Staple the housewrap up taught, then tape over the staples to seal things. That's probably the quickest and cheapest way to go.
Bill
I appreciate the responses! I respect the work of both of y’all very much. I was able to convince all parties to go with a stapled up house wrap with taped seams and sealed as best as we can around cans, speakers, wall edges, etc. Unfortunately, this was a bit of an afterthought on this project but hopefully this experience helps lay the groundwork for it to not be the case so much in the future.