Tongue-and-groove boards for a cathedral ceiling
I’m planning to build a house in central PA which will have cathedral ceilings for all of the rooflines and large dormers. Because of the dormers, I’ve ruled out a vented ceiling and am thinking I need to go with closed cell spray or flash and batt. For cost reasons I’m leaning towards the flash and batt option with R-21 closed cell spray and R-19 unfaced batts for cost reasons. We are also considering doing T&G wood in some of the areas…so my question is do I need to put drywall under the T&G wood? From what I’ve read, when using closed cell spray foam you want it to “dry to the inside” so does that mean the drywall could be omitted? Any suggestions would be appreciated!
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Yes you need drywall.
Pajon,
If you are installing closed-cell spray foam on the underside of your roof sheathing, your roof assembly won't be drying to the inside (because closed-cell spray foam is a vapor barrier). The assembly will work, but there is no interior drying.
Because you are using spray foam, you don't have to worry about air leakage. So the numerous air leaks through the tongue-and-groove boards won't be a problem.
The main reason that you probably need drywall on the interior side of this assembly is as a so-called thermal barrier -- a layer required for fire safety. For more information on this issue, see Thermal Barriers and Ignition Barriers for Spray Foam.
Once you have installed and taped the required layer of 1/2-inch drywall, you can install tongue-and-groove boards on the interior side of the drywall if you want.
If you have further questions about code requirements for thermal barriers, consult your local code enforcement official.
Even at R49 most closed cell foam doesn't quite hit vapor BARRIER status, but it's close, and ~0.15 perms, give or take. That's about 3x as vapor open as 6 mil polyethylene. At R21 it's about 0.3-0.4 perms, which is sufficiently vapor open for seasonal drying in most locations. (Vapor barrier latex paint runs about 0.5 perms, and works pretty well in most climates.) That's not to say it's a good idea, or very green, but it's not exactly a vapor barrier.
Even 2" of closed cell foam is sufficient for protecting the roof deck in most locations & designs, but may not be enough to keep the fiber insulation dry. In US climate zone 5 parts of central PA as long as at least~40% of the total R is the closed cell foam the fiber insulation is pretty safe, as long as you aren't actively humidifying the house during the winter, and keeping it under 40% RH. I zone 6 portions bump that to 50%.
At those levels the vapor permeance between the fiber insulation and the interior only needs to be under 10 perms. If using drywall under the t & g, paint it with a standard latex primer. Kraft faced batts the facer on the room side will also get you there from a vapor retardency point of view ,but are impossible to make sufficiently air tight- the air-tight drywall under the t & g is still needed.
R19s are only R19 at 6.25" of loft, and only R18 at 5.5" (standard 2x6 framing deptth). They are also among the least air retardent insulation materials available. R20s batts (designed to meet currrent code minimums) may not sound much better, but in fact they are. They perform at R20 at 5.5", and are much more air retardent. R21s are even better still, but cost-compare them against R23 rock wool, which is a better material in many regards (it's far more fireproof, lower friable fiber particles that can stay suspended in air, etc. If the remaining depth space under the closed cell foam is between standard batt thicknesses, go up a step in thiickness and compress it into place, leaving no voids. eg: A low density R25 designed for 2x8 framing compressed to say, 6.5" is going to perform better than an ultra-low density R22 (manufactured loft- 6.75") in the same space, both on R value and air retardency. (An R22 compressed into 5.5" 2x6 framing isn't even as good as an R20 or R21 batt in the same cavity!)