Interior wall sheathing options
My question is about the suitability for 3/4″ maple or similar plywood for the interior cladding of the exterior walls for a cabin in climate zone 7. No walls require drywall for fire code related reasons. I would like to do plywood to add stability (high winds exposure), because I enjoy it more than drywall, and because I don’t have to tape, mud, and sand it in place. There are many options to deal with the seams. Specifically, I am wondering about the vapour ramifications? My wall from exterior to interior is: Hardie Board & Batten, 3/4″ spruce furring strips & Coravent for rainscreen gap, 1.5″ Rockwool Comfortboard 110, regular Tyvek house wrap, 1/2″ exterior grade plywood sheathing, 2×6 stud wall insulated with 2×6 Rockwool batts, Certainteed Membrain vapour retarder. The next layer would normally be the drywall. Considering that 3/4″ dry plywood has a vapour permeance rating under 1 perm, am I setting the wall up to trap moisture between the plywood and the Certainteed Membrane vapour retarder layer? Thanks in advance for any advice.
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Replies
Your layer of 1/2" plywood on the exterior should already be giving you plenty of shear/racking resistance to help with the high winds. Just make sure you orient those panels so that they tie the studs together with the top and bottom plates so that each panel/wall section is locked together structurally, and follow the planned nailing schedule.
I don't really see any need for additional plywood on the interior to help beef up the assembly beyond what you already have with the exterior panels, but I also don't see that interior plywood will hurt anything. Plywood is not a vapor barrier, so you're not creating a moisture trap. Your bigger issue is going to be air sealing. With drywall, the tape/mud you do to finish the wall doubles as air sealing between panels. With plywood, you don't have an easy way to seal the edges between panels. I suppose you could use battens on those seams, then some sealant to seal the adjacent panels that way if you want to. Another option would be to use one of the more durable smart vapor retarders (Siga Majrex, etc.) between the plywood and the studs, then detail that as your air barrier. You could use CertainTeed's MemBrain, but it's not as durable, and you need to make sure your air barrier stays intact here if it's going to be your primary air barrier layer in the assembly.
So basically I see zero issues with moisture here when compared to drywall. I think your primary concern should be around air sealing, since you will have to do that differently with plywood than you would if using drywall.
Bill
Matt,
I agree with Bill, the plywood itself will not make any difference to the wall from a moisture perspective. Unfinished it is much like your variable-perm vapour retarder, becoming vapour-open when necessary.
However that all changes if you use something like polyurethane on it. If it can't open up, there is no point in having the MemBrain (and if it can open up, why have the MemBrane doing the same function?) The wall is still safe - except in the summer if you use air-conditioning for an extended period of time. Depending on the finish, the plywood may act as a wrong-side vapour-barrier.
Interior plywood used as a wall finish uses the smallest fasteners you can get away with (trim screws or brads). It won't add any useful structural strength to the walls. 3/4" is too thick. I'd use either 1/2" or 5/8".
I've had two projects where it was used as an interior finish - and thank God it was someone else doing the installation. It is extremely unforgiving of out of square framing, and very time consuming. Unlike drywall it is also not possible to repair or patch it as it gets damaged over time. I like the idea, but wouldn't want to live with it.
Thanks for the reply Bill. I had planned to use CertainTeed's MemBrain between the plywood and the studs. I was also planning to seal all the plywood edging. My concern was if I detailed both the CertainTeed MemBrain and the plywood very carefully to , would I be trapping any moisture between the two?
There is no issue with trapping moisture between MemBrain and the plywood. The only concern I would have is Malcolm's regarding how you finish the plywood. I had assumed the same paints would be used as you would with drywall, but Malcolm is absolutely correct that if you use something on the plywood that turns it into a vapor barrier, then things will be different.
I would stick with drywall myself, it's easier to work with in some ways. I don't think the plywood really gains you anything here. Usually when people want plywood up, it's to make it easy to hang heavy things on the walls, like shelving in a garage. I personally haven't seen it used as a finished wall surface in a normal living space, but I'll defer to Malcolm as to what's in style in that regard since in my usual work, aesthetics are pretty far down the list (for industrial type facilities! :D)
Bill
Thank you as well Malcolm. I appreciate all the advice. For code reasons, it did not seem adequate to have only the plywood which is not among the usual accepted products for this part of the wall and it seems more challenging to ensure long term air-tightness with plywood. I was planning to prime and two coat paint it with a breathable interior paint - probably a board and batten look? Still brainstorming. Haven't installed anything yet. I understand the challenges you mention with the large rigid unforgiving panels! Thanks again.
Matt,
Most of the difficulties I mentioned occur when you do the clear-coated minimalist look which is very popular right now. Painted and trimmed it would work well.
I would consider using your exterior sheathing as the primary air-barrier. That takes the worry out of sealing the perimeter and any penetrations in the plywood. It is then is only acting as a variable-perm vapour retarder, you can eliminate the MemBrian, and are left with a very well performing wall from a moisture management perspective.
My final follow up question would be about drywall for remote cabins that may experience a considerable amount of time with low heat (for energy conservation when not being occupied) or even no heat if something should happen and the building was not accessible during that time. Is standard drywall appropriate for that type of building? Is "mold-resistant" but still paper-faced drywall a better choice or is something like a glass fibre gypsum panel the best option?