Is my friend making a mistake with his wall assembly
I have a friend/builder who is building his own home on Cape cod. I’m wondering about his envelope construction. He’s putting comfortboard 80 directly on studs with conventional plywood over the comfortboard 80. Most assemblies I’ve seen have comfortboard over the plywood. My guess is he’s going to use 30# tarpaper over the plywood then white cedar shingles. Inside he will have latex paint over plaster, over blue board and Rockwool between the studs. He’s an older builder and set in his ways but wants to build a higher performance home than his usual old school structure. The possible problem is he’s old school. He has a general mistrust of anything new. Is he making a mistake here? I’d hate to see him blow it on the vapor management.
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I believe he is in Zone 4 Marine or 5, not sure which. Offhand I don't see a thermal/vapor/moisture problem with his plan.
My only real concern is structural. Placing the Comfortboard 80 between the plywood and the studs negates the value of the plywood for resisting lateral loads (wind and seismic). I know seismic is not a problem in his area. But wind may be. So he will need to make sure his walls are adequately braced some other way.
I'd like to see him do a rain screen under the shingles, and use a variable perm barrier inside but strictly speaking I don't think it's required in his area. Certainly the rainscreen I would do regardless.
There are 3 ways that I know of for doing a rainscreen with cedar shingles.
1. Install the shingles over a polypropylene mesh over the entire wall surface. Benjamin Obdyke Cedar Breather or similar.
2. Install vertical furring at 16" oc, and then put horizontal strips over that per the shingle coursing. He could cut the furring from scraps of his plywood.
3. Use shingle "panels" such as Shakertown's Craftsman Panel. These go on like lap siding and only require vertical furring strips, with no horizontal strips.
The third option is my favorite as it is less labor overall, but the materials may be more expensive. Anyhow, I hope these ideas are useful.
What is old school about comfortboard between the studs and plywood?
I think he probably needs diagonal bracing if he insists on that construction
However, I have seen houses built where the sheetrock was structural.
Not that I would do it, but I have seen it done.
Wind is definitely an issue on Cape Cod. Is he using diagonal bracing, either 1x4 or metal straps, to resist lateral loads? If not, he's making a big mistake.
Vapor management is less of a concern; ideally the framing would be protected from condensation with the mineral wool board on the exterior, but it's a lot easier to install shingles over plywood.
this is a poor use of the most expensive insulation you can buy....
Also rockwool has varying density throughout a board, I imagine his wall will be a bit wobbly...
I'm not as comfortable as others with the vapor management in the proposed wall. While plywood does act a bit like a variable vapor retarder, it is definitely the first condensing surface in that wall. The studs will probably stay nice and dry but I worry about the plywood. If the insulation is outboard of the sheathing you get both the structural benefits and the benefit of keeping the plywood warm and dry. Even a smart membrane on the inside probably won't help much, as plywood is not nearly as permeable as the membrane and moisture wants to move from inside to outside all winter long. And I do agree about the structural issues. Cape Cod is in a pretty serious hurricane zone. He'd better have an engineer design his bracing systems with that wall design.
Peter, approximately 99.9% of existing homes in cold climates have the cold-sheathing problem. Plywood has a higher perm rating than variable-permeance membranes and would help greatly with outward vapor drive.