What’s the “Good Enough” wall assembly under cedar shingles?
I’m doing a major addition to our smallish house in the DC area of Maryland (Climate Zone 4). We are using cedar shingles (WRC) for siding, the shingles being installed over classic slicker by B. Obodyke, applied over Henry Blueskin VP100 applied to the 1/2″ 5 ply plywood sheathing. I would like to have a thermal break in the studs, and I would have wanted to use continuous foam on the exterior, but I don’t want to have to put a second layer of sheathing on the outside of the foam for the shingles. My options seem to be a double stud (2×4) wall structure with the appropriate gap, or a staggered double stud wall on 2×8 plates (which seems like more work and heavier walls to raise than doing a double stud wall). Any thoughts or advice while I’m at the drawing board? Thanks!
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Heat flow doesn't care about thermal bridges, what matters is whole assembly R value. Of course it is always better to aim to reduce thermal bridging but if you can't don't sweat it.
In your milder climate, a standard 2x6 wall with high density batts (about R18 assembly) is good enough:
https://effectiver.ca/calculator/wall.php?id=3954
If you want better, you can always go up to 2x8 24" OC which is close to an R24 (using R30 batts) assembly:
https://effectiver.ca/calculator/wall.php?id=3964
2x6 24" OC is about the same weight as 2x4 16" OC, 2x8 24" OC is a bit more but not a whole lot. Unless you are using something heavy like 5/8" gypsum sheathing either wall is relatively easy to lift up.
Staggered studs tend to be a pain to insulate unless you are dense packing. I would avoid it.
Going over plywood a fully adhered WRB is a will definitely work but bit overkill. Taped seams on the plywood followed by regular sheet WRB is also good enough. You can also use one that has the built in drain mesh (ie Slicker HP).
Apparently it's possible, but maybe labor intensive, to install shingles over foam using battens:
https://www.jlconline.com/how-to/exteriors/sidewall-shingles-over-foam-sheathing_o
https://forums.jlconline.com/forums/forum/jlc-online-expert-forums/exterior-details/33314-cedar-shingle-siding-over-xps-foam-sheathing