Wall Assembly for Cedar Shingle Siding
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).
Thanks. This is immensely helpful. Since my posted question, I've been all over the map on approaches. Zip R6 sheathing is available now locally, so was considering that. But my engineer really is wanting plywood (as do I) due to the bracing needs we have with the design. In the end, I think I've decided on the 2x8, as that is only about $1000 more (than 2x6) total in framing to achieve the net R24 assembly, while a 2x6 with the 1" Zip R adds an enormous amount of cost to achieve nearly the same net R value and since I'm transitional between zone 4 and 5, I'm reluctant to not have R7.5 from the foam (as zone 5 calls for).
Any suggestions for the headers on a 2x8 wall (my engineer just asked)? I was thinking 1 2x, 1/5" polyiso, and then 3 2xs toward the inside (I'm doing mineral wool insulation in the 2x8 stud wall). Better to put the polyiso near the outside?
For best strength, generally you want your headers on the outside so they are nailed to the sheathing. So I would go for a two ply outside header (good part, this now comes straight out of code tables), 2.75" of rigid or a 2x4 batt squished followed by a single ply on the inside.
You can also leave out the interior single ply and fill the space with a squished 5.5" fluffy batt.
Depending on the details of your situation, keep in mind the option of NO headers. In other words, you may be able to use the rim joist of the floor above as a header. Another "no header" option is the plywood box beam. Both approaches are fully described in section 602.7 of the residential code. Also, you don't need headers in non-structural exterior walls. Remind your engineer of these options.
These approaches would allow you to use the identical insulation scheme above your openings as you use in the rest of the wall. Simple is sweet.
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
Another approach over battens is to use panelized shingles. They're a bit more pricey for materials but save considerably in labor. They can go right over vertical furring strips.
Shakertown's Cedar Cove is an example. But there are others.
http://shakertown.com/cedar-products/cedar-cove-shingle/
T-studs are another good option for simplifying thermal breaks (and whole-wall R-value): https://www.tstud.com/barenaked-tstud.
Built something similar a couple years ago... 2x6 (+Bonfig =2x8), Zip, mortair vent (similar to Obdyke), and shingles. Rockwool insulation.
If you're looking to save a buck price out the 2x4 size of Rockwool and see what the bulk discount is vs 2x8 size. A little more labor to install two batts vs one, but for me the savings were significant as 2x8 Rockwool was special order and the 2x4 was regularly stocked and discounted if you bought enough.