Acheiving shear resistance in a wall with lots of windows, when using Zip R sheathing
I’ve designed a one-story, cathedral-ceiling timber-frame addition for my home. I live in climate zone 4A. It is a seismically active area (seismic zone D1) in Southern Illinois.
I’m leery of SIPS due to stories of moisture failures. I also doubt they’d provide the needed lateral resistance. After toying with more complex wall systems, I ultimately decided on simplicity- 2×6 studs with 2″ Zip R sheathing. After doing a lateral load analysis, an engineer said that the eave walls would be fine, but gable end walls wouldn’t be stable using Zip R because they have a lot of openings.
We COULD switch to true exterior insulation on the gable ends, then attach battens for mounting the cellular PVC board and batten siding. But that’d take more labor. Plus, I’d prefer to have the same window installation details on ALL of the walls.
I tried to get the engineer to let me get the shear resistance from a layer of wood T&G or shiplap on the INTERIOR of the studs, since we like how that looks (even on the diagonal to maximize shear resistance). But unfortunately, the engineer didn’t bite at that… likely because the actual resistance of such materials are hard to know (as noted in the GBA post at https://www.greenbuildingadvisor.com/question/vapor-open-exterior-sheathing). Despite its title, this post also yielded no high-tech “silver bullet” vapor-open structural sheathing.
The ENGINEER’S proposed solution is to put a layer of OSB directly on the gable wall studs with dense nailing patterns to handle most of the shear, then apply the Zip R on top of that with lower nailing density. I’m leery of this, since yet another GBA post (https://www.greenbuildingadvisor.com/article/permeability-of-oriented-strand-board) notes that OSB’s permeability, while it improves at the RH levels of a wetted condition, does not rise nearly as high as plywood at those levels.
Given all this, my intention at this point is to ask the engineer if we can use plywood rather than OSB for extra layer of sheathing on the gable ends.
Does anyone have a better suggestion?
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Replies
Tony,
Plywood sounds like a good idea to me.
I guess another option would be a small dedicated shear-wall, like Simpson Strong-wall.
There is a whole section of the IRC dedicated to different ways to handle lateral loads: https://codes.iccsafe.org/content/IRC2018/chapter-6-wall-construction#IRC2018_Pt03_Ch06_SecR602.10. As you can see, there are a lot of variables to consider, but if you work through it step-by-step, you might find a solution that your engineer missed.
https://basc.pnnl.gov/resource-guides/lateral-bracing-gable-end-walls might give you some ideas as well. To add on to Malcolm's answer, Simpson also offers site-built portal frames, which may be less expensive than the Strong-wall option. https://www.strongtie.com/products/lateral-systems/strong-wall-shearwalls/site-built-portal-frame-system
Thank you all for your suggestions.
I'm more talented with Sketchup than building codes, so I don't think I'll try to go toe-to-toe with an engineer on codes. And shedworkshop's resource on bracing gable walls was more about handling perpendicular wind loads than lateral racking. For performance reasons, I don't want to rob space from my wall cavity or increase the framing factor, so that rules out Simpson's Strongwall and portal frames. Steel Strongwall could work, but it would be a bit of a thermal bridge and might complicate the blowing of cellulose cavity insulation.
I don't mind a second layer of material to resist shear. So I guess I was seeing this more as a question about water/vapor management. Specifically, will an assembly of latex-painted drywall, cavity insulation, plywood, polyiso, Zip sheathing, and PVC siding (WITHOUT battens for the rainscreen gap so many GBA regulars talk up) dry in both directions? And is there some other material which would dry out BETTER than the plywood? I THINK Malcolm's answer as "Yes" to the first of those questions, and "No" to the second. But I'd welcome other more input.
Tony
That portal frame doesn't add much steel and most of it is not across the wall. I would at least ask your engineer if a couple of your window headers replaced with the portal frame would add enough bracing.
In terms drying capacity, it doesn't matter what you have behind the Zip R. The idea is the interior surface of the Zip R is warm enough that you don't have condensation (or at least not much) so having a layer of wood there, either CDX or OSB, won't change anything. Both OSB and CDX are also much more permeable than the foam on ZIP so any moisture that does make it to the foam layer can easily dry to the interior.
No rain screen can work with siding as long as it is a horizontal install. Vertically siding has issues with water wicking along the seams which is best handled by installing over a rain screen.
As Malcolm already said, plywood's fine.
The IRC code gives details for achieving quite a lot of shear resistance using simply prescribed methods, especially if you use the ABW or PFH approaches. You'll find these in R602.10.6. But I'm sure your engineer is well aware of them. For additional shear strength you can also use plywood on the inner face of the studs. But the code has no prescribed details for that. Engineers must work it for themselves (easy to do based on American Wood Council and APA guidelines).
I suggest you use a rainscreen approach to your exterior finish. That would be resilient and largely eliminate risk of persistent moisture inside the wall.
As far as SIPS, that ship may have sailed in your case, but when properly built (as is the case with any construction system) there absolutely shouldn't be water problems. The major manufacturers provide the relevant details for how to properly assemble them. The failures occur when contractors don't follow the guidelines. They are also perfectly capable of satisfying strong seismic codes. I've seen multiple projects in the San Francisco Bay Area using them.
I’m also seismic D1 and used Plywood for its strength and vapor permeability. Be aware of over nailing, especially in older lumber. Double the studs on all edges, even if not specified by the engineer as 2” and even 4” patterns will shred studs
Sorry for my delayed response, but I've now done enough additional GBA research (see below) to ask the engineer if I can just switch over to plain Zip sheathing on the studs with continuous insulation on top. The insulation will be held on with horizontal wood battens on top of strips of corrugated plastic to allow vertical airflow. I realize it's more labor, but it will give me a rain screen gap... which your input has identified as more important with vertical siding.
Thank you all for your insights.
Research notes from various GBA rabbit holes:
- This change moves my wall air barrier inward, and I'll be sure to tie it to the roof air barriers
- https://www.greenbuildingadvisor.com/article/installing-windows-in-a-foam-sheathed-wall was very helpful with window detailing. I lean toward "outie" windows in "picture-framed" openings, since I now have battens anyway.
- Diagonal battens or lattice battens with vertical siding
- Battens needn't be pressure treated