Lstiburek double-stud wall: What does outer wall sit on?
Hi all,
I’m preparing to build a small (32 x 22) single story house in central Vermont next summer, and debating wall assemblies. I’m interested in doing double stud construction, and in doing my research I’ve come across numerous articles about Lstiburek’s preferred double stud wall, with the bearing wall on the inside, sheathing on the outside face of the inside wall, etc. I like it! But, my question is, what supports the outer wall?? I’ve seen it drawn as a 2×4 wall and as a 2×3 wall, but I haven’t seen any detail about what it’s sitting on or hanging from. I realize it’s not a bearing wall, but surely it’s not just hung from the top plate…right? Maybe this is a detail intended to be figured out on a job-by-job basis. I don’t really want to increase the size of the foundation, or add a brick ledge, or anything like that. Maybe cantilever the floor joists over the stem wall?
Thanks in advance!
Sam in Vermont
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Replies
Sam,
I believe I've heard/read Joe Lstiburek describe the outer, non load-bearing wall, as "hanging" from the roof. I'll see if I can track down a source for this.
Ultimately, I think the bottom plates of the outer wall would tie in to the slab edge insulation. If you are using a stem wall system, then here is a way to build Joe's ideal wall (see attachment). This is from the award winning Rochester Passivehaus:
https://rochesterpassivehouse.blogspot.com/p/our-passive-house-some-of-goals-we-had.html
Basically, they set the interior load bearing wall further in from the rim board. They also oversized the mud sill to allow the outer wall to bear on it. Great design, in my opinion.
Hi Rick, thanks so much for your response! That is quite an assembly! It looks great. I don’t think I have the budget for all that, and we’re not aiming for passive, but that gives me some good ideas to go with, especially the oversized sill. If you do come across the source of the ‘hanging’, I would be very curious, especially if there’s a drawing/detail. Thanks!
Rick,
I can't speak for Joe Lstiburek, but lots of wall designs include an exterior framed component that hangs off the inner bearing wall. For more information on one approach -- the Larsen truss approach -- see "All About Larsen Trusses."
A primary difference between Larsen Trusses and other double studwalls is that with the Larsen Truss usually only the interior studs are structural, directly above the foundation and supporting the rest of the house, whereas with most other double studwall designs it's the exterior course of studs that are structural.
Of course it's possible to design it for both sets of studs to be structural, but it's simpler analysis and easier to get code sign-off if it's just one or the other. The second set of studs are still providing some structure even when it's resting on joists not directly above the foundation but there's no need to do the math to prove how much of the load is being carried by the off-foundation studs if the primary set of studs meets the load requirements.
With Zip sheathing attached to the inner studs, that is going to act as both the structural support wall and the braced wall for lateral shear from wind and seismic forces. I don't love the detail above, because the load-bearing wall is not sitting on the rim joist. This makes shear strapping tiedown very difficult, and depending on the floor truss design, that might actually have the load bearing wall sitting just on the plywood subfloor. That's not good. You have to make sure your load paths are managed as well as the air barrier layer.
If the inner wall is truly carrying all of the loads, then the outer wall can actually hang from the ceiling joists. Hurricane clips from the rafters/joists across the top plate and onto the studs would easily carry the weight of the outer wall.
Thanks all, that is helpful. Peter, have you seen a drawing floating around of what it would look like to hang the outer wall from the ceiling joists or rafter tails?
Thanks,
Sam