Are there any examples of Joe Lstiburek’s Ideal Double-Stud Wall in the wild?
This is the wall I’m talking about: https://buildingscience.com/documents/enclosures-that-work/high-r-value-wall-assemblies/high-r-value-double-stud-wall-construction?topic=doctypes/enclosures-that-work/etw-high-r-value-enclosure-assemblie
It’s a double stud wall with the interior studs bearing, and sheathing applied to the exterior of the interior studs to form the air barrier layer in the middle of the insulation.
When I’ve seen this wall get brought up in the past, worries about constructability issues are often raised for two-story (or higher) buildings.
I was curious if anyone was familiar with a project that used interior bearing double studs and knew how the constructability issues played out in the field. Bonus points if the build is documented online!
I guess conceptually, this wall isn’t that different from a larson truss wall, but it seems like the way the larson trusses hang off the interior wall potentially simplifies construction compared to the essentially balloon-framed exterior wall in Joe’s design. But perhaps not! I’m no framer.
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Brendan,
Good question. I'm adding this comment to bring your question near the top of the list, so that other GBA readers might see your question and share what they know.
Brendan,
I think you've identified the main drawback to the wall: it's a lot harder to build when the project is over one storey - if you want to balloon frame it. The work to the exterior wall can't be done on the floor deck, and the wall has to be assembled stud by stud. One solution is to break up the exterior walls and connect them with the second floor sub-floor. They can then be framed before the interior walls, and all the work done on the floor deck.
The other problem with moving the structural sheathing to the interior wall, is what to do on the exterior. None of the alternative are very palatable. You can spring for more plywood, which is expensive. Fibreboard or no sheathing also have their attendant drawbacks.
Larson trusses could be applied to the interior walls before standing them, eliminating a lot of ladder work, but you still have the same choices to make about exterior sheathing. With Larson trusses you also may face approval problems, as Truss joists aren't typically designed for vertical loading.
Brendan,
In the absence of any built examples, you might want to look at the details proposed in this thread:
https://www.greenbuildingadvisor.com/community/forum/green-building-techniques/95256/cost-effective-double-stud-wall-interior-wall-load-b
Thanks for that link Malcolm.
I wonder if this could be a simpler way to achieve the mid-wall air barrier and vapor retarder of Listeburek’s detail with simpler build ability.
2x6 loadbearing exterior wall with air barrier and vapor retarder membrane on the interior of the 2x6. Then an interior non-bearing 2x3 or 2x4 wall. The ratio of fluff outside the membrane to fluff inside the membrane would want to be appropriate for the climate zone to keep the membrane warm enough to avoid condensation on the inside of the membrane.
Does this seem like a good idea? Are Joe’s concerns about conventional double stud walls overblown?
This is all hypothetically speaking, but I think it’s an interesting discussion!
.
Just spitballing here with my lack of practical framing knowledge... but if you supported the floor joists on a ledger then running your air barrier up to the deck above is easy peasy. This detail is like what you see when you google “Swedish platform framing”, but adapted to a double wall.
What I don’t know is what new issues supporting your floor on a ledger board like that raises.
Brendan,
The permutations are almost endless.
From a framing standpoint your first detail is simpler. Both walls are the same height and can be built and stood at the same time. It may make the air sealing more difficult though. Typically these types of double walls incorporate something to tie them together, like common plywood top plates or gussets.
Supporting the joists on ledgers does mean a bit more work, as instead of a rim-joist you need to block in between them for lateral stability. Is there any real advantage to running the joists through to the exterior wall to support them, rather than just bearing on the interior wall? That leaves the exterior face of the inside wall continuous, except for the second floor subfloor, and the air barrier can be placed there.
Thanks for bearing with my permutations Malcolm :)
I think now that you've said it a second time, the solution you proposed in your first post:
"One solution is to break up the exterior walls and connect them with the second floor sub-floor. They can then be framed before the interior walls, and all the work done on the floor deck."
Finally sunk in and makes sense. That seems like a nice simple solution to the platform framing issues. Took me a while to realize what you were saying haha.
Say you're doing the method you described, with the interior wall bearing the floor joists, but the subfloor extending out to the exterior wall: If you're on the second floor, and you've built the exterior non-bearing wall, and now you're ready to build the interior wall, how do you attach the air barrier to the outside of the interior wall? Would you attach it to the interior walls when they are flat on the ground then tilt them into place? How do you seal the sections of wall together from the inside? Could you tape from the inside, or get on a ladder and do the final bit of taping from the outside?
Thanks again!
Just to triple check I’m following, attached is my understanding of what you’re describing.