Cost-effective double stud wall with interior wall as load bearing?
For a two-story home with double stud walls in a cold climate, it seems you avoid a lot of thermal bridging if the rafters/TJIs for the second floor are supported by the inside stud wall. I have questions about how to do this “load-bearing inside wall” assembly most cost-effectively. The approach needs to be something a small crew with no crane can implement (so ballon framing is out).
Joe Lstiburek, worried about condensation on the inside of the wall, advises placing the structural sheathing on the outside of the inside stud wall, making it the primary air and vapor control barrier. But this seems a very labor intensive/expensive way to do things: (1) it requires a second layer of sheathing and house wrap on the exterior stud wall, and (2) it requires two steps to install the insulation (inside and outside the structural sheathing). It seems debatable whether the condensation risk warrants all this effort, and I’m wondering if the following wall assembly (which accepts the condensation risk) is a cost-effective alternative:
— build a standard 10.5″ double stud wall, with load on the inside
— put a class II vapor barrier on the inside of the inside wall,
— put the other control layers on the outside of the outside stud wall.
— let the cavity dry only to the inside.
— use metal strapping on the outside of the inside wall as sheer bracing.
This wall requires only one layer of (non-structural) sheathing, and insulation is done in one step, which seems more cost-effective than the Lstiburek wall. But unlike the Lstiburek wall, it requires installing the class II vapor barrier and the metal shear bracing. Thanks for any advice about the proposed wall, and in particular about how easy it is to build relative to alternatives.
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Replies
A double studwall with the interior studs being the structural wall is a Larson Truss wall, and it works. Details of the interface to the foundation will be an important performance factor. A discussion of the merits of double studwalls and Larson Truss walls would be Case 4 and Case 5, beginning at the bottom of page 37 in this document:
http://buildingscience.com/sites/default/files/migrate/pdf/BA-0903_High-R_Value_Walls_Case_Study_rev_2014.pdf
It's not clear what is meant by:
"-- put the other control layers on the outside of the outside stud wall."
???? Is this where you are placing the primary air barrier? If yes, what is it made of?
This would be a mistake:
"-- let the cavity dry only to the inside."
If you make the exterior sheathing vapor-impermeable it reduces the resilience of the assembly. When there is a Class-II vapor retarder on the interior, allowing the assembly to dry toward the exterior generally results in lower moisture content, and the greater ability to withstand incidental bulk water incursions, and incidental air leakage at the interior vapor retarder layer.
John,
I think you are better served by simplifying the construction so that the floor extends out to the exterior and building both walls on the second floor.
If you keep the floor back you you do get the benefit of reducing thermal bridging, but that is offset by the problems of increased complexity to build. The other advantage to keeping the floor back: that the sheathing on the inner wall acts both structurally and as a continuous well protected air barrier, disappears in your proposed assembly anyway.
The complexity comes from a couple of things: Rather than building two identical walls (often at once) with your method you end up having to build a completely separate outer wall that probably can't be built on the floor deck. Also, without balloon framing the outer wall you are left with a hinge at the second floor. You need to tie back this joint by either running the subfloor out, or providing some bracing back to the rim joist. Because the two walls will have differing amounts of lumber that can shrink across their grain you need to take this into account too.
There may be ways around these problems but I'm not sure this is a simple wall system.
John,
Dana and Malcolm have given you good advice. There are lots of ways to build a double-stud wall, but in all cases, it's important to think through (a) buildability, and (b) the question of the load path from the roof load, through the bearing wall, to the foundation.
If you are considering the Larsen truss approach, you may want to read this article: All About Larsen Trusses.
Thanks for the helpful responses. Malcolm highlights a buildability issue and Dana a performance one. On buildability, I should clarify that the TJIs would be hung from the interior wall, as in Wall 1 in the attached figure. My thinking was that this would be easier than Larson trusses because both the upper and lower walls could be built on the deck and raised. Is that wrong-headed? On the questions Dana raises, the details are in the attached Wall 1. I don't want to do air-tight drywall and wonder if there is an internal membrane that could make this work. Here I am following Paul Bieber's recent Fine Homebuilding article (but without the balloon framing). Bieber doesn't say what he does on the inside, and perhaps his wall details are asking for trouble with condensation?
http://www.finehomebuilding.com/membership/pdf/147939/021260032-balloon-frame-construction.pdf
An alternative that might be easier is attached wall 2: staggered double stud wall on a 2x8, TJIs resting on outside wall, zip sheathing and 2" of Roxul (I'm trying to avoid foam). Again, if the wall does what it is supposed to performance-wise, my focus is on understanding how easy it is to build. Seems like the stud wall is very easy and my understanding is that the labor and materials cost of Roxul climbs quickly with multiple layers and substantial thickness, but one 2" layer might not be too bad and will help with thermal bridging between the two floors. My hope would be with the right membrane on the internal wall, any condensation could dry to the inside (and of course can dry to outside with Roxul).
Thanks for any thoughts/reactions.
John,
Both your alternatives anticipate the problems I brought up. I'll make a few observations you have probably already thought about too, but maybe one has slipped by.
- Your second option involves both double stud and exterior insulation. Each involves a lot more work than a standard exterior stud wall. I'd be inclined to choose one approach.
- Ceiling heights are usually set at 8'-1" or 9'-1" to facilitate the standard dimensions of drywall. Your first option means to achieve this you will probably need to individually cut all your wall studs. The second allows you to use pre-cut studs but you would need a second top plate to hit the required height - although walls that are 1 1/2" too short are a lot better than 1 1/2" too tall.
- If you are comfortable using zip sheathing on the staggered stud wall, why not use it on the double stud wall and forgo the diagonal bracing? There appears to be a consensus that as long as you don't use OSB and include a rain screen, that the sheathing will be safe without the exterior mineral wool.
Thanks so much. At the risk of taking too much advantage of your generosity, I have one last question. You write:
"- If you are comfortable using zip sheathing on the staggered stud wall, why not use it on the double stud wall and forgo the diagonal bracing?"
I'm not sure I understand. If the inner wall is load bearing, I thought you can't put the structural sheathing on the outer wall? If you make the outer wall load bearing with Zip, then you have the problems of thermal bridging and of insulating between the TJIs. If you mean put the zip on the interior wall, then you're back to having to nail two layers of sheathing and installing insulation twice? My hope was to avoid two layers of sheathing and doing insulation twice. Sorry for my confusion, and thanks again for your willingness to engage about this.
John,
If you are tying the two walls together with 1" of plywood, there isn't much difference in how they act from a staggered stud wall. We probably wouldn't get away with sheathing the outer-wall here in the PNW because of our heightened seismic concerns, but almost everywhere else I'd bet it doesn't matter.
Malcolm,
Thanks again for the useful advice, to me directly, and indirectly as well (though your responses to others).
John
John,
I take a lot more from this site than I contribute. Good luck with your build.