Adapted double stud walls
Hi,
I am in climate zone 6 in Maine. I am looking to transition away from using foam in my wall assembly and I am (obviously) trying to optimize cost, performance, and durability. I have a sort of adaptation of a double stud wall that provides a good R-value and is cheap and easy to build. However, I am a little concerned about condensation. The wall, from the inside to the outside, is drywall, 2×6 dense pack cellulose cavity with studs 24″ oc, ZIP sheathing, the same 2×6 cavity with dense pack, 1.5″ rigid wood fiber, and then a rain screen. Should I be worried about moisture accumulating on the outer 2×6 studs or should they be able to dry adequately with the air gap near by? Attached is a rough plan view of the assembly.
I have read many of the other threads about double stud walls but if I missed something don’t hesitate to let me know.
Thanks
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Replies
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James,
One of the benefits that double-stud walls enjoy over those the simply use deeper dimension lumber is the reduction of thermal bridging through the framing by including a gap. Have you considered using 2"x4"s for whichever wall isn't load-bearing, or perhaps both, and keeping the wall the same depth?
With the inclusion of a rain-screen cavity, and high-perm wood fibre sheathing on the exterior, I think your wall will have no problems with moisture accumulation.
Curious why using double 2x6 studs if you are using dense pack cellulose? Assuming you are standing the outer wall up, sheathing and air sealing the inner face of that wall with osb, and then standing up the inner wall? Why not use 2x4 (or 2x3" ) inner studs and just space them at 5.5" from sheathing? Also, how are you detailing the air barrier at the top plate and rim joist? I'm just a home owner, so trying to understand why designed this way.
Edit, didn't see above reply
David, one reason would be to allow for thicker slab edge insulation. With 2x6 outer studs, you can cantliever them 2" over slab + the 1.5" wood fiber board. This allows for a 3"-4" space for slab esge insulation.
2x4s would be fine structurally but some code officials may not like it- even for a non load bearing wall.
Malcolm,
Thanks for the quick reply.
I have looked at that variation but wouldn't the outer layer of sheathing (between the outer stud and the rain screen) become a condensation plane and make drying difficult?
James,
David has made the same suggestion. No change in your proposed wall assembly except for the size of the studs.
A lot depends on which wall is load-bearing, and that can be informed by whether it is a slab on grade, one or two stories. If you could post a rough wall section from foundation to roof you will probably get more useful advice.
David,
The inner stud bay is our structural wall so the rim joist and top plate rest flush with the sheathing--it should be able to continue straight over both. I used two 2x6 bays so that I could attach the outer rigid insulation directly to the stud without having to introduce an outer layer of sheathing, which I worry would cause condensation problems (we'll see what Malcolm says).
James,
We are all happily posting at the same time.
I agree you don't want an additional layer of exterior sheathing, but don't see why that means you need a 2"x6" wall to attach the insulation to.
If I reduce the outer 2x6 to a 2x4 so that there is a thermal break between the two stud bays then I would need two layers of sheathing--one for each stud--for structural reasons. But, as we discussed, I don't want an outer layer of sheathing. Are you suggesting the outer stud could be floating there with no sheathing?
Yes. It gets attached to the inner-wall in a variety of ways. One simple one is to run the sub-flooring out from the second floor. Again a full section would work out these details.
Double stud walls with no exterior sheathing have been around for decades.
I assume you have a WRB over the wood fiber sheathing.
In terms of moisture management, the wall should be fine. The strong air barrier in the middle of the wall makes prevents excessive condensation within the wall, and drying potential to the exterior is strong. By the looks of it, your wall qualifies as wood fiberboard sheathing under a vented cladding by the IRC's standards, and so you only need a class III vapor retarder on the interior (e.g. latex paint) -- a further sign that this wall has very good moisture management. Just make sure you meet their definition for vented cladding.
I agree with Malcolm and David that the two 2x6 walls is a very strange setup. If you need 2x6s on the exterior as a cantilever, consider making the interior wall non-structural so as to use 2x4s.
I am also confused by the inclusion of zip sheathing inside the wall. Taped plywood would be able to the same job cheaper; even taped OSB is probably sufficient.
Edit: modified to catch up with new posts.
James,
There are so many variations of double stud walls, you may get overwhelmed with all the changes that get suggested. As Aedi said, you wall works, the rest of the comments we make are just tweaks.
Have you thought through how each wall gets dense-packed? That may also inform some possible changes.
You sure are right about that; so many variations. I may have to go back and do some more research and tweaking before I take this thread any further. Thanks (both Aedi and Malcolm) for the affirmation that it should work and for the suggestions on how to tweak it.
Ok, Still curious how this gets built if inner stud bay is structural, with sheathing attached exterior to that inner stud. Frame and stand up exterior non load bearing wall. Frame and sheath (and air seal) the load bearing stud wall. Stand that wall up. Add rigid fiber board to exterior and dense pack. Net interior bay and dense pack. Add dry wall.
Seems difficult to air seal at top plate and rim joist. Why not just put sheathing on inner face of interior stud wall after you stand that framing up, and have the wall and ceiling the air barrier. Fur out for dry wall so you have a space to protect your air barrier.
Is it just because it is easier to sheath the wall while it is on the floor prior to standing it up?
We actually prefab all of the walls. It's all done in a shop and trucked out to site which makes it much easier. Right now we use 2x6 walls, sheathing, and 6" EPS so the whole thing is assembled in the shop with the edges of the sheathing exposed. Then once the panels are on site and stood up they are air sealed (sheathing taped together) and the remaining EPS is put in. More info here if you're interested: http://thegohome.us/how-its-made/panelization/
Ok, I see. So are the 2x6 studs in your plan view above lined up and fastened together to give the wall sufficient rigidity to move it with out deforming? And how do you air seal at wall and roof junctures? I have struggled with how to air seal our remodel/retrofit, so it is a topic I am a bit overly focused on now.
David,
I think double-stud walls with the sheathing on the outer-face of the inner-wall were the brainchild of Thorsen Chlupp. https://www.greenbuildingadvisor.com/question/the-sunrise-home
The sequence is usually to frame and sheath the inner-wall as though it was conventional 2"x construction, then add the outer-wall as a space for more insulation - almost as you would Larsen trusses.
The advantages are: Continuous air-sealing of the sheathing including the rim joist, and continuous insulation outside that in the outer-wall.
If it is a two storey house, the outer-wall can be balloon framed and connected by brackets, or attached at the floor level by extending out the subfloor.
Lucas Durand did a variation on Chlupp's walls with no sheathing. His blog shows how he detailed the exterior: http://ourhouseuponmoosehill.blogspot.ca/p/details.html
Ok, thanks for the links Malcom. Air sealing still looks too complicated for me compared to being able to just air seal at wall and roof sheathing plane which can be done if using thick externally applied insulation. But if you live where it gets really cold I can see why double stud wall would be easier to build rather than screwing 8-12" inches of insulation to exterior. On our remodel addition I have had plenty of initial trouble screwing into studs through multiple layers of roxul comfort board. Thinking about switching to Larsen trusses when I insulate the original house walls.
David,
Lucas Durand's wall is probably pretty difficult to seal, but Chlupp's should be straight forward. Conceptually what you are doing is framing and sheathing a house much as you would any other. The exterior sheathing gets taped as the air-barrier, then a second wall gets built outside.
I agree about thick exterior insulation with furring. I haven't done a lot, but what I have done was time consuming and difficult.
Like Malcolm said, and in response to you question from earlier today, Chlupp's wall makes it very easy to air seal. The layer of sheathing in the middle of the wall assembly--plywood or ZIP on the inside framed cavity--can be taped directly to a sheathing layer that will go below the roof truss. This means the only real sealing that must be done is taping at seams.
Take a look at:
https://www.greenbuildingadvisor.com/article/monitoring-moisture-levels-in-double-stud-walls
Takeaway is with "standard" double stud walls, the sheathing accumulates moisture during the winter but dries out quickly in the spring.
I don't see the benefit of complicating the wall for a pretty marginal moisture issue. If you are worried, the extra money is better spent on getting moisture resistant sheathing (CDX or DensGlass).
Your idea of extra space for insulation around the rim joist is a good one. You can also accomplish that by building with a structural 2x6 inner wall over either 2x12 or plywood bottom plates and cantilevering the non structural outside wall. This would give even more depth.
"Air sealing still looks too complicated for me". I want to empasize that the moisture issue is directly tied to air sealing. The first defense against moisture issues is having a very tight house and keeping the moisture a) out of the house to start with and b)managed by correct detailing and ventilation. A leaky house with thick walls can become a great mold factory.
James,
One thing with the wall you propose, without offsetting the 2x6 studs or adding in a gap, your wall ends up around R37. Offsetting the studs is around R40, gaping the wall with 2x4 exterior studs R42. Either option buys you enough R value that you can reduce the exterior wood fiber to 3/4 and save a bit.
Maybe I'm missing something, but I still don't see how you can get a decent air seal in the middle of the wall with prefab unless you are danging plywood of the edge of the studs, which adds its own structural issues.