Double stud walls, again
I’m sure many of you are really tired of discussing wall sections so I apologize ahead of time. I have read lots of the blogs and discussions on here about walls, insulation, double-wall construction, etc. I’m designing a house in South-Central Pennsylvania (Zone 5 near Zone 4) and am having difficulty with the wall. My initial wall section was a 2×6 wall filled with R22 Roxul, sheathed with plywood, followed by 2” of R8 Roxul Comfortboard, a rainscreen and fiber cement siding. When we met with the builder, he looked at the wall section, said it was a “solid” wall, that it would be expensive, that we might as well build two walls, and that he didn’t think the insulation or siding guys would enjoy it—long screws, finding stud edges, window trim, etc. After more research, I don’t think R8 out the outside is sufficient anyway. So I’ve been redesigning the wall section to try to address constructability and cost while optimizing the wall.
I think the builder would prefer to just meet the Code requirement by putting insulation in the cavity. The builder’s solution to energy efficiency is to offer SIPS. While I admire the energy efficiency of SIPS, I think they are an over-engineered system that is too dependent on perfect installation techniques. I’d also like to avoid using foam for a variety of reasons. We are not trying to create a Passive House, but at least a pretty good house. (this is not yet fully defined, but better than code is a start.) I don’t think placing insulation in the stud cavity alone is going to meet our expectations. The builder seems opposed to adding insulation on the exterior, so it seems to me that a double stud wall is our option. This would make the siding and window installation more like the builder’s team is used to.
I started with Lstiburek’s ideal wall combined with some Swedish framing. See Image 1. Since the house will have a concrete floor, there’s 4” of foam at the edge that needs to be covered by something. (I had originally shown the insulation angled back to the wall to avoid that, but the builder didn’t think that was needed—I had shown 2” insulation, and he said the trim and drywall would cover it.) So I put the interior wall over the slab edge insulation (sort of like the one in Dan Kolbert’s recent post), and made the exterior wall the load-bearing one. I like the idea of having a resilient air barrier on the inner side of the interior wall as well as the idea of a service cavity. But this also means that, in terms of efficiency of material, we’re “wasting” a structural material there. There’s also a very difficult air sealing situation at the second floor joist. The wall has a max R value 45 with R15 inside the sheathing and R30 outside.
So, I angled the slab edge insulation, and made the interior wall the bearing wall. See image 2. This required making the foundation wall 10” instead of 8”. This seems to use material a little more efficiently (above ground), reduces thermal bridges, and seems like the air barrier details would be easier (located at the plywood on the inner side of the interior wall. However, the walls would have to be built separately with the inner one being fully completed and air sealed before the outer one. The air barrier at the windows might be problematic, assuming that the windows are placed in the outer wall. The wall also has a max R value 45 with R15 inside the sheathing and R30 outside.
On this last one, I have shifted more toward Dan Kolbert’s wall, eliminating the plywood at the inner side of the interior wall, the exterior wall as the bearing wall, and using 2 5.5” Roxul batts. See image 3. This is an attempt to make the wall easier to build. Does this work in climate zone 5? Is cellulose necessary to make this work? Would it be necessary to put an air barrier on the inside?
I would welcome any feedback. I’m particularly interested in constructability issues. What are your thoughts about tapering the insulation at the slab edge? Are there other suggestions for a cost effective way to get at least R30 walls?
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