Foundation detail for double stud wall with FPSF
Hello.
We are designing our house with a frost-protected shallow foundation with 4″ of EPS for vertical insulation (maybe using warmform for this) and a double stud 2×4 wall system, with the outer 2×4 wall being the load-bearing wall (because this makes sense for our construction sequence). With 4″ of vertical insulation, we are struggling with how to position the exterior load-bearing wall without having an awkward step-off to the foundation insulation below. Even if we use a warm form with a tapered top edge, I don’t see how we can get the bottom plate anchor into enough concrete that the outside of the wall doesn’t land too far inside the vertical insulation.
I see how others have solved this problem by putting perimeter insulation on the inside of a stem wall or frost wall, but we are hoping to do this without a 48″ foundation wall (in order to avoid excess concrete and multiple pours). How have others solved this problem with a double stud wall system and a FPSF with 4″ or thicker vertical insulation?
Thanks for helping me not try to reinvent the wheel!
Jenny
ps – climate zone 6a (Maine)
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Replies
jenny_davis,
I'm not sure it being a double-wall affects much. The problem of dealing with the thick exterior insulation would be similar whatever wall you were using.
I'm also don't think the problem is a big as you fear. Suppose you overhang the exterior of the foundation with your plate by 3/4". Add 1/2" sheathing, 3/4" rain-screen gap, a 1 1/2" water-table at the bottom of the siding = 3 1/2" You are getting pretty close.
Thanks Malcolm. This is helpful. As for other wall systems, for example, our current home has 8" of vertical insulation on the foundation and 8" SIPs outside the framed 2x4 walls. So I can see how with SIPs or rigid insulation outside the framing, it's easy to get close to flush with thick foundation insulation. Will keep working on an elegant solution . . .
Thanks again.
Food for thought - you can do a FSPF with stem walls too, it would just be a much shallower stem wall than 48". Still requires two pours for the stem walls and the slab, etc. Just uses a bit less concrete than 48" stem walls.
Thanks. We have considered this. Seems like we would have to put code-minimum vertical insulation (r=4.5) on the outside of the stem wall in order to make it a FPSF, even if there were 4” perimeter insulation on the interior of the wall. That would certainly be less concrete than a full depth frost wall so maybe still worth considering.
Jenny, this is the detail I came up with, and a licensed engineer signed off on it. Ironically he would not accept the Warmform so we did a site-built system (as shown) instead. Basically the interior wall is structural, with an extra-thick mud sill and wall gussets to support the exterior wall.
At the second floor, I had transitioned the structural wall to the outer layer, but the engineer wanted to keep the interior wall structural all the way up.
Interesting he wouldn't accept the Warmform system, did he mention why?
If your inner wall is load bearing, where is your sheathing in this? It looks like it's on the outer wall.
For a FSPS in my area, the bottom of the footing needs to be 14-18" below grade, depending where the project is. The Warmform system relies on freely draining fill to reach those depths, and not all engineers are comfortable with that approach--they want to see something solid.
The inner and outer walls perform as a system so the sheathing/racking resistance is on the exterior and vertical loads are carried on the interior. That doesn't meet prescriptive code and may not work in a seismically active area. In the past I have designed metal strapping to be used as wall bracing, which is a prescriptive approach and would work with this design.
Thanks Michael for sharing this detail.
I don’t think this would fly in my area (seismic) as you said. This makes me think of a Larson truss through, is there a reason you went this route instead?
You might be surprised, or you might not: https://codes.iccsafe.org/content/IRC2021P1/chapter-6-wall-construction#IRC2021P1_Pt03_Ch06_SecR602.10.4.
Double stud walls are easier to build--fewer different steps; more time inside when framing through the winter as this crew was; there's a clear location and demarcation for the WRB and air control layer. I also like various forms of exterior insulation, including Larson trusses and other outrigger systems, but they are more complex, especially for a crew not used to building high performance homes.