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Double wall over stepped concrete wall

user-7114797 | Posted in General Questions on

Hi my name is Matt, from zone 5a in upstate NY.

I’m designing/building a PGH on a challenging lot, requiring a stepped frost protected shallow foundation

I’ve attached my CAD details & the exterior elevations. The detail I’m most concerned with is where our double stud wall meets 8″ concrete walls above the slab.
Could anyone advise how they would handle these areas?

Some of these detail notes are incomplete as I’m working on them now, but they should clearly convey the situation for the framed basement walls.
The slab is the same level throughout the basement.

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  1. GBA Editor
    Martin Holladay | | #1

    Can you give us a specific question? Examples include, "I'm worried about air sealing the vertical crack between framed walls and a concrete stem wall," or perhaps, "I can't figure out whether to make my interior wall the bearing wall, or to make the exterior wall my bearing wall."

    Unless you state a question, it's hard to help.

  2. user-7114797 | | #2

    Hi Martin,

    I suppose it helps if I give a coherent question.

    I'm unsure how best to isolate moisture from the concrete walls from entering the framed walls, horizontally & vertically.
    And then how to sheetrock flush over both wall systems.

    For the horizontal, initially I figured we could run the 6mil sub slab poly up and over the concrete wall, ending under the z-flashing. The anchor bolts would poke through the poly, but that seemed like a good solution still.
    The 2x10 pressure treated plate hangs 1.25" to the interior, and I can put 1" and .25" sheets of XPS there to flush things up. The XPS would be fastened to the concrete (through the vapor barrier) and then the sheetrock would be glued to the XPS.
    Does this sound functional and buildable?

    Vertically, the seam between framed walls and concrete walls, I don't have a solution yet. Any advice there would be helpful too.

  3. Jon_R | | #3

    I'd give some thought to what happens if you get vertical differential movement between the slab and walls.

    An alternative is here:

  4. GBA Editor
    Martin Holladay | | #4

    Q. "I'm unsure how best to isolate moisture from the concrete walls from entering the framed walls, horizontally & vertically."

    A. The usual approach is with sill seal, which forms a capillary break and helps with air sealing. For horizontal joints, gravity helps. For vertical joints, the sill seal works fine as a capillary break -- but not so well for air sealing, because you don't have the beneficial effects of gravity. So on the vertical joints, additional caulk or a high-quality European tape would help with air sealing.

    Q. "How to sheetrock flush over both wall systems?"

    A. Shim as needed -- with either rigid foam or wooden shims -- to get everything co-planar, and then install your drywall. Make sure that you have enough R-value in your concrete stemwalls before you begin shimming.

  5. seabornman | | #5

    Your knee wall needs to be designed as a retaining wall. It's taking soil loads, wind loads and vertical loads. It might be easier to make those walls like your other basement walls (full height) and put windows through them where needed. Simplifies your detailing also.

  6. Expert Member


    Since it's your outer stud-wall that is load-bearing, i'd consider either using a wider flashing (or putting a cant strip in your forms to slope the top of your foundation wall) so that the outer-wall falls where the inner one is now. That allows the inner one to be continuous from floor to ceiling, making the framing simpler, providing a cavity for both more insulation and services, a path for drains,and solid backing for drywall and trim.

    I'd also consider moving that basement door on the right elevation away from the concrete wall for easier detailing.

    Good luck with your build.

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