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Community and Q&A

Insulating Wall Between two Different Slab Elevations

I have a basement with 2 different slab elevations with 2′ differential between the two. There will be a stair with 3 risers connecting them. I am insulating the basement from the inside. We are getting ready to pour the slab next week and in preparation I have placed 4″ of EPS on top of the sub-slab stone. I am trying to decide how to best insulate the concrete wall between the 2 elevations where the stair will be. While I am planning on using 2″ of roofing polyiso for the basement walls, I am am thinking I would be better off using something with a higher compressive strength where the stair will be. I am trying to avoid XPS, I do have a 25 and a 60 PSI sheet onsite that I can use for this application. I am using GFRC without wire mesh for the slab and I want to avoid placing the concrete is a compromising position. I am thinking I should adhere the XPS to the wall between the slabs and have it extend 4″ above the top of the EPS I have already placed on the upper elevation to match the final height of the upper slab. That way the edge of the upper slab stops at the edge of the concrete wall dividing the two as opposed to extending 2″ over and resting on the wall insulation.


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  1. User avatar
    Dana Dorsett | | #1

    Why do you think it needs higher compressive strength? (It's not as if it's footing for an 8' tall concrete wall that's bearing the weight of the house or anything...)

  2. Jonathan Lawrence CZ 4A New Jersey | | #2


    Its not so much the slab pressing on the foam that I am concerned about, but the stair compressing against the foam (plus drywall) where it will be attached to the wall. I guess once it is attached and torqued into the wall, the future compression from people walking on it would be minimal anyways. I still plan on using the roofing polyiso for all the other walls.

    Where I would be concerned with compression as I noted above is if I stopped the vertical insulation at the height of the EPS and then used a nailer to keep the concrete in place when poured. That 2" cantilever over the wall onto the foam seems like a possible recipe for cracks. My mason prefers this method.

    Btw, I do have some steel columns that are bearing the weight of the house sitting on polyurethane bearing blocks to act as thermal breaks. Those are 1,000 psi, but they make them as high as 4,000 psi. They are a lot easier to install than insulating the footings, which I also did for 2 other columns why I have the leftover XPS.

  3. User avatar
    Dana Dorsett | | #3

    OK, how heavy is a poured concrete 3 riser staircase? It's minuscule compared to an 8" foundation wall with a house resting on top.

    Or maybe I'm not understanding the problem- a picture might help.

  4. Jonathan Lawrence CZ 4A New Jersey | | #4


    I did a quick Sketch-Up, which does a much better job of describing the issue than my written words. I did not add the stairs, but they are metal stairs that would be attached through the wall foam into the concrete. I don't see any thermal bridging issues with this stack-up and I think the upper slab is better served if not resting on the vertical foam as shown. My mason disagrees.

  5. Jon R | | #5

    I don't see the problem with a 2" cantilever in concrete - with foam (or air) under it.

  6. Malcolm Taylor | | #6

    i agree with Jon that a 2" cantilever isn't worth worrying about. If you do run the foam up, it's worth remembering that once a stair (or roof rafter or any similar diagonal element) is secured at the top and bottom there are only vertical loads at both ends, no horizontal ones. So the force will be downwards trying to bend or shear off the bolts, not trying to compress the foam.

  7. User avatar
    Dana Dorsett | | #7

    Even a 4" cantilever (maintaining the thickness of the sub-slab EPS at the vertical section) is not a problem here.

  8. Jonathan Lawrence CZ 4A New Jersey | | #8

    Thanks for all the comments and I am now comfortable with the cantilever if we go that route.

    I finished placing the sub-slab EPS today so hopefully we will get this poured next week.

  9. Malcolm Taylor | | #9

    Does the metal stair extend right up to the slab? Most stairs are one riser short so that you don't end up with an extra tread at the same height as the upper floor. That would probably the stair attachment would be well below the slab.

  10. Jonathan Lawrence CZ 4A New Jersey | | #10


    I was envisioning that connection to be made on the vertical wall portion, not the upper slab. Maybe a steel ledger bolted to the wall that the stringer is then welded to. I have another steel staircase going between the 1st and 2nd floor and I was going to have the fabricator make this 3-riser stair as well.

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