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Slab Insulation

Jeff_Garnelis | Posted in General Questions on

Hi all,

 I am in Connecticut, Zone 5. I am building a house with a walkout basement. My original plan was to use 2inches of Halo’s Subterra GPS foam under the slab, but I was unsure how to insulate the edge of the slab on the walkout side. I was doing some digging and found this article:

While reviewing the article, I noticed that Steve Baczek suggests putting the insulation above the slab. This would allow me to thermally break the slab from the foundation and would allow a “cushier” walking surface in the basement.

Is there any downside to putting the insulation on top of the slab? I am still leaning towards putting the insulation below the slab strictly because putting two layers of Advantech across the floor of the basement would be very expensive, but I am still curious. Thank you in advance.  

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  1. Jeff_Garnelis | | #1

    I am also curious if its necessary to do the vertical insulation to thermally break the slab. Should I just pour the slab all the way to the walls? If I install the foam, I feel like it will not sit flush against the foundation walls, allowing radon to escape from behind the insulation. I've seen guys wrap the poly up the wall and use Siga's Fentrim tape to seal it against the foundation. Is the cost benefit worth taping the poly up the wall?

  2. Expert Member


    Except for renovations where there is no sub-slab insulation, I'm not sure what moving the foam above the slab gains you.

    Steve Baczak's detail works but can cause some complications. I would suggest instead that if you decide to use foam above the slab that you:
    - Drop the slab so that the top of the sub-floor is level with the bottom plate of your exterior walls. Otherwise you a) up with the exterior and interior walls being differing heights, and b) need taller walls (both concrete and framed) to end up with the same ceiling height.
    - Figure out how you will frame interior load-bearing walls that need to go down to the slab or interior stem-walls.

    The slab edge foam will be buried under the interior insulation on the regular basement walls, and can be hidden on the walk-out portion as shown in the link you posted. It is definitely necessary, unless you have exterior insulation on your stem-walls. Concrete has an R-value of 0.1 per inch. Un-insulated that leaves you with a huge thermal bridge. The thickness of that slab-edge insulation may be specified in your code.

    Concrete slabs should always have a slip-surface between them and the stem-walls to deal with differential movement and shrinkage. You need to use the poly as your air-barrier and radon seal, so it makes very little difference whether there is foam there or not.

    1. 5Stud | | #3


      I am usually impressed with your answers.
      This one is right up there!
      I love Baczek but sometimes ...

  3. Expert Member
    Michael Maines | | #4

    If you're considering a concrete-free slab, you don't have to use two layers of Advantech. A single layer of Advantech over 2x4 sleepers also works.

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