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sub slab insulation and vapor barrier over center-beam footers?

mmoogie | Posted in Energy Efficiency and Durability on

I know this has come up in various forms before, but can’t seem to find a definitive answer by searching. I doing an ICF crawl space, 24 x 60. There will be a center beam for the floor platform. I’ve spec’d four sub-slab piers down the middle of the floor. Foundation guy is planning to just scoop out some rock at those spots and leave out the 2” sub-slab foam and poly and pour a monolithic floor and internal piers. I’m feeling uncomfortable with the break in insulation and particularly the break in the vapor barrier. What’s the best way to handle this detail? Foundation plan attached below.


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  1. plumb_bob | | #1

    If you can cast the top of the pad footings to the same elevation as the top of your wall footings then your poly sheet and rigid insulation details should line up well. Your drain rock can then get leveled to the top of the footings, the insulation laid down and cut around the piers, and the poly goes on top of everything and sealed to the walls and around the piers.

    My understanding is the ICF wall acts as both a air and vapour barrier, the poly sheet should not have to cover all of the ICF walls as shown in the detail.

  2. mmoogie | | #2

    Perimeter footers are poured. Trying to pour the internal footers and the slab in one pour as we are racing the clock on weather. Trying to decide if it’s worth annoying the foundation guy to do other than as a one-shot pour.

    XPS is only a class 3 vapor retarder, not a vapor barrier, I believe, as the perm rating for 2” XP’s is greater than 1.

  3. Expert Member


    What about this?

    Pour the interior pads low enough that you can run the sub-slab vapour-barrier and insulation through continuously, then cut out an 8"x8" square where the posts will go. You will have a very minimal lack of continuity.

    Edit: I think plumb bob and I have described the same thing.

  4. Expert Member
    Akos | | #4

    24' is easily spannable with I-joists or trusses. If you look at the cost and labor of the center beam, columns and concrete, you are probably ahead plus you get a much flatter floor with engineered joists.

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