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Conditioned Crawlspace with added Rigid & Concrete Flooring

ntrail | Posted in General Questions on

Hello – 

I need to insulate a portion of my crawlspace (currently unconditioned) that will be conditioned and serve as a mechanical room moving forward. 

Right now there is a concrete half-height (~4 feet) wall above dirt (10-mil).  The plan is to put down 4″ of concrete on top of R10 over this dirt, with R15 (3″ of XPS) on the concrete wall portion – the question here is how to tie the wall into the ground layer pending. 

I think there are two logical approaches – see graphic here for current state and path A vs B options – delineated by putting a portion of the wall XPS down to the interface joint or not. 

TLDR – Is one of these (or another path altogether) recommended?  

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Replies

  1. matthew25 | | #1

    They both have a portion of the rat slab touching the uninsulated footing. Check out Jake Bruton's "raft slab" where they put the insulation under the rat slab and also over the footing for continuous coverage:
    https://buildshownetwork.com/contents/jake-concrete-raft-slab?page=168

    If you tape the seams I doubt you'll have the "floating insulation" problem that people usually bring up whenever the insulation is above the vapor barrier.

  2. Malcolm_Taylor | | #2

    ntrail,

    - As Matthew says, you want the foam to extend right to the stem-wall over the footings, both for continuity and to avoid the slab cracking from differential settlement.

    - I see no downside to going with option A.

    - Having the poly above the foam isn’t just to stop it from floating doing the pour, the main reason to do it is to avoid bleed water from the concrete pooling under the foam, which can weaken the concrete and take months to dry.

  3. Expert Member
    BILL WICHERS | | #3

    Another vote for Option A, but you do have a little bit of thermal bridging there (but less than in Option B). There are some details on GBA that show how to put a thermal break there.

    I would be sure to put reinforcing mesh in that slab. This can greatly reduce cracking, and doesn't add much cost. Do be sure that the remesh is properly supported during the pour so that it ends up in the approx middle of the slab though. I see this done incorrectly all the time, with the remesh ending up UNDER the slab where it does no good. The usual ways to support it are on wire "chairs", or tied to stakes or pieces of rerod pounded into the ground in a grid.

    If you'll have heavy mechanicals on there, rerod can help keep it from cracking. My concern is settling of the slab with the edges resting on the footings if this is a retrofit of an existing structure.

    Bill

  4. ntrail | | #4

    All this makes sense - I will push for an insulation moat (call this option A*) - thanks for the feedback and links.

    RE Poly below/above insulation - In general is the preference to have the poly on top of any insulation moving forward? The current call-out provided by the architect is to have it beneath for my build.

    RE reinforcing mesh - the plan is to put #4 rebar on 2-foot centers in the mix to give it a bit more strength overall.

    1. Malcolm_Taylor | | #5

      ntrail,

      Poly often gets called out below the foam as a way to protect it, but it should be placed on top for the two reasons I gave in post #2 above.
      https://www.greenbuildingadvisor.com/article/polyethylene-under-concrete-slabs

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