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Slab on Grade Construction

B W | Posted in Green Building Techniques on

I am sure this question has been asked a million times and I apologize beforehand for any repetitiveness. I am starting a house build within a few weeks and need to know the correct/best way to construct the Slab on grade. I am in zone 5, a mile from the ocean (Rhode Island). I am building a 2500sq ft, one-level home with SIP walls. The slab will be 4″ thick and have PEX tubing for heat. Although I have done a ton of research, I am confused as to the sub-slab construction. As I stated, the slab will be 4″ thick and have 2″ XPS on the bottom and sides. Should the 6mil poly (or equivalent) be under the XPS and then gravel or stone? Should there be anything else I should consider? I just want to make sure this is done right as I’m sure you can imagine, this will be the only time to get it right. -Thanks-

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  1. User avatar GBA Editor
    Martin Holladay | | #1

    Who is in charge of the foundation work -- you or a concrete contractor? If this is your first slab, you should know that concrete is unforgiving of errors and there are a lot of ways to make mistakes.

    Is this a thickened-edge slab or have your already poured frost walls? Perhaps it is a frost-protected shallow foundation? There are a lot of ways to do it, but you can't just place 4 inches of concrete on your subbase without considering perimeter loads and frost action.

    The poly vapor barrier always goes directly under the concrete -- never under the rigid foam.

  2. John Klingel | | #2

    "The poly vapor barrier always goes directly under the concrete -- never under the rigid foam." Which is a topic of debate, actually. B.W: Search here and various other places about that. I don't see that the issue is clear cut yet.

  3. B W | | #3

    I will be having 4' frost walls (on a 16" x 8" footing) 10" thick...5.5" for sills and a 4.5" ledger for 2" of XPS and 2.5" for the concrete slab (which will also be pinned). My question is the subbase makup...dirt then 4" (?) of gravel then 4" (?) of stone then 6 mil poly then 2" XPS and finally the 4" concrete slab? I am doing the slab with a relative that works for a concrete contractor and I just don't want to make any mistakes. -Thanks-

  4. John Klingel | | #4

    BW: I am not a pro here, but that sounds like what we have always done, and what I see done and read of. Make sure you remove any disturbed dirt, then compact whatever gravel you install, usually in 4" layers (lifts) at a time. The poly under or over the foam is your call. Mine will go under, as others do, but, as Martin said, it is recommended by some learned building science cats to have the poly against the slab. The important point there, as I see it, is to not have ANY WAY for any water to get against the slab and wick up. That will kill materials put on slabs that are not vapor/water open.

  5. User avatar GBA Editor
    Martin Holladay | | #5

    Don't install any dirt inside your frost walls; only install gravel or stone, free of organic matter, carefully compacted with a tamper in short lifts. At least 4 inches of clean crushed stone is a good idea, followed by horizontal rigid foam, with vertical foam at the perimeter, followed by the poly vapor barrier, and then your concrete.

  6. John Klingel | | #6

    Oh. And don't forget radon pipes, if that is an issue in your area. Also, if you are heating the slab and are in a very cold area, more than 2" of foam may be advisable. Maybe 6 or 8".

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