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Gravel vs. Sand Substrate for Sub-Slab Rigid Foam

bintou | Posted in General Questions on

Im doing a slab on grade with: 2″ foam insulation + 15mil vapor barrier and wire mash.
For the base my excavation guy put compact sand but I been reading that most use gravel or 3/4 crushed rock.

Are there any disadvantages of putting the foam directly on top of compacted sand vs gravel ? Im in a colder area by the way. Vermont
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  1. Expert Member


    There is no real disadvantage to using sand as fill as long as it is properly compacted - and sand does make installing sub-slab services easier. The problem is it also allows moisture to move up to the foam though capillary action. So while it's fine to use sand as fill, it needs to be top-dressed 4" to 6" deep of some clear-crushed aggregate directly under the insulation.

    To finally get round to answering your question: No, the foam should not be placed directly on sand.

    1. bintou | | #2

      So add crushed rock and then put vapor barrier then add foam on top?

      1. Expert Member
        MALCOLM TAYLOR | | #5


        No the vapour-barrier needs to go over the foam.

        1. bintou | | #7

          @Malcolm Taylor

          Vapor Barrier on top of foam? Wouldn't concrete not cure well or move if on top of plastic sheet like that?

          1. Expert Member
            MALCOLM TAYLOR | | #10


            The article I linked to explains what is in play.

  2. Expert Member
    BILL WICHERS | | #3

    It would probably be a good idea to put some geotextile fabric down between a sand layer and a gravel layer to prevent migration over time where the sand could clog the gravel.


    1. bintou | | #4

      Hi Bill - I dont have gravel on grade its just sand that is compacted. Do you think is necessary to add gravel on top of sand before I put the vapor barrier on top ?

    2. Expert Member
      MALCOLM TAYLOR | | #6


      I know what you are trying to avoid, but I'm not sure there is a mechanism to make the sand and rock mix. If the sand is compacted to sufficiently support a slab, it will also support the gravel. If the slab can exert enough force to push the gravel into the sand, it would have to be compacting the sand, and the whole substrate sinking. I've never heard of mixing as being a problem worth addressing.

      1. bintou | | #8

        So then putting the foam directly on compacted sand and is ok then ?

      2. Expert Member
        BILL WICHERS | | #9

        You're probably right, as long as there isn't a high amount of groundwater that could cause problems. I've seen some strange stuff when there is a lot of ground water (i.e. you could poke a hole in the sand and see it fill with water). I don't think I've ever seen issues with dry areas.

        For the OP, regarding your post #7:
        Concrete does not dry out to cure, the moisture in the concrete is used as one of the components required for the chemical reactions that are the 'cureing' process. This is why it's sometimes desireable to "tent" concrete, to cover it with poly, to prevent it from drying out while it's curing. There have even been some paving projects in my area where they keep a layer of liquid water on top of the concrete road surface while it cures to ensure the concrete doesn't dry out at all.

        You won't have any problems with concrete curing on top of poly, and the slab is going to weigh so much that it isn't going to slide around, eithere.


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