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no vapour barrier under slab?

Trevor_Lambert | Posted in General Questions on

The engineer who designed my insulated slab is giving me advice contrary to what I’ve read on here, and frankly contrary to what makes intuitive sense to me. When I asked him about a vapour barrier, because none was shown on the drawing, this is what he replied: 

“We don’t generally put vapor barrier under shop floors, particularly with the rigid insulation in place.  You could tape the seams of the insulation if you want to be extra thorough.  It’s not even all that common to put vapor barrier under basement floors in Ontario.  It makes a big difference to the person finishing the floor as it takes a lot longer for the bleed water to disappear when it can only go up.  Regardless, you’ll want to talk to have a plan how to best cure the slab. Keeping it wet and covered with tarps for a couple of weeks is the best practice.  Otherwise you risk having the slab ‘curl’ due to differential drying.  If you do decide to put poly vapor barrier down it should go on top of the insulation before the gravel topping.”

I think the concern about “bleed water” could be addressed by just adding less water. It makes more sense to me to have the proper amount of water for curing, and try to keep it in there, rather than having extra and wanting it to drain away. But I am no concrete person, that’s just my intuitive take. I’d appreciate some expert feedback on the advice above.

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  1. Expert Member
    Michael Maines | | #1

    You are correct in that it's better to use less water than to eliminate the vapor retarder. The concrete contractor can use water-reducing admixture to make placement easier. The engineer is correct in that a slab left open to the air is at risk of curling. I always specify damp-curing for a week, meaning covering with a tarp, or ideally with burlap that is misted a couple times a day. A tarp can lead to laitance, a whitening of the surface from standing water, whereas burlap allows evaporation, just more slowly than happens with a slab open to the air. A slow-cured slab is stronger and less likely to crack than one that is allowed to dry quickly.

    In a shop space, aesthetics and vapor-retarding probably aren't as large a concern as they would be for a home with polished concrete floors. Taped foam is probably ok as the vapor retarder, but a membrane is more reliable.

    If your engineer is so concerned about bleed water, maybe ask them what they recommend for a water-reducing admixture.

    1. Trevor_Lambert | | #2

      Laying a sheet of poly would be a lot less work and probably cheaper than taping the foam. The pieces are only 21.5 x 63.5, so that's a lot of seams. But what do you think about the gravel going on top of the poly? That would be easier to do, but just seems wrong to me.

      The parts about keeping the concrete from drying too fast are questions I never asked. I think I'll leave those details to the concrete guy. I'll get his opinion on the poly placement as well and take that into consideration.

      1. Expert Member
        Michael Maines | | #7

        I don't like it but it's probably ok.

        Did your engineer spec a water-cement ratio? If not, they are leaving too much up to the concrete contractor. You can ask your engineer for a specification for reduced water content.

  2. freyr_design | | #3

    Your engineer is wrong and should stop detailing slabs this way.

    Here are some resources:

    Just because it’s a shop does not mean you don’t want vapor protection in your slab. I dont know what kind of tools you have but they are generally more susceptible to rusting than most other things we deal with day to day.

  3. Expert Member


    Bleed water under slabs is only problem where there is a void for it to accumulate - like when the poly is under unsealed foam. If the concrete is poured directly onto sealed foam or poly there is nowhere for the water to pool, it stays in the slab and any excess not needed for the cure diffuses through the top.

  4. Trevor_Lambert | | #5

    Ok, I am convinced that poly is necessary and that it should go directly beneath the concrete. I'm leaning toward 10 mil poly since it's going on top of gravel, but would two layers of 6 mil be as good? It seems much more readily available.

    1. Expert Member
      MALCOLM TAYLOR | | #6


      As it is a workshop it is outside the OBC requirements for sub-slab vapour-barriers so the thickness is up to you. If you want it to also be a radon barrier it needs to be continuous and sealed.

      1. Trevor_Lambert | | #8

        Keeping radon out would be nice, so I'll try to keep it perforation free.

  5. tjanson | | #9

    The reason why the bleed water taking longer to evaporate can be a concern, is it takes longer to finish the slab. Concrete gets placed, screeded, and bullfloated. Then you wait for the bleed water to dry up. This can take a few hours. Then you mag float it, and then you steel trowel it. You can't mag float it before the bleed water dries up or you work the water into the surface of the concrete which weakens it. The extra time to wait for the water to evaporate could add some labor cost from the crew, or maybe other problems.

    Just for reference, 4000 psi, no air, 5-6" slump, mid-range water reducer, 3/4 aggregate is what I ordered for my hard-troweled addition slab.

  6. Trevor_Lambert | | #10

    Does it matter what type of gravel is between the insulation and the slab? The drawing specifies 3/4 clear, but he also indicated no vapour barrier. I can't imagine the concrete being affected why what type of gravel is under it if it's separated by a vapour barrier. It seems to me a smaller size would be easier to rake into place.

    1. Expert Member
      MALCOLM TAYLOR | | #11


      The size isn't as important as that it is clear with no fines, so it its as a capillary break stopping water from wicking up to the vapour-barrier.

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