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Vapor barrier over slab material and details

jonesi | Posted in General Questions on


ive been looking through the archives of the Q A here trying to find some answers to a remodel/conversion im doing. I havnt been able to find an answer yet so figured id reach out to the community. Its a slab on grade carport im converting into shop space. 

ive cut the perimeter out and placed a continuous stem wall and bituminous water proofing. in doing so i saw there isnt a vapor barrier below the slab.

the floor assembly i want to do is floating, comprised of continuous  rigid insulation with 2x sleepers over that layer, subfloor, then a finish t and g floor. 

I get lost trying to determine: what vapor barrier i should use, be it fluid applied, taped poly, or if the rigid insulation is enough? And then whats the detailing to make it continuous, do i tape the seams? How is it terminated at/or above the stem wall? Does the rigid insulation need to be sealed with foam?

any information would be greatly appreciated. And thanks for yalls time. 


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  1. brian_wiley | | #1

    Hi Ian,

    I’ve got a similar project going, so I’ll be following this thread with interest. So far in my research it seems like an epoxy-based coating is the preferred method. There’s a post on Christine Williamson’s Instagram feed (@buildingsciencefightclub) with details.

    In instances where I’ve seen a vapor barrier like poly or stegocrawl used, the sheet was run a few inches up the stem wall and sealed with a bead of sealant and secured with fasteners or term bar, similar to how you might finish a crawlspace vapor barrier.

    I’m not sure if taped foam would work. It’s seems like sealing the foam to the wall at the edges might prove tricky, but I’m far from an expert so hopefully one offers their opinion.


    1. jonesi | | #2

      Hello Brian,

      Thank you for responding. I went and looked at that post about using the epoxy, thats really helpful. I called and spoke to an Ardex tech support this morning about their products. And with the epoxy they said i needed to mechanically grind the surface, but with their VR 98 i wouldnt need to. So im gonna start looking at that product as well. Let me know which assembly you go with.

      1. brian_wiley | | #3

        Hi Ian,

        Is there any way that you can link to that post from Building Science Fight Club? I tried all morning to find it without any luck.

        I'll definitely follow up with the assembly I land on. Would love to hear how your's turns out.




  2. jonesi | | #4

    Hi Brian,

    You bet. Heres the one i found that was helpful. Not sure if its the one you had in mind.


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