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Community and Q&A

Liquid Vapor Barrier vs. Epoxy

thegiz | Posted in General Questions on

So after reading this article which preferred a liquid barrier over poly

I decided to go that route. I want to use it under carpet padding that has air channels in case product fails. I figure if barrier debonds worse case scenario would be liquid vapor dissipates into air. Dehumidifier will be running.

Is epoxy what you use in this situation or some other product? Any recommendations on something you have used before? Bill gave me ideas on some good epoxies just making sure that is what I should be using in this situation. Something resilient and fairly easy to apply.

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  1. Expert Member
    Akos | | #1

    Since you won't be directly walking on it, you can use any of the many tile decoupling liquid applied membranes. These are usually much cheaper and easier install than epoxy.

  2. exeric | | #2

    This may be kicking a dead horse but it seems important. Hammer, you seem to be "stuck" on Lstiburek's article. Do you know if your concrete slab had poly installed UNDERNEATH your existing basement slab? That article's advice is all predicated on that. If you do not have it installed then you have a whole world of difference in the amount of water that will be transmitted through the concrete from the earth. Epoxy would have to resist much more hydrostatic water pressure in that case. I do not think Joe has made a good case for saying polyethylene sheeting shouldn't be used in that situation. You are not going to be making holes in the polyethylene by nailing wood flooring planks through it. You may just not be in the same category that Lstiburek is arguing and his resistance to poly on top would then not apply to you. You need something that is completely resistant to vapor drive if you don't have that poly BELOW the concrete.

    Poly is also very tough. Much tougher than Joe implies. I have noticed in the past that though Joe is very knowledgeable he imputes perfect installation conditions prior to adding on to existing building assemblies. That is seldom the case. Just have the poly extend a few inches up the wall so water won't migrate out from under the poly on the perimeter and get on top of it. You can't have it both ways. You can't resist all vapor drive from below but allow water to dissipate if it comes from above and goes on top of the poly. That's just the situation and you'll have to accept it at some point.

    Akos'' suggestion is also a good one because I think that would also act like a complete vapor barrier and it would seal at the edges using thinset to the bottom of the walls. I'd forget about liquid applied vapor barriers. They just won't hold up to the hydrostatic water pressure you're going to have without poly BENEATH the slab.

    1. maine_tyler | | #7


      I suggest you re-read Joe's article. You are misrepresenting what is said in it. He says nearly the opposite of what you claim. Here is a quote:

      "Today, if we are smart, we don’t need to separate the wood flooring from the concrete as long as we separate the concrete from the ground and we allow the concrete to dry before we install the wood flooring."

      In other words, he is saying when poly is beneath the concrete (and the sides as he points out later) there is NOT the need for the VB above. If there is no vapor/moisture decoupling beneath the slab, however, he suggests to use an adhered vapor barrier.

      I think one could make the argument that poly could be well detailed to avoid the pitfalls Joe lays out, but part of the issue Joe is getting at has to do with having an air film that is capable of communicating vapor via air-flow into the floor assembly, which is inherent to a sheet product, unless one glues it down (but then why not just use an adhered product to begin with). This risk (it's surely simply a risk and not a guarantee) is mitigated by adhering the VB.

      You certainly want the VB, if liquid applied, to be robust and stay adhered rather than flake and disintegrate into non-existence.

      Note that I personally don't have a strong opinion on whether Joe's concern is valid (I also have no empirical evidence one way or the other). I am just clarifying Joe's argument the way I see it.

      1. exeric | | #9

        "I suggest you re-read Joe's article. You are misrepresenting what is said in it. He says nearly the opposite of what you claim. Here is a quote:

        "Today, if we are smart, we don’t need to separate the wood flooring from the concrete as long as we separate the concrete from the ground and we allow the concrete to dry before we install the wood flooring.""

        Well, that's true. But he went on to denigrate poly and people who use it via sarcasm; "that advice to use it is on the internet so it must be true". But he himself is advising using it underneath the concrete. It has to be used somewhere and in most cases in retrofit situations it will be on top of the concrete because it wasn't installed beneath it.

        He fails to address that and also fails to say poly is the simplest, easiest, and cheapest solution for those situations as long as you use proper detailing, like extending it up the walls a tiny bit. If you read that article as the first thing you read you'd assume using poly over the concrete was only for dummies.

        As has been said here concrete foundations don't dry from the interior. It's been said by Martin Holliday that even Joe has changed his mind and agrees with that now. So the whole point of not putting poly down on top of concrete when you don't have it below no longer makes sense. I don't think most people would have gotten that from the article which to me is a shortcoming of the article.

        1. maine_tyler | | #10

          I will not defend Joe's tone or use of sarcasm. He certainly has a way. Sometimes it's enjoyable, but it probably depends on which side of the joke one feels they're on.

          I'm also not 100% convinced poly above is an absolutely terrible idea, but I can understand his points and would enjoy more empirical evidence either way.

          Just for clarity on this:
          "But he himself is advising using it underneath the concrete. It has to be used somewhere and..."

          He his not slamming it as a product but for the specific use. Poly below is sandwiched by the concrete and dirt (or crush or foam). The concrete is an air barrier above the poly.
          The specific issue he cites with poly above concrete is that it can (theoretically) harbor mold and then communicate spores into the floor assembly (and beyond). In a way it cannot when beneath the concrete. I imagine this is most prevalent when the poly is poorly detailed and also if there is any warping to the floor allowing for a 'pumping' action of the membrane.
          A perfect installation with tight poly is perhaps less likely to have these issues. (?) Ruminations.

  3. thegiz | | #3

    I’m 100% sure I don’t have poly underneath the slab. The reason for the liquid vapor barrier was because there was debate over if mold underneath the poly could escape into the air. There is a side that says it’s risky, so trying to be cautious. I’m a bit confused because you said poly would apply in this situation but you said Akos had a good suggestion. Is a liquid membrane different than a liquid vapor barrier?

    1. exeric | | #4

      I may have misinterpreted what Akos said. There are two Schluter applied membranes that are really really effective. Those are Ditra and Kerdi. They both are completely waterproof once applied with thinset. Ditra is also a decoupling agent, but you really don't need it if you are only putting down carpet. Tile is a different story. I think the Schluter membranes are on a whole 'nother level of quality compared to standard liquid applied membranes. I for some reason thought Akos was referencing the Schluter products but was probably wrong.

  4. Expert Member
    Akos | | #5

    The coating in this case is a vapor barrier, not a water proofing. Liquid water needs to be dealt with other ways. Since it is only blocking vapor, there is little pressure, as long as it sticks to the concrete it will work. Doesn't have to be anything fancy, any of the roll on ones (redguard, laticrete, aquadefense).

    The idea here is to keep soil moisture away from your flooring so the only source of moisture would be from the air itself which can be dealt with proper humidity control. If the concrete is warm enough and the air is dry enough, it will not condense on the concrete surface.

  5. thegiz | | #6

    I know I have over analyzed this whole idea but if I really want to be 100% safe I could use the membrane, use ecocork foam with 6mil vapor barrier on top, the vinyl floor with area rugs on top. Probably a whole bunch of unnecessary steps but any vapor from ground would have to go through membrane, 1/8 of foam, 6mm poly, and through vinyl. Interior air would be on a nearly waterproof floor with a dehumidifier. Rugs could be tossed if need be. I’m sure someone could find a risk here but I would think it is low.

  6. Deleted | | #8


  7. thegiz | | #11

    I kind of worded this wrong. If I use a liquid applied membrane on top would vinyl with an underlayment work as well. That would be a vapor barrier, following by foam, followed by an impermeable surface.

    1. Expert Member
      NICK KEENAN | | #13

      I would assume that no drying is possible, so make the floor out of materials that can't absorb water so never will need drying.

  8. user-2310254 | | #12


    I think this is analysis paralysis. I'd pick one of the reasonably workable strategies you outline and go for it. I'd also check my work periodically to make sure there were no moisture and/or mold issues.

    As we discussed before, your bigger issue may be that mysterious void under your slab. But who knows?

  9. thegiz | | #14

    Yes true the mysterious void may be an issue, seems to be only in one spot though that I can see without ripping out entire slab.

    So yes I’m going to go with one of my strategies either carpet or vinyl but was just wondering if adding a foam layer over a vapor barrier would be pointless or stupid if I’m using lifeproof vinyl flooring, I think lifeproof comes with attached pad. I also have never heard of another product or similar set up where you would put a 6mm vapor barrier on top of your foam instead of underneath which is basically what this product from the orange guy is doing:

    300 sq. ft. 3 ft. x 100 ft. x 3.2 mm Waterproof Premium Plus 10-in-1 Underlayment for Vinyl Laminate & Engineered Floors

    Is that actually smart or are you trapping the foam and growing mold. I know underneath the slab you would want your vapor barrier underneath your foam but this is above the slab.

    1. maine_tyler | | #16

      " I know underneath the slab you would want your vapor barrier underneath your foam but this is above the slab."

      For clarity, the generally recommended assembly is (from bottom to top):
      Compacted or existing undisturbed soil --> free draining crush rock --> foam insulation --> poly --> concrete.
      So the poly VB is above the foam, immediately adjacent to the concrete.
      In this scenario, you have a (thick) air barrier above your vapor barrier.

      But you are right that a VB above the foam when above the slab is a bit of a different animal. I don't really know. I think you are in territory where you will find many different schools of thought on which specific products to use and how to use them (unfortunately for you).

      In the absence of confidently enrolling in any specific school of thought regarding where PRECISELY the lowest perm VB should be placed (or what type it should be) my advice is similar to DC's: choose moisture resistant materials, especially if sandwiched by the (potentially wet) concrete and any significant (class I) Vapor retarders. In your assembly, it sounds like you could have a few of those.

      Did the use of actual rigid foam get ruled out in an earlier thread? Because that seems like the obvious standard for this procedure.

      Another option is to find a local specialist/contractor who has a good reputation; the value of having someone on-site to see reality can far exceed any of the arm-chair advice given here (both have value of course).

    2. Expert Member
      Michael Maines | | #17

      The way I have done this on many projects is to cover the floor with poly and run it up the wall a few inches. If the walls might leak you need to leave a drainage gap. Over the poly I install sleepers and between the sleepers I place EPS foam. Over that I install 3/4" subflooring, either Advantech or plywood. I have not installed myself but have drawn and have seen others float the subfloor over the foam, either two layers of 1/2" plywood screwed together, or a single layer of 3/4" screwed through the foam into the concrete. The poly blocks water vapor coming up from below and the foam keeps the floor surface warm enough to prevent condensation. Because there is a chance the floor could flood, I would not use an impervious material like vinyl flooring because it won't allow drying to the interior, which could be important if groundwater leaked through the walls or up through the slab. (No product will resist hydrostatic or vapor pressure forever.) I would also not use carpet because it harbors dust and micro-critters. But if you don't think there is a risk of even minor flooding you might be ok with the vinyl flooring, it's hard to say. It might be a risk I'd take myself but not one I would recommend to clients. If I didn't have the ceiling height or finances to do it properly and safely, I would choose a different project.

      I'll attach a couple of examples from last year. The first is a lot like your basement. I neglected to include poly but talked with the builder that it could go above or below the foam. The second one is a garage conversion so not quite the same situation but with more details shown.

  10. user-2310254 | | #15


    Have you performed a calcium chloride test or taped a square of plastic to the slab? I don't recall if we have established whether you have a "wet" or "dry" slab.

  11. thegiz | | #18

    I ruled out rigid foam because in order to use lvp on top according to advice given was that you need a substrate over the rigid foam. That would mean raising ceiling height. The ecocork product is about 1/8 thick with attached 6mm poly on top. So because of how thin it is I’m assuming the weight gets transferred to the concrete and you wouldn’t need an additional substrate. However that is a very thin layer. So not sure how that would perform sandwiched in between concrete with applied liquid barrier, then 1/8 foam, then 6mm poly with vinyl on top. This set up would go against the group that believes in having semi permeable surfaces. That would mean sticking to carpet pad and carpet or some other setup if I followed that train of thought.

    I did open the floor last week to put in a post and there was no liquid water but yes I need more testing to determine how much vapor is coming up.

  12. johnarch1 | | #19

    Hello Everyone,

    I am look for some advice:

    I have just finished reading the article " walking the plank".
    Here is our specific conditions.

    We are renovating an older house.
    The existing flooring system is a 3/4" ply-wd on 1 1/2" sleepers on concrete slab.
    The slab does have a plastic vapor barrier below the conrete.
    The house is devided into a newer and an older part.
    The older part of the house there is a bituminous layer applied above the concrete between the sleepers. The newer part of the house is only concrete.

    The ply-wood and sleepers are to remain. An existing solid hard-wd floor on the both the newer and older part of the house has been removed and is getting replaced with a new wideplank engineeered flooring. The hard-wd floor that was removed was in relatively good condition.

    The existing slab was cut into several locations for relocating plumbing pipes. The slab and vapor barried below the slab are to be patched. The vapor barrier is also patched as best of possible but will no longer be continous.

    Considering the information above and that we will be replacing the existing solid hard-wd floor with a new engineered wood floor on the existing ply-wd and sleepers in the entire house should we install an new vapor barrier above the slab in the area of the slab that have been exposed and patched - in some area this area is quite large. We are also addition a small addition were the plan was to match the existing construction. The GC thinks i am being overly conservative as the vapor barrier would only get installed in the patched area and not everywhere - so he sees no reason to do it. I my gut tells me if it is open and we can easily install a vapor barrier above the concrete in the ares where the conrete is exposed then perhaps we should do it. In this situation I think it would only make sense to install a liquid applied vapor barrier above the concrete. Please let me know your thoughts. Also any recommendation on exaclty which product to us would also be helpful.

    Thank, J

  13. thegiz | | #20

    I had to read this several times to understand what you mean but how are the sleepers and plywood remaining and areas are being patched? If you had to remove a section of floor and patch it you are better off imo using 1 1/2 inch foam board insulation and forget about applying the vapor barrier. In my original post I was concerned with applying anything directly to the concrete because I couldn’t give up the headroom. In your case the floor is separated from the concrete. Your only concern would be mold under the wood sleepers.

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