GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Finishing flooring on cement slab

Christopherpcampbell | Posted in General Questions on

I have used plastic to test the unfinished slab in my basement. It comes up dry. So the advice is to treat the slab with sealant and then put a vapor barrier, and then apply any insulation and flooring strates. But I am concerned about creating a greenhouse for vapor under a plastic vapor barrier. Yes, the sealant should prevent moisture from coming up. In fact, I even have a partial but broken vapor barrier (plastic sheath) under the cement slab. But sealants break down over time. And my vapor barrier is not without imperfections.

So 1- isnt it unhealthy to have a vapor impermeable sheet above a dusty concrete floor, which might trap vapor over long time periods?
1a) if this is true, then what is a good permeable floor system that would allow any moisture vapor that does get in to pass through it?

2- Many moisture mitigation systems claim to be green, but how much should I worry about outgassing? Is there one system that you recommend most?

Thank you.


GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. davidmeiland | | #1

    What kind of finish flooring do you want to install?

    Don't worry about vapor being trapped underneath or in your slab. Do keep it from migrating into the house.

  2. GBA Editor
    Martin Holladay | | #2

    You don't want a vapor-permeable floor system. The dirt under your concrete slab is damp, so you want a vapor barrier.

    Don't worry about what's under the barrier you install. After all, there is fungus living in your soil, as well as mold and various creatures. Who cares? All of these things are on the exterior side of your barrier. That's why you want a barrier -- to separate the clean, dry interior of your home from the damp, messy exterior.

    You ask about "moisture mitigation systems," but I'm not sure what you are talking about. Fans? Polyethylene? Drainage pipes?

  3. peaceonearth | | #3

    I thought Chris was asking more about trapping moisture. He has a vapor barrier below the slab, -poly I think with some imperfections. But it must be working fairly well since he did the poly test and has no obvious migrating moisture above the slab. The advice he has gotten was to put another poly layer above the slab, potentially trapping moisture between two layers of poly. This may be okay in this instance if stuff is (mold, moisture, insects) truly trapped by the upper poly. Up until now he has had no problems with his system that may be permitting some drying to the interior.

  4. GBA Editor
    Martin Holladay | | #4

    Neither David Meiland nor I advised Chris to install a second layer of polyethylene above the slab. The person who suggested that measure was Chris (based perhaps on advice from an unnamed person).

    In general, imperfections in a poly vapor barrier under a slab don't matter too much. Even if 10% of the area of the poly consists of holes, it will still be 90% effective in reducing vapor transport. The poly is supposed to be a vapor barrier, but in this application, it doesn't have to be an air barrier. The concrete is the air barrier.

    It's up to Chris whether or not to depend on the (possibly compromised) old poly under the concrete, or to install a new layer of poly above the concrete.

    I am trying to drive a stake through the heart of the old misconception that moisture can be "trapped." If there is a decent layer of polyetheylene somewhere in the floor assembly, then clearly (a) conditions above the poly should be relatively dry, and (b) conditions below the poly are likely to be damp. That's why you want a vapor barrier! So the moisture under the poly isn't "trapped" -- it just is.

    Unless you have a plan to dry up all the moisture in the soil outside of your foundation -- and I'm not sure how you would do that -- I would stop worrying about "trapped" moisture. It's damp out there. Get over it.

  5. user-1135248 | | #5

    I assume the referenced "plastic test" is like my own science
    experiments, where I caulked an expanse of clear plastic against
    part of the basement floor and wall and left it there for a
    few months. I expected plenty of moisture to show up on the
    backside, but it didn't at all for which I was grateful and
    better assured that the surrounding soil drains well.

    Still, after going through untold angst about whether to insulate
    the basement wall with something impermeable or not I totally
    agree with Martin [and indirectly, Joe's recent thoughts on
    all this]: basement walls are effectively part of the dirt, and
    if you let them breathe to the inside they'll just keep diffusing
    more of an infinite supply of moisture out there in toward you.
    Best to block as much as possible and keep it outside where it
    belongs. If incoming moisture got "trapped" it would result in a
    higher concentration at the membrane than the local soil, so physics
    says it would then diffuse right back out where it came from!

    This implies that the "dampproofing" applied to foundation
    exteriors doesn't make much difference in the slow diffusion
    case -- leading to another question: what exactly is dampproofing
    supposed to guard against? Bulk water? Steady hydrostatic
    pressure will pretty much take any defense off the table...


  6. peaceonearth | | #6


    I wasn't thinking you had advised Chris to do poly above the slab, -I realized it was advice from elsewhere and that he was asking about it. I just wasn't sure you addressed that specifically (until your second post), and I was interested in that issue as well. I'm now clear on your feeling on the issue (and good advice from H as well). Years ago I had a dirt cellar floor, which always looked nice and dry in winter (my wood heat was in the cellar), until one day I noticed an inadvertant piece of plastic on the floor (the test!) with much vapor beneath it, unable to evaporate like it was doing everywhere else. I had lots of excess moisture housewide, especially obvious on inner window surfaces when it was cold. I later poured a slab, putting 6 mil plastic below, and it is amazing the difference this made.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |