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Basement moisture clarification

josephny | Posted in General Questions on

I’ve been reading other threads regarding dealing with moisture in a concrete-lined basement and learned a lot.  

Mostly:  Never let the basement walls breath to the interior.

In my case, I have a 45′ x 34′ concrete wall basement in upstate NY and will be spraying 2″ of closed cell foam.  My understanding is that the CC will create a vapor barrier in addition to providing a nice amount of insulation (I have 1″ of XPS on the outside of the concrete wall, for a total of about r-19 on the walls).

But, the basement floor, which is a poured concrete slab that is about 6″ thick and starts at 7′ below grade (down to 7′ 6″ below grade) probably doesn’t have any insulation or vapor barrier underneath.

So, my question is:  Should I be concerned about moisture vapor coming up from the floor?  Is there something I should do to the interior of the floor?

Thank you!

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  1. Expert Member
    NICK KEENAN | | #1

    Yes you should be concerned about vapor coming out of the floor. A barrier as simple as a sheet of poly plastic will work.

    1. josephny | | #3

      So I'm clear, by vapor we are talking about water vapor, right?

  2. Expert Member
    BILL WICHERS | | #2

    You generally need a little more than a sheet of poly on a basement floor. A sheet of poly alone is not only dangerous to walk on (it's slippery stuff), it will probably accumulate water in spots.

    Better is an epoxy floor coating. Best is an insulated subfloor arrangement.


    1. josephny | | #4

      So epoxy coating the basement concrete floor will create a good vapor barrier?

      While the basement is inside the conditioned envelope, it's not habitable space. It will be used for the home's mechanicals and for storage.

      Epoxy sounds like a nice solution.

      1. charlie_sullivan | | #6

        Even with a 100% vapor barrier epoxy layer, you can get condensation on it. If you store cardboard boxes, there's mold potential from that. A better solution is a layer of EPS foam over polyethylene as a vapor barrier, with a layer of OSB on top. I've even heard of using cement board as the ultilty room surface to walk on over the foam. You'll get energy savings as well as less potential for moisture and mold.

        But you can also go with the epoxy and just keep stuff off the floor.

        1. Expert Member
          BILL WICHERS | | #9

          Correct, epoxy can help to prevent moisture migration THROUGH the slab, but it won't do anything about moisture condensing ON the slab when the slab happens to be below the dew point of the indoor air. You need insulation to deal with condensation.

          Regardless of the intended use of the basement, you really shouldn't use a sheet of poly on the floor since it's hazardous. It will look like crap too. Epoxy will give you a nice finished floor surface. If you put down rigid insulation and a subfloor, that gets you the best overall arrangement, but it's also the most expensive and involved to install.


  3. thegiz | | #5

    So question on the epoxy coating. Do they make an epoxy that can be applied to a damp service? I also came across this today and had me wondering:

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

    The big orange store sells it, same company that makes lifeproof carpet pad that I was interested in. It is made of polyethylene with a built in 6mm poly. However they want you to install in with the poly on top. So 1/8 of polyethylene and then your 6mm vapor barrier. Here is its tech on water vapor:

    Does that even make sense. Isn’t that similar to laying down rigid foam and then laying poly on top before your flooring?

    1. charlie_sullivan | | #8

      That's an interesting material. It says it's a mix of polyethylene foam and cork, with some, presumably toxic, antimicrobial treatment. The SDS says at least 60% poly and <10% cork. So I'm not sure the cork is doing much other than giving them a marketing angle.

      As far as the vapor barrier location, I suppose the foam doesn't mind being damp so you can have the vapor barrier layer on top, and I guess it's on top because it's easier to seal the adjacent strips together that way, with the edge flaps on top. The R-value of 0.48 isn't much to get excited about.

  4. josephny | | #10

    The basement height (2/3 of the basement is 7' and 1/3 is 6'), combined with the added work/cost of a subfloor (which would always worry me, perhaps baselessly, because of the once in a blue moon leakage/spillage that I'd inevitably cause to happen -- and now knowing how much is just sitting invisibly between slab and subfloor assembly), sure make me lean towards epoxy.

    Can anyone recommend a good product?

    Thank you so much (as always) everyone!

  5. maine_tyler | | #7

    what's going on here? Is this a bot? EDIT: this is in reference to a post that no longer exists here.

  6. josephny | | #11

    I hope you don't mean me.

    I'm not a bot -- a bot would be far more rational and reasonable (;-)

  7. maine_tyler | | #12

    No, the post I was referring to has been removed. It was posting what appeared to be clips from generic and unknown resources (almost like a google search answer to the question). And the profile was empty. If it was a real person, I was not trying to offend; it was just displaying some... unusual behavior.

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