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Community and Q&A

Carpet on concrete slab–vapor barrier?

Mark Waldron | Posted in General Questions on

We are working on a :budget restricted” fix-up of a house near Dayton, Ohio (Zone 5). The house is slab-on-grade with solid masonry walls (we are adding 1.5″ of polyioso to the interior of them). The floor is concrete slab on grade, and has had asbestos and then vinyl tiles applied to it in the past, and most recently had carpet over this. When I tore out the old carpet it was quite filthy, but I can’t say I saw any visible mold, etc. on the surface or backing.
Approximately 75% of the slab is covered with these tiles, and the rest has various amounts of mastic or glue.
We’re going to put wall-to-wall carpet down ( a long story, but that’s the outcome).

My question: In this situation, is it advisable to put down a vapor barrier (e.g. 6 mil polyethylene) under the carpet pad and carpet? I can’t think of a down side–it won’t make any top-side moisture problems worse, and it might reduce them (if moisture migration up from the slab is making them worse). Since I can’t expect any drying from the inside down under any circumstances, it seems best to at least reduce the possibility of moisture coming up.

I wish it were practical for me to get a warm surface under the carpet and pad to reduce the amount of time the fibrous material spends below the dew point (an insulated sub-floor layer, etc) , but I think I’m going to have to be content with something less ambitious.



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  1. GBA Editor
    Martin Holladay | | #1

    I don't see any downside to installing polyethylene, as long as you make sure that the interface between the polyethylene and the carpeting doesn't squeak when you walk on it.

    Ideally, you will excavate the perimeter of your house about 2 feet, so that you can install a 2-foot-high strip of vertical rigid foam (1.5 or 2 inches thick) at the slab perimeter. The perimeter of your slab is responsible for most of the slab-related heat loss. You'll need to install metal Z-flashing at the horizontal seam between the top of the rigid foam and the bottom course of siding.

  2. Expert Member
    Dana Dorsett | | #2

    There is a down side to installing carpet on an uninsulated slab in your location. The carpet is insulating, and the subsoil temps are below 55F, whereas your mid summer outdoor air dew points are north of 65F more than half the time:

    With an insulating carpet above the slab the slab temperature can be low enough to create a mold problem at the deeper levels of the carpet.

    The solution (which you already alluded to) would be to install 3/4-1" of EPS over the slab held down with a subfloor TapConned to the slab. The R3-R4 is enough to keep the temperature of the underside of the carpet warm enough to mitigate that issue.

    The simpler solution would to use something other than carpet, something that won't grow mold, or trap materials that can grow mold the way a carpet can.

    If you're going with just the vapor barrier under the carpet approach, use something a LOT heaver than 6 mil polyethylene, since it will behave a slip surface under the carpet underlayment and will likely abrade through in high traffic areas well before the carpet needs replacing. (30 mil polyethylene is probabaly enough, or perhaps EPDM membrane roofing.)

  3. Jon R | | #3

    See here for an expert who recommends not using polyethylene.

    Note that summer slab temperature will be above deep subsoil temperature and that it's indoor (controllable by the occupant with AC or dehumidification), not outdoor dew point that will determine if there is condensation in the carpet.

  4. Expert Member
    Dana Dorsett | | #4

    Designing a house to work well without mechanical dehumidification or air conditioning would be best practice, and that requires at least keeping the carpet underlayment above the outdoor dew point. Without air conditioning it's likely that the average indoor dew point will track somewhat above the average outdoor dew point.

    Yes, the slab in mid-summer will be above the deep subsoil temperatures, but determining how much above the deep subsoil temps is a complex to model with lots of important factors that can't be known without intimate knowledge of the local soil strata & types, the distance from the water table, etc. But a mere inch of foam renders it irrelevant.

    It reads as if Mark is going forward with the carpet after considering the risk, and will just have to manage the indoor humidity carefully in summer.

  5. Mark Waldron | | #5

    Thanks very much for the input.
    Martin, I had plans to install some perimeter insulation, but now wonder if, in the particular case of this house, it is worth the effort. The walls of the home are solid masonry (a wythe of concrete "bricks" on the outside, with an inner wythe of CMU blocks covered with stucco/plaster in the interior). We have now insulated the interior of these walls with 1.75" of polyiso, but that thick, cold block still sits atop the slab/footer and will be effectively keeping that slab edge cold all winter long even if I insulate the slab edge above grade and a foot or two down.
    Dana, I wanted to go with a foam/plywood subfloor Tapconned to the slab, but it just wasn't possible due to resource limitations (incl time). My Manual J calcs indicate the floor will constitute 35% of my winter heat loss, which is a LOT considering the modest insulation of the walls and ceiling, and that the house has a relatively compact ground footprint (square shape, 1.5 stories). It may turn out to be slightly less than this (my Manual J program didn't give me credit for the approx. R3 of carpet and pad), but it's still major.
    Jon,--thanks. It turns out Joe isn't much of a fan of plastic sheeting anywhere, I guess!

    1. Deleted | | #7


  6. John Clark | | #6

    Would this work?

    Home Depot (and probably others) used to sell a hardwood flooring system comprised of plastic underlayment w/adhesive strip and foam backed hardwood planks. I helped a friend install them in his basement (CZ3) after he pulled up the carpet installed by the previous owner. The previous owner had pets and of course pet urine soaked through into the concrete.

    Good luck.

    1. Bryce Nesbitt | | #8

      A similar product is DriCore R+ : a foam plus OSB click lock tile (so no tapcons needed).
      It still needs a vapor barrier, else all the moisture ends up in the room anyway through the edges.

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