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Installing Foam on Concrete Slab

thegiz | Posted in General Questions on

My basement is unfinished but would like to make the concrete floor a bit more comfortable. I plan on finishing the basement last after I finish remodeling the rest of home. I came across EVA foam interlocking tiles that would be soft and I think can go directly over concrete. Will mold be a problem? I have mostly a condensation issue no flooding. I also saw interlocking carpet tiles with a pvc base. These look more durable against mold but the foam would be so much more comfortable for my kids. Any thoughts? Could I simply swap out and clean foam or will mold just grow regularly? Can I use a plastic dimple matt under foam to help. Thanks for advice. Joe

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  1. onslow | | #1

    Joe G,

    If pulling up latex foam back carpet in a friend's basement is relevant experience, then I would suggest caution before proceeding. The basement in question was "dry" in that no visible water had ever shown up. However, due to the floor most likely being poured without poly or a washed stone layer (older, mid 60's) there was a small and persistent amount of moisture present. The latex foam backing of the carpet impeded moisture escape and resulted in mold growth. The carpet smelled musty to me anyway, but my friend was nose blind to it.

    I would recommend testing for how much moisture is being transpired by your basement slab with an old test. Take a 3-4' square of clear poly sheet and tape it to the floor. Wait a week or more and see what collects under the poly. What you see will be collecting under the PVC backed carpet tiles for sure. The EVA foam is closed cell and will act the same, so opting for ones that allow some transpiration of moisture would be a safer choice. Breathable foams may not make for good floor tiles and simply may not be made. Test any foam by attempting to pour water through it. If it is open like a car sponge, it might work. Over time even the "open" foam will likely start smelling a bit. Kid goo is just a fact of life and will offer its own issues.

    The dimple sheet will act like a lumpy sheet of poly, so if moisture is collecting under the test sheet then it will under the dimple sheet. The illusion of air exchange with such a tiny a gap is a magnified (mini-fied?) version of undersized roof venting. Plus there are no air pressure variances to help pull air across the sheet area. Think of trying to pull air through a 12 foot long soda straw. The average humidity levels maintained in the basement will determine the rate of moisture transfer from under the dimple sheet. Diffusion is a very slow process.

    If you have ceiling height available, then creating a slightly elevated play area covered with a foam tile might be a successful work around. Treat it as a floating floor, 3/4" sheeting over 2x2 or at least 1x3 furring. No need to anchor to the floor, which saves a lot of misery with concrete fasteners and removable for when you go to sell the house or the kids grow beyond playing on the floor.

  2. thegiz | | #2

    Thank you for the advice, found this gem of an article about installing rigid foam on a concrete slab written by Martin

    So ceiling height is scarce in my 1900 home. According to article if ceiling height is an issue you should stick to 1 inch rigid foam at least with a 1/2 to 3/4 inch plywood subfloor screwed to the ground. Could I get away with 1 inch rigid foam, with 2 layers of 1/8 inch plywood staggered And floating with carpet on top? That is thinnest floor I can think of that would be kid friendly. That would add only an inch to height of floor, carpet I’m assuming is not much of an addition to that inch. Is there a better way? What would be thinnest possible combo? Thanks, Joe

  3. Expert Member
    BILL WICHERS | | #3

    You'd want to use rigid foam rated for compressive strength here, and not a type that would have issues with moisture. That basically rules out polyiso, and possibly type-I EPS too. You don't need extreme compressive strength here, but you want it to be sufficient that it doesn't get mashed down from people walking on it and light furniture.

    I don't think you'll have a problem with two layers of 1/8" plywood, although I might consider hardboard instead. You want those layers staggered so that none of the gaps between panels line up, and you want to remember that you can't put any heavy furniture down there (no pianos or pool tables :-) without risking damage to the floor. Normal subflooring is 3/4" and is much stiffer and more durable than what you're planning on.


    1. thegiz | | #5

      Bill, so the lowest point in my ceiling is about 77 inches, under my main beam. I’m 6’1’ so have a few inches to work with but not much. I do have weights in the basement. Would it make more sense to stagger 2 1/4 inch plywood over 1/2 inch eps type II. I would lose some R value but the floor would be stronger. I could also use 1 1/2 piece of plywood instead of 2 but then I would have to attach it to concrete floor instead of floating it. Also is foam a better option than carpet

      1. JC72 | | #6

        Two layers of 1/2 " plywood on top of the foam (Staggered seams). First layer attached to the slab and second layer glued/screwed to the first layer. Foam can be unfaced XPS (1") seams taped.

        IMO two layers of plywood with staggered seams is really important when it comes to load distribution.

        1. thegiz | | #7

          Could I get away with 1 inch XPS with staggered 1/4 plywood to save some head room? The article I linked to above said that you can either stagger 2 1/2 inch plywood boards or if you are saving ceiling height attach one layer of 1/2 attached directly to concrete. Is floating 2 1/4 inch plywood not as strong as 1 layer of 1/2 plywood? Trying to avoid having to screw into concrete. I think it would be a lot easier to float the floor. What size screws would I use?

  4. _JT | | #4

    I have the same setup - foam mats on my basement floor. For me the concrete is painted with a concrete paint and on top of that I put a vapor barrier - I also inspect it once a year - a few areas I've reapplied the concrete paint where it seems like the paint is peeling.

    These are the ones that I use:

    When I first did it before the vapor barrier I had the musty basement smell. The vapor layer took care of that. It also brings up the floor temperatures in the basement considerably - and does make it comfortable to walk around in.

    I'm sure it's not to code. (Although since it's "temporary" - I don't think the code is applicable.) I'm sure full up floor assemblies with vapor barrier + plywood + flooring will last longer and be more durable.

    I think eventually I will transition to plywood + foam + vinyl floor sheets. But I'm not in a rush - the tiles work well. But I would not put them directly on bare untreated concrete.

  5. Andy_ | | #8

    I've done a couple garage conversions that were on old concrete slabs. The standard there is to put down poly, then rip 2x4 or 2x6 sleepers to level the floor (remember garages are sloped), put whatever rigid foam we can get off of Craigslist, then 3/4" tongue and groove subfloor.
    Works great. The floors are warm since the temperature of the slab isn't as extreme as the outdoor air temperature. I just don't think you'd need as much insulation on a basement floor as you would an upstairs wall. Ideally you'd like to get more insulation in your basement, but I wouldn't stress out on it too much if you don't have the headroom.

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