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Community and Q&A

Floating plywood floor on rigid foam insulation on concrete?

user-1067956 | Posted in GBA Pro Help on

I am proposing a project for a slab on grade Habitat house with 2″ of foam under the slab and 1 1/2″ or 2″ of foam on top of the slab with 3/4″ t&g plywood or osb floor. I want to float the plywood rather than use screeds to minimize thermal bridges. The interior walls are all non-bearing so they can be fastened to the plywood. I am certain that this is not an original idea so I would like to know if anyone has tried this and what the down side issues might be.

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  1. GBA Editor
    Martin Holladay | | #1

    1. Why not put all of the rigid foam under the slab instead of putting some of it on top?

    2. If you really want to put some of your rigid foam on top of your slab, here are some links to show you how other people have been doing it:

    Fine Homebuilding: The Stay-Dry, No-Mold Finished Basement

    Fine Homebuilding Q&A: Finishing a basement floor

    GBA Q&A: Basement floor insulation retrofit

  2. user-1067956 | | #2

    Thanks. I guess the short answer for the concrete sandwich is that it is more comfortable for the occupants to stand on a wood floor rather than concrete. Just that little bit of give in the material seems to make a difference. I have no specific data on that just my opinion. Thanks for the links. I read though them and I think the trick is to have some way to tie the pieces together.

  3. Expert Member
    KOHTA UENO | | #3

    I definitely agree with Martin--putting all the foam under the slab is preferable and a lot less work. Some friends and I recently worked on their basement, where we did an assembly along these lines--no existing basement slab insulation. The original design intent was 1" XPS with taped seams, polyethylene, 3/4" T&G plywood "floating" above it. However, we found that the plywood curled or "potato chipped" too much, and we had to fasten it down to the concrete with Tapcons, about 6 per 4x8 sheet. Note that Tapcons won't self-countersink--you need to drill a countersink in order to get them flush. Perhaps you could do it with two layers of 3/4" or 1/2" plywood, but at that point, you're getting into a pretty ridiculous amount of material to lay down just for a bit of cushion.

    As an FYI, our recommendation for an ideal slab can be found at these web pages:

  4. user-1067956 | | #4

    Thanks again. I was hoping that someone had installed this system before without issues but I suspected that the potato chip effect might be a problem. I don't want to go there. I think I will explore the foam fastened down with screeds and then plywood or maybe just put all the foam under the slab. The other issues with the concrete slab on top is the sheet vinyl they typically use in the kitchens and baths. The low VOC adhesives just won't work on new concrete without much fussing around.

  5. user-980774 | | #5

    You might also try two layers of 7/16 OSB with staggered joints both directions (glued and screwed).
    Less likely to curl and no more expensive than 3/4' T&G.

  6. user-1067956 | | #6

    Thanks again.

  7. gusfhb | | #7

    There is a flooring product that is an adhesive and vapor barrier that I don't have the name of at my fingertips, bucket is in the garage.

    I used that and then discovered a wonderful tool called a "T nailer"

    Air powered concrete nailer, works great but you must have the right length nails as they don't drive more than 1/2 inch or so. Fussier than a framing nailer, but you can drive 20 nails a minute instead of 20 an hour for tapcons. Great for this application where you are really just holding while the glue dries and in case it fails. Absolutely love the thing

    Insulate under tho....

  8. jklingel | | #8

    I did exactly what Richard P suggested, 11 years ago in our basement bedroom. Were I to do it again, I'd use plywood instead of OSB. No issues have as yet surfaced.

  9. user-1067956 | | #9

    This is awesome. The first time I have ever submitted a question to a blog and you all have been hugely helpful. Hope to return the favor at some time. Philip

  10. Boro | | #10

    Personally I'd place the insulation underneath to maintain the thermal mass. Then just squeeze out a thick bead of PL glue 12" on center. That will eliminate the unforgiving concrete and save a bundle. Finish the underside of plywood with shellac.

  11. gusfhb | | #11

    I think the product was 'Dritac 1001' but will double check

    [edit 12/16] that is in fact the stuff, seems to work great

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