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

Foam over a concrete floor

kenorakq | Posted in General Questions on

I plan 4 – 6 inches of EPS under the slab and now am considering an inch of either EPS or XPS on top with 5/8 or 3/4 tongue and groove plywood or OSB to provide a nicer floor to walk on as well as a warmer floor in the walk out basement. I think this will make it easier to attach partition walls and tile in the bathroom and hardwoods in the sunroom.
Question. .. Is this a good idea, and do I glue the foam (EPs or xps) to the concrete and the sheeting to the foam or leave it free floating?
Any gotchas in this?

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. milwaukee | | #1

    i personally would spot glue it with foam adhesive. Tape the foam joints. Foam the foam edge to wall intersection. Then tapcon the plywood on top of the foam every 16 to 18 inches square. Making sure that the tapcon gets at least 1 inch of penetration in the concrete floor. leaving 1/8 gap between the plywood sheets, then caulk in between each sheet for even more air sealing. I would NOT use polyethylene vapour barrier anywhere. But is just my take on it.

  2. GBA Editor
    Martin Holladay | | #2

    A few thoughts:

    1. If you are planning to install 4 to 6 inches of rigid foam under your basement slab, that's enough insulation. There is no need for more insulation above the slab. Your slab will be at room temperature.

    2. If you want to install tile in some of your rooms, a concrete slab is the ideal substrate for tile.

    3. If you want hardwood flooring above a concrete slab, you'll need to either install sleepers or two layers of plywood subflooring over the concrete before installing the hardwood -- but you don't need any rigid foam. Of course, if you go with hardwood, you may need to come up with a plan to raise the level of the floor in your tiled rooms to keep the flooring co-planar.

  3. Expert Member
    Dana Dorsett | | #3

    Martin: What is the point of double-layering the subflooring in suggestion #3?

  4. kenorakq | | #4

    I was thinking that the foam under the plywood would replicate the OSB product with the blue styrofoam (sold as 2'x2' interlocking slabs) on it...I was thinking it would be easier on the feet...kind of a built in cushion, If I'm adding sleepers wouldn't it be better to omit them and use the foam say 1/2 or 3/4 or 1 inch?
    I don't want to spend extra $$$ or effort if the foam under the slab will be sufficient..... and there will be no bare concrete in the basement walkout level,,,either tile in the washroom or hardwood in the rest of the basement.

    I know its been discussed before but since we are discussing it again.... Is there any point in going from 4 - 6" or even more...I know that I have to add the "wings" of foam from the footer sloping down and outward since this basement have the east side footer just below grade in zone 7. I plan on the foam wings around the perimeter.


    recommendations regarding foam...I plan on EPS...however the local recycler can't guarantee the foam is type 1 or 2 or anything for that matter.. It is recovered from commercial building ceilings... does it matter if its not a known grade?

  5. GBA Editor
    Martin Holladay | | #5

    Hardwood flooring nails will bottom out -- hit the concrete before being all the way set -- with just one layer of plywood.

  6. kenorakq | | #6

    couple of followup questions...

    1- instead of 2 x plywood; given that the nails would go through the first layer of plywood and strike the concrete below is it feasible to use foam (the same thickness as the 2nd layer of ply) as its cheaper and affords a bit of insulation? The nail would still set in the first layer of ply (3/4" ply).

    2- ,.....recommendations regarding foam...I plan on EPS (for below the basement slab)...however the local recycler can't guarantee the foam is type 1 or 2 or anything for that matter.. It is recovered from commercial building ceilings... does it matter if its not a known grade?

  7. GBA Editor
    Martin Holladay | | #7

    I was always taught to install two layers of plywood when installing hardwood flooring over a slab. It turns out that you can use one layer of 3/4" plywood and shorter flooring nails -- as long as (a) the plywood is well-secured to the slab with power-actuated fasteners or TapCon screws, and (b) the flooring manufacturer approves of this method.

    Note that some hardwood flooring manufacturers specifically state that their flooring cannot be installed on below-grade slabs. Here is an example of one that says "not below grade":

  8. kenorakq | | #8

    I never thought of that...thanks

  9. kenorakq | | #9

    NEW thought....

    engineered flooring might take care of the nailing issue... I'll talk to my wife

  10. Expert Member
    Dana Dorsett | | #10

    If going with a layer of foam instead of doubling up the subflooring, the density of the reclaimed EPS or XPS doesn't much matter. (The subflooring distributes the load.)

    With reclaimed foam there can be thickness variations to deal with- you may have to spin or swap out sheets to get the edges to match up within 1/8" of one another, which is about the maximum deviation that would still work.

  11. kenorakq | | #11

    Thanks...I am now thinking of 3/4" XPS from one of the big box stores under a single layer of 3/4 ply instead of double ply in the main areas where the engineered flooring will go (If he boss agrees)...and double plywood (2 x 3/4 in the bathroom where the tile will go).
    Regrading the foam density; what are your thoughts on the unknown density of 4 - 6 inches of EPS ---under the slab---...the supplier can't tell me what is what and its not labeled!

  12. GBA Editor
    Martin Holladay | | #12

    The worst-case scenario for the sub-slab foam is that the R-value of the EPS might be a little less than you had hoped. So -- assuming the EPS is cheap, which is why you are buying it -- just go with 6 inches instead of 4 inches, and stop worrying.

  13. charlie_sullivan | | #13

    In steady-state conditions, the extra bit of insulation above the slab won't make much difference, but it could help or hurt in transient conditions:
    1) You have the thermostat set back and want to warm up the room quickly. That's much easier and the floor becomes comfortable much sooner if you have some insulation above the slab.
    2) Not very likely but possible is that If you have the windows open in the summer, and a cool night is followed by a hot humid day, you could get condensation on the floor with no insulation above the slab.
    3) On the other hand, if you have solar gain, the room will overheat faster with the insulation above the slab.

  14. kenorakq | | #14

    great advise from all here thanks a million..... I just bought an enclosed cargo trailer and will start moving EPS this week..

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |