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Community and Q&A

Hardwood floor on slab

mk10 | Posted in General Questions on


I plan on installing hardwood on a concrete slab.

Slab was poured ~1968
Zone 6, Vermont
At some point looks like someone laid 4-6 mill plastic on the original slab then poured ~1″ concrete on top.

Slab seems very dry, there is a clik cork floor on top of thin white foam for floating floors laid right on top of the slab with no plastic, no mold or wet cork.
Also there is some existing wood flooring that i will take up, under it is: paper, 1×3 pine, 6mil plastic the carpet glued directly to slab, smells musty but not wet or moldy.

I would like to create a thermal break with minimum height, here is my plan:

– 1″ polyiso with foil face cut into 12″ strips with a 3/4″ deep by 3″wide lap cut on one long edge This will give 1/4″ foam under the wood strap creating a small thermal break between the strap and the concrete. It adds an overall height of 1/4″ over just installing the straps directly to the concrete, not sure if that is worth it or not. 1/4″ isn’t that much but i have transitions to deal with.
– Will power nail 3/4″ x 3″ plywood strips in the lap joint thru the foam directly to concrete
– Will caulk between each rigid foam piece.
– Will nail the 3/4″ hardwood to the straps

Is there a better way to do this? Cheaper?
Is poly iso the right stuff? or xps? I don’t even think the xtra r-value of the polyiso would make a difference plus it costs more.
Should i use Plastic on the slab? is it cheap insurance, or a waste.
Should I use something between the hardwood and the foam for squeaks, etc?

In the end I don’t think I would gain much, just comfort level of hardwood on foot, do you agree?

Thanks! Mark

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  1. GBA Editor
    Martin Holladay | | #1

    Your method will work.

    Q. "Is there a better way to do this? Cheaper?"

    A. The more rigid foam, the better. There is a tradeoff between ceiling height and R-value. Only you can decide how thick you want the foam to be. Another approach is to install a continuous layer of T&G 3/4-inch plywood on top of a layer of rigid foam -- which can be 1/2-inch foam, or 3/4-inch foam, or 1-inch foam. Of course, you need a lot of Tapcons to hold down the plywood, which is a pain.

    Q. "Is polyiso the right stuff? or XPS?"

    A. Either one will work. If you use plywood nailers instead of a continuous layer of plywood, the weight will be more concentrated on the foam. High-density EPS or XPS can bear more weight than polyiso, but if you put a continuous layer of plywood on top, that won't matter -- any type of foam will work.

    Q. "Should I use plastic on the slab? Is it cheap insurance, or a waste?"

    A. Cheap insurance.

    Q. "Should I use something between the hardwood and the foam for squeaks, etc?"

    A. You should install rosin paper or asphalt felt under the hardwood.

  2. dankolbert | | #2

    I would also make sure the slab is really dry. Here is an article on glue-down, but it has good info on moisture-testing the slab.

    And to me, having 1/4" of foam over the sleepers doesn't seem worth the trouble.

  3. mk10 | | #3

    Do you think I would gain much going from 3/4" to 1" foam? My other thought is could i just nail 3/4 strapping directly to the concrete floor with 3/4 foam in between? This would eliminate the 1/4" foam under the strapping that i was planning on for a thermal break between the concrete and the strapping, I am not sure it will make a difference to have 1/4" foam under the strapping.

    Or if I wanted to get crazy what do you think about 1/2 poly iso and 1/2 Ply on top, I would biscuit the ply to keep it flush. the 1/2" continuous ply would allow me to use hardwood "shorts" flooring. is 1/2" enough for nailing the hardwood?

    thanks! Mark

  4. Expert Member
    Dana Dorsett | | #4

    Get RID of the strapping, put the iso in continuous sheets on the floor and use a real 3./4" or 23/32" T & G subfloor TapConned to the slab 16-24" o.c.,with the seams staggered by a foot or more with the seams in teh iso. Even 1.5lb polyiso has more than sufficient compressive strength to handle residential flooring loads when there's a substantial subfloor distributing the weight, which would be far less localized pressure than if using strips.

    Half inch polyiso is better than nothing, but woefully short of what is appropriate for a zone 6 climate. At 1.5" it would start looking pretty good, but even 1" is a measurable improvement over 1/2", and sufficient R to avoid getting musty-stuff growing in the subfloor from being too cold and retaining moisture during summers.

  5. mk10 | | #5

    Is there any concern with the polyiso taking on moisture?

  6. GBA Editor
    Martin Holladay | | #6

    Any time that you are installing finish flooring on a concrete slab, the first order of business is to ascertain whether the slab is dry. Ideally, you want a slab that is fully cured. You also want a slab that has no blotter sand underneath it. Ideally, you want a slab that has a polyethylene vapor barrier directly underneath it. And you want the slab to be above grade. If the slab is below grade, you want to be sure there is a layer of crushed stone under the slab that is connected to drainage.

    In your case, the slab was installed in 1968, so it should have cured by now.

    You don't tell us whether the slab is above grade or below grade.

    You do mention that the slab seems "very dry," and that there are no signs of mold, after many decades. That all sounds good to me.

  7. mk10 | | #7

    Hi Martin,

    The slab i suppose is at grad-ish. the house is on a hill so the back wall is in the hill (about 6' of dirt against that wall) the each end of the wall have a return of about 4'. I only ever see some condensation on the floor against that wall but usually intermittent for about a month in the summer (heck, summer is only a month here anyways).

    Also the slab is on some ledge, i don't know how much but it is, not sure what is under it but it looks like they backfilled the area with concrete then built forms on top and poured the slab.

    My biggest problem is my transitions, if i go with Dana suggestion (which i like better than fooling around with strapping) it would be a total of 2.5" which i need to transition to a 1/1/2" existing slate floor and a 1/2" slate. That would be a lot of drop, 2" for the latter.

    What about using 3/8" solid? that would help lower the height but I am not sure how well it installs and feels underfoot.

    So what would be the most reasonable work around?

    Can I use 1/2" ply on top of the foam? can I nail 3/4" hardwood to that" . This is crazy but i could biscuit the ply (no glue) so it is flush sheet to sheet
    Could i then go to 1/2'' polyiso"? separately from that, would it be that big of a difference between 1/2" to 1" foam? I know it's more r value but is it enough to make a difference in my situation?

    Also as far as the crazy strapping idea goes it is one of the lowest profiles (1.5" finished) so its worth considering.
    - Would I really even gain anything by having 1/4" of foam under the wood strap? (rabbiting 1" foam to allow for the 3/4" straps)
    - Is attaching the wood straps directly to the concrete and putting 3/4" foam in-between the straps defeating the whole purpose of insulating the floor in the first place?

    thanks for everyone's help in advance!

  8. mk10 | | #8

    A few final questions

    I do need to keep the final assembly as low as possible, I figure currently i have 3/4" hardwood on top of 1x3 sleapers with no insulation and it isn't that bad. about an r2

    I would almost do the same thing but with 3/4 iso in between and for get about the losses thru the sleeper,it is cheaper but i like the idea of a ply sub-floor for more installation options.

    So Here is what i am thinking:
    6mil plastic > 1/2" poly iso > 5/8 Ply (or OSB?, screwed to slab) MP products "Quiet Walk" > 3/4" or 3/8" t&g hardwood.

    That would give me ~ r5.5 more than double what is there now, which ain't to awful bad, it's the trade off for a lower transition.

    Is that assembly going to be enough to ward of condensation?
    Is the 5/8" ok for nail down hardwood? can it be osb? should it be sealed (to add water repel) or does it need to be ply.
    The Quiet Floor says that it act as a vapor barrier, will trapping the ply between that and iso (another vapor barrier) be a problem?

    Any other thoughts appreciated!

    Thanks, hopefully this will be done next week so i can stop bothering everyone!

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