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Insulated floor on top of concrete slab

user-917907 | Posted in General Questions on

I am considering building a slab-on-grade, or slab with short perimeter footers, superinsulated house in a cold climate. Most slab insulation schemes I’ve seen put foam insulation under the slab, often with fancy insulation schemes at the slab edge. I very much dislike having a hard, cold concrete floor (and, yes, even a 60*F concrete floor is cold to bare feet). Is there a simple cheap way to make an un-insulated slab with all the insulation placed on top of the slab, then a plywood sub-floor placed on top of the foam? Or maybe 1×6 “floor joists” set directly on the concrete, with insulation (foam, fiberglass, cellulose?) between the joist? Any other ideas?

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  1. GBA Editor
    Martin Holladay | | #1

    It's certainly possible to install a layer of rigid foam on top of a concrete slab, followed by a layer of plywood. People do it all the time when they remodel their basement. However, most people wouldn't call this solution "simple and cheap" -- two adjectives you mentioned.

    If you decide to go this route, pay attention to the continuity of your thermal insulation at the perimeter of your house, where the floor foam meets the walls. Otherwise you'll end up with thermal bridging at this critical location.

  2. jinmtvt | | #2

    Martin : what about condensation in cold climate ?? do you believe it could become a problem at the slab or at the edges ? the slab will get much colder than subinsulated one nah ??

    Jack : in which "cold" zone are you at ??

    I would make sure there is a serious water barrier on top ..but ever there i am unsure how this situation is affected by condensation.

    If you want to use cellulose, still use at least 1 " of XPS under the wood studs to cut off thermal should calculate wheter using celllose + wood studs VS high density foams such as xps or type 2+ EPS ( will be affected by thicness here if you want high R value )
    As you will need to use plywood on top in both situations neway.

    What about polyiso Martin ??
    I've used firestone HD guard 0.5" fiber faced polyiso under a membrane flatroof recently, and it is extremely stiff !! i would use it under plywood on top of other insulation ( R5.5 but only 1/2" )
    and it was not really expensive ..could let you use cheaper polyiso underneath and still get good total R .

  3. GBA Editor
    Martin Holladay | | #3

    I think it's a lot easier to put the rigid foam insulation under the slab than over the slab. I'm not advocating this approach. As you point out, the concrete slab will obviously be much colder if the insulation is on top of it.

    However, if someone wants to insulate on top of the slab, and they are worried that the slab is cold, all they have to do is add more insulation.

    If you want R-40 rigid foam on your slab, nothing is preventing you from installing it -- except the difficulty of attaching the plywood on top. As the foam gets thicker, you reach a point where the most logical way to establish a floor on top of the foam is simply to pour another 4-inch slab on top of the foam. At that point, the whole approach just gets ridiculous.

  4. user-1101088 | | #4


    I stumbled on this post while searching for a quick answer to a similar question I have. I'm just looking for a clarification...

    I live in Montreal and already have the basement slab poured on top of some rigid insulation. 2" of XPS I believe, then a vapor barrier then 4" of slab. There is also 1" of XPS on the walls going down to the footing before the slab was poured, so thermal bridging is cut off. All is good with that.

    Now I'm getting ready to do the floor and was thinking of throwing down another 1" on rigid foam on top of the slab with plywood directly on top. I was thinking the second layer of foam would be to take that edge off from the concrete. What I wanted to make sure is that the sandwiching of concrete between two layers of foam isn't gonna cause any issues... Just want to make sure I don't do something stupid there... :oP

    Also, if it's ok to sandwich, is there a particular type of foam better suited? XPS, vs EPS as far as rigidity or perm?

  5. GBA Editor
    Martin Holladay | | #5

    There is nothing wrong with your proposed sandwich. Most people use XPS for this application, although EPS will work. Depending on your anticipated loads, you may want to choose one of the high-density varieties of EPS if you prefer EPS.

  6. user-1101088 | | #6

    Great, thanks Martin.

  7. jinmtvt | | #7

    Martin : just for informatie purposes ( je suis curieux! )
    would you think that humidity levels within the concrete slab could pose any problems ?

    Thinking about it afterwards, i do not see why...still would like your thought on that !

    Martin, why is polyiso never suggested around here?
    I just recently discussed with 2 EPS manufacturer and my main roofing distributor,
    and i can get Firestone Polyiso for cheaper per R than EPS and XPS
    and they've got some that is good for rooftop weight loads, must be usable for underfloors ??
    I remember readin that they didn't recommend using it below grade where it might get wet ??

  8. GBA Editor
    Martin Holladay | | #8

    If you include a capillary break (4 inches of crushed stone) under a slab, plus a layer of polyethylene, your slab should stay dry.

    Q. "Why is polyiso never suggested around here?"

    A. On the contrary, I often suggest that polyiso makes sense. See, for example, this article: How to Install Rigid Foam Sheathing. In that article, I wrote, "Polyiso ... enjoys a solid reputation as the most environmentally friendly type of rigid foam insulation."

    You can't use polyisocyanurate under a slab because it absorbs moisture. GBA has plenty of information on these topics; I suggest that you use the "search" feature on our website. (I think that 80% of your questions could be easily and quickly answered, Jin, if you first tried to search for the answer.)

    For example, see this article in our encyclopedia: Rigid Foam Insulation. That article notes, "Because it can absorb water, polyiso is not recommended for use under slabs or on the exterior of foundation walls."

  9. jinmtvt | | #9

    MArtin : thanks again for the answer

    Yes i tend to ask too quickly, and then find out the related info/articles while waiting for and answer,
    as i just read the Rigid Foam Insulation article before this post :p

    around here .... i meant around here in Quebec
    I've never ever seen it use in ANY residential project
    i've only seen it use in flatroof when contractor buys from Firestone distributor
    otherwise they use EPS or just a layer of XPS .

    As for the slab, i understand that it will not get wet from underneath,
    but i was more referring as it drawing moisture and condensate from interior air because of its temperature difference if insulation is added on top of it .

  10. GBA Editor
    Martin Holladay | | #10

    Assuming that someone wanted to put between 2 inches and 6 inches of rigid foam on top of a concrete slab, the foam would be a fairly effective air barrier as well as a fairly effective vapor retarder. There is no reason to believe that there would be air movement through the foam; nor would there be much (if any) vapor diffusion.

  11. jinmtvt | | #11

    Superb, that is exactly what i had conclude from reading posts here, but wanted to make sure !!

  12. user-917907 | | #12

    Thanks to all for your comments and suggestions. I can see that the devil is really in the details.

    If most of the foam insulation was under the concrete slab, but a one-inch layer was put above the slab, then a layer of plywood (as Tom W suggested), what density would be recommended for that above-slab layer be?

    How would the plywood be attached to the foam/slab? I can think of attaching sleepers to the slab, putting foam between the sleepers, and then screwing the plywood to the sleepers. Or laying the sleepers on top of the foam, then screwing the plywood to the sleepers (would the foam tolerate the increased pressure of the sleepers?) Or perhaps putting two layers of plywood down on top of the foam, at right-angles to each other with joints staggered, then screwed to each other. Are there other ways to attach the foam/plywood to the slab?

    Back to my original idea of placing four to six inches of foam above the slab. Maybe the slab isn’t really needed at all??? How putting a 15 mil layer of vinyl ( for the vapor/radon barrier on top of the leveled and compacted sand/gravel, then four to six inches of foam, then either sleepers and plywood or two crossed layers of plywood? What is that expensive cold slab of concrete needed for anyway?

    Here is an outfit in Alberta, Canada that sort of does that, but uses steel “studs” for the sleepers:

    But their foam isn’t as thick as I would like, it’s manufactured at the wrong end of the continent for me, and I suspect my DIY version might be cheaper and have less thermal bridging. But if I am inventing a new mousetrap I’d like to know what unintended consequences I’m not thinking of?

  13. jinmtvt | | #13

    never noticed their "slab replacement" section until u pointed it out!!

    I don't see why this wouldn't work for residential basement
    that will end up getting a wood floor neway

    How much is worth a basement slab ?? ( concrete + work ? ) and u still gotta install insulation under it neway ,, why not skip the concrete at all!! what exactly do we use the concrete for in a residential building neway ??

    Worth a tought i'd say ... get thicker foam with the money saved on the slab ..

    Martin could you chime in on that ?

  14. jinmtvt | | #14

    Just a quick thought, if this is not fastened to the ground ( and it sure doesn't see like it is )
    Once could do the equivalent using poly foam gun dispenser and large block of EPS/XPS .
    Glue them together (side and overlap if overlay )
    then glue the plywoods on top of it with the same foam

    I've tested it on type 2 EPS, and Dow professional foam adhesive on plywood/EPS rips out the EPS itself when u try to pull it off

  15. GBA Editor
    Martin Holladay | | #15

    Even if you convince a local code inspector that you don't need a concrete slab -- and the convincing will take a lot of work -- you still need concrete at the perimeter of the house to bear wall and roof loads.

    Concerning rigid foam on top of a slab: if you want to avoid sleepers, there are two basic approaches: either fasten the plywood through the foam with Tapcons, or install two layers of floating plywood (screwed to each other to prevent "potato chipping.")

    Many of these issues have been discussed before on the GBA forum. See, for example:

    Where to put the vapor barrier?

    Basement subfloor retrofit insulation options

    Floating plywood floor on rigid foam insulation on concrete?

    The Stay-Dry, No-Mold Finished Basement

    Finishing a basement floor

  16. Oak_Orchard | | #16

    You are making an interesting challenge to convention …

    It's all doable but unpopular and not well supported by the building materials industry or contractors. Plan to do the work yourself of pay much more than conventional construction.

    Frank Lloyd Wright was fond of rubble trench foundations and they are structurally fine but you need an engineer's signature. Excavate to local frost line clearance depth, pour in, and compact crush rock. A perimeter beam can be formed with concrete-at-grade (called a grade beam) or use a rated ground contact PT lumber or rough dimension timber.

    Same with the floor/slab issue: ground contact rated PT dimensional lumber, plywood and timbers. Then insulate the floor box. You can create a center beam or any specific lay out for intermediate structural supports (under your walls) with the rubble trench or PT framed foundation.

    The reason for slab-on-grade is usually to have in-floor radiant. The reason for XPS and vapor barrier underneath is to isolate the mass to keep it from losing heat and absorbing moisture. Otherwise why have a concrete floor. (Except no concrete means unsurmountabel insurance, mortage and resell issues). It has no specific structural advantage. It is cold, condensing and it’s a sponge unless its part of your designed and moisture- and mass-managed heating and space conditioning system.

    Big issue with slab-on-grade is your mechanicals. They all have to be in the walls and that is not as easy or convenient as one imagines. I tried it and no contractors like to work in this complicated environment of piping, wiring and heating system plumbed in walls. It's more expensive to work on and hard to get to later. Everything ends up behind drywall. You will need to thicken the walls to protect the pipes. All piping and wiring and supply/returns, venting and waste evacuation are greatly complicated without a basement or a decent and conditioned crawl space (i.e. one you don’t mind crawling on for hours and doing work on your back or a stool).

    Using PT ground contact rated lumber is expensive and now a-days might not be any less expensive than a 4" on grade concrete slab. You must stack and climatize PT lumber for at least 6 months. It must have no sun exposure to it at all during this period.

    XPS is expensive so no matter which foundation system you chose (grade beam, rubble trench, perimeter concrete, PT'ed or hybridized), you need to put out for the insulation. Some say there is no advantage to poly-iso as it apparently loses R rating over time.

  17. user-917907 | | #17

    Thanks for all the helpful links. It sure would make the GBA site more useful if there was some sort of automated indexing system for all the blogs and forum topics. You must get very tired answering the same-old same-old day after day. Thanks for sharing your knowledge, and for being so patient.

    I’m aware of the rubble-trench foundation and am thinking along those lines too. My possible building site is on a knoll and should be very easy to drain.

    Could you explain why pressure-treated lumber must be stacked and climatized, with no sun exposure, for six months? I’ve never heard of such a requirement.

    I think that in-floor radiant heat might be doable with my proposed sans-concrete foam/plywood floor. After laying down the foam and the first layer of plywood, lay out the heat tubing on top of the plywood. Then screw/glue 2x sleepers into the first layer of plywood between the tubing. Then screw/glue the second layer of plywood to the sleepers. Should make for a very stiff floor. But the tubing wouldn’t be as well protected against a stray nail or power saw as it would be if it were imbedded in concrete. Or maybe the tubing could be placed in grooves in the upper layer of foam, just under the lower layer of plywood? Not sure what sort of heat tolerance Styrofoam has? (This is all a moot point for me as I’m not planning to install radiant heat)

    I’m thinking you could run electrical conduit, water lines, and wastewater lines in the foam, or certainly down in the gravel below. Maybe that is easier than doing the same with concrete? If the sleepers were 6" or 8" maybe it would create a plenum between the two layers of plywood large enough to run forced air ducts?

  18. GBA Editor
    Martin Holladay | | #18

    You wrote, "It sure would make the GBA site more useful if there was some sort of automated indexing system for all the blogs and forum topics."

    When it comes to searching for previous Q&A threads, I can't think of a system that will work better than the search box. The search box works pretty well, although it helps to choose the right search words, and sometimes you have to try a few combinations of search words before you get good results.

  19. Oak_Orchard | | #19

    Jack …I also favor sans-concrete building. Concrete is expensive and since it is not waterproof and since it is so dense, it must be either completely isolated from the ground or completely isolated from the house. There is no in-between. Isolating concrete slabs and basement/perimeter walls is expensive and involves a lot of unnecessary materials. It is certainly not "green".

    And by eliminating concrete you can also eliminate a lot of XPS and EPS.

    But you need to stabilise the house on some thing that gets you below frost. That can be gravel or PT posts or even concrete posts would be a small inexpensive concession.

    Raising an insulated box completely off the ground and isolating it on posts/piers reduces going-in costs substantially. If you have no envelope-to-ground contact, you don’t need the PT at all (unless you're using PT posts or piers).

    PT lumber comes in several grades. Top grade is required for ground contact. Is expensive and it's also uses better structural lumber, not just top grade treatment. But it comes to you wet. PT dimensional lumber and PT plywood must be stacked properly to dry and acclimatize or they sag and twist.

    I recently used the in-ground certified and rated PT 4" by 8"'s … they took 5 months last winter to dry in an unconditioned space. You could speed it up in a conditioned space. We used them for heavy sills.

    This summer I set a loose bundle of ½" ground contact rated plywood against a building for a few days on a job where it was exposed to wind and sun. It twisted and warped much more than other grades/types of ply, making saw cuts and assembly difficult.

    The in-floor radiant questions/suggestions need to be posted here as a separate topic.

  20. Scot69 | | #20

    Many thanks to Martin and others that impart knowledge,it is much appreciated.I thought you might be able to affirm or redirect my plan to better heat a three story open space that is very cold on the first floor due to heat rising.I plan to lay 1 inch of XPS on unisulated concrete slab,apply 3/4 strapping,adding 1/2 inch pex with s and covered with 3/4 plywood and laminate flooring,yes headroom is an issue,but am willing to sacrifice some to not compromise it.

  21. GBA Editor
    Martin Holladay | | #21

    Whenever a house has extreme stratification issues -- with a hot ceiling and a cold floor -- the cause is usually air leakage. I'm guessing that your house is leaking a lot of hot air out of the top floor ceiling, and that a lot of cold outdoor air is entering through your basement, crawl space, or cracks near the bottom of you house.

    Rather than installing a radiant-floor heating system, you would be better off investing the money in blower-door-directed air sealing.

  22. jinmtvt | | #22

    Oak Orchard: not to sure what you are inferring here ...

    building green = building that lasts
    I don't see anything other than concrete or lbocks or stones , getting through a flood or a very bad leak.
    If the foundations of a building are not solid, how good is the building ??

    Now the basement floor slab is something else.
    Other than load points, what is the necessity for concrete floor in a basement if it is going to be covered by some other flooring finish later on ( in a finished basement of course ) ??

    Even for a slab on grade here, u still need to install footings at ~5ft depth, so again either bocks or concrete is required there ..posts are probably much more expensive.

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