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

Building a Subfloor on Top of Foam Over Slab

mstaj | Posted in Energy Efficiency and Durability on

Hello, I have read so much on this forum and learned a ton. I am renovating my house (2 hours north of toronto, ontario). I am converting my old slab on grade (uninsulated) garage to a 4 season living space.  I am going to be heating the space by attaching it to my existing HVAC system, and so I need to have heat runs in the floor. I would need about 5-6″ of space for the heat runs.

I want to know if I can lay 35PSI foam on top of the concrete and THEN build a traditional style subfloor with 2×6’s on edge so that I have space for my HVAC runs in the floor? (I have tons of ceiling height to work with so the floor being thicker is not an issue). ie have the 2×6 on edge directly on the foam.

The other way I have seen this done is by having 2×2 sleepers on the concrete and then I can insulate between them with the foam – this brings up a thermal bridge from my cold uninsulated concrete slab – but if it has to be done this way then I will.


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. Expert Member
    Michael Maines | | #1

    Matt, unless you're planning on very heavy loads, your approach should work well. I went through the math in this article: I do foam-between-sleepers when there is concern about ceiling height, but otherwise I prefer for the foam to be continuous. I recommend using EPS or GPS for environmental reasons; polyiso isn't a good choice where there could be standing water at any point, and XPS blowing agents are potent, persistent greenhouse gasses. Over time, XPS thermal performance will drop to be almost equivalent to EPS, so you might as well just start with EPS.

  2. Jon_R | | #2

    > have the 2×6 on edge directly on the foam

    Another way to analyze this is that 25 psi foam is commonly used in floor panels where there is direct/continuous bearing from surface to support. Joists spaced 16" will concentrate pressure by about 11x - which suggests far more than 35 psi foam. Some discussion by Malcolm and Martin (see below) that I take as "not recommended". Maybe 35 psi foam changes things enough.

    Without seeing more analysis by structural engineers, I'd use something stronger to transmit floor pressure directly to the concrete. Your 2x2s option(perpendicular to joists) would do this. So would slices of 4x4 here and there (through the foam, located under the joists)- this also provides a leveling method (the concrete will not be perfectly level and will not bear evenly on the joists). Lag screws part-way into the bottom of the joists would also support and level. The thermal bridging isn't very significant (it's not a simple ratio of foam R value to steel x area).

    1. Expert Member
      Michael Maines | | #3

      Typical floor loads are 40 psf live load and 10 psf dead load. With 16" o.c. joists, that's 67 pounds per linear foot, or 5.6 pounds per linear inch along the joist. That load is spread over the 1.5" joist so the load on the top of the foam is 5.6/1.5 or 3.7 pounds along the joist, or 5.6 pounds per square inch on the foam.

      Foam bearing capacity is rated at 10% deformation, which is fairly linear with load. To account for long-term creep under sustained load, engineers use a 300% safety factor. So 35 psi foam is really good for a 12 psi load, more than twice the actual load, which means deformation would be more like 5% of the foam thickness. For 2" foam, that's less than 1/8", which is more than ideal if it deflected that much when you walked on the floor. But you wouldn't have the full 50 psf load for extended periods, joists will spread out any loads, and most joists NOT sitting on foam will sag more than 1/8" over time. So I don't see the problem. I would be interested in Malcolm and Martin's comments but your link isn't working.

      1. Jon_R | | #4

        This site has been removing links, after they are entered and then tested. Fixed (for now). Hmm, broken again, let's try this:

        Isn't concentrated/point load (eg, 1000-2000 lbs) the right input (vs 40 psf which is specified as a uniformly distributed load)? Below they came up with 800 plf = 44 psi, much more than 35 psi foam derated to 12 psi.

        1. Expert Member
          Michael Maines | | #5

          It's not working for me, using Chrome. I know they're struggling with spammers. Maybe posting the whole address instead of an alias would work?

          1. Expert Member
            MALCOLM TAYLOR | | #7

            I may be wrong but I think there is a quantitative difference between the loading experienced by the foam when it is below the floor joist as Matt is suggesting, and above them as it was in the discussion Jon linked to. In the first case any repetitive point loading (from say a good sized guy doing jumping jacks) is distributed by the joists before impacting the foam. When the joists are directly under the foam those forces are only mitigated by the unsupported plywood subfloor. In that situation the other concern is that with each deflection of the foam the fasteners are also pushed upwards into the finished flooring, or loosened.

          2. Jon_R | | #8

            Good point.

            Given that the garage floor won't be perfectly level or flat, is it even buildable with joists directly on foam?

          3. Expert Member
            Michael Maines | | #9

            Malcolm, could you post a link to that discussion? Jon's link isn't working and I'm really curious about the details.

          4. Expert Member
            MALCOLM TAYLOR | | #10


            it's the first link in Jon's post #4 above.

        2. Expert Member
          BILL WICHERS | | #6

          A point load would be something like a table leg. There was a thread on here a while back where I ran the numbers for someone for a pool table, and that ended up being a bit of a problem, so the recomendation was something like two layers of 3/4" plywood to spread the load out over a larger area.

          Point loads concentrate large loads into a small area. An example would be a 100 pound table with four 1 inch square legs. Assuming the table is evenly balanced between all four legs, that means each leg carries 25 pounds, which works out to a 25 PSI load per leg. If you put the table upside down, assuming a 4 foot square table, the 100 pounds is distributed over 16 square feet (4 feet multipled by 4 feet), which is 2,304 square inches. that means you have a distributed load of 0.0434 PSI for the same table, because the load is distributed over a much larger area.

          Foam is rated for compressive strength in PSI. 35 PSI foam here would be good for 35 pounds on any given square inch of foam, or distributed loads of the same amount which works out to 5,040 pounds per square foot. As far as the foam is concerned, there is no difference between a point load and a distributed load as long as the total load on any given square inch of foam doesn't exceed the 35 PSI rating.

          I was going to work out the numbers just as Michael did, but since he beat me to it, I'll just say I agree with his math. All you need to do is take the floor load in square feet, and convert that to an equivalent load in PSI distributed over the total cross sectional area of whatever is going to be in contact with the foam and make sure the result doesn't exceed the rating of the foam. In the OP's case, you're taking the floor load and concentrating it onto the edges of some 2x6s, so it's pretty easy.

          You can extrapolate this out into a multiplier too. With 16" OC joists, you can consider this to be 1.33 square feet of floor area supported by each lineal foot of floor joist. A lineal foot of dimensional lumber (which is 1.5" wide) on edge has a total cross sectional area of 18 square inches. This means 18 square inches divided by 1.33 square feet gives a multiplier of 13.5 -- whatever you put on the floor in pounds per square foot gets concentrated into that much PSI from the perspective of the foam compared to putting the load on the foam evenly, without the joists.

          Working in reverse, this means 35 PSI foam is good for 35 * 13.5 = 472.5 pounds per square foot of evenly distributed floor load. That's almost twelve times the usual residential design load of 40 pounds per square foot. 40 pounds per square foot divided by 13.5 gives a bit shy of 3 PSI of concentrated load on the foam directly beneath the joists.


  3. mstaj | | #11

    Thanks for all the discussion so far. I appreciate all the viewpoints. I think generally it looks like it will be ok directly on top of the foam, especially with the load being spread by the overlying floor. I don't understand the levelling comment though. The garage floor isn't perfectly flat but it's close and I was thinking the foam would allow for some wiggle room given the floor isn't out by more than 1/4" . I'm going to put a glue down cork floor on top so I don't think it has to be perfectly level. (Also given the quality and amount of crowning in so much of the lumber were getting right now, I don't knowif having the concrete under perfectly level is going to make any difference).

    Thanks again

    1. Expert Member
      Michael Maines | | #12

      Matt, when I've considered using this detail I planned to put shims under the joists at any low spots. If the joists are undersized for the span, the floor would feel bouncy if not in direct contact with the foam or other support.

      1. Jon_R | | #13

        Start adding small shims and the foam load calculation in #3 is way off. But with only 1/4", maybe shims aren't needed.

    2. Expert Member
      BILL WICHERS | | #14

      It's probably best to use floor leveler prior to putting down foam. That way you avoid any issues with load concentration on the foam. If there are just a few small bumps, the foam compression can actually help you since it will tend to even out small irregularities in the floor.


      1. mstaj | | #15

        This is the idea. There really isn't many spaces in the floor and the couple small bit are not huge.spots, my thought was the foam would push into small irregularities. I could also put in some longer/wider shims if once I build it I find I will have a gap.

  4. Expert Member
    PETER G ENGLE PE | | #16

    You can also grind down any high spots. You might want to do a survey of the floor with a long (8') straightedge. Just a guess, but deviations up to 1/8" will probably not be noticeable, even maybe up to 1/4". Much above that, you might start to see some deviations in the floor above. Likewise, small deviations in the joists themselves will tend to be de-emphasized by locally deforming the foam, but larger deviations won't be able to deflect the foam enough and could telegraph through. Overall, I think you'll be OK with your approach.

    One other thought, though: In IRC code areas, garage floors are pitched to drain. Is yours pitched or level, and if pitched, how are you going to account for that in your foam/floor sandwich?

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |