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Building code question: 2×4 floor joists supported by foam

matt2021 | Posted in General Questions on

Framing a floor above a concrete, tiled slab in New Jersey. NJ has adopted the International Building Code. Does anyone know whether the IBI allows for floor joists that span over 6’-7’ (indeed mine will span 14.5’) to be made of 2x4s, provided that adequate support is provided?  And does the code anywhere mention the option of rigid foam as under-joists support?

The assembly I am looking at is as follows:

3.5” of rigid foam (XPS 25 psi)
2×4 joists (16 O.C.) sitting on the foam at one end, and, at the other end and in between, because of a slight slope (approximately 1”-1.5”) in the floor above which the new floor is being framed, some additional foam blocks, or if needed wood blocks, secured with spray foam (from cans, or indeed the same spray foam that will be professionally installed in that room’s ceiling). It’s quite possible that the 2×4 joists, rather than being supported, say, every 5’, will be supported by the foam (pieces of rigid foam and spray foam) along their whole length.

I like this assembly a lot. Yet, I don’t want to run the risk of having to face objections from the inspector, in case using 2×4 joists for such spans, supported by foam rather than wood blocks that would be sitting against the concrete, is not contemplated by the code. It seems that, every time something is not included in the code, that risks triggering the demand for an engineer‘s study. On the other hand, I suspect that these assemblies are becoming more and more common. I just don’t know whether I could point to a code section, were any objections to be raised. 


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  1. plumb_bob | | #1

    Unless the slab you are building on top of is a structural slab, concentrated floor loads should be directed to the foundation, not the slab.
    Also, 2x4s make poor floor joists because acceptable deflection limits are quickly exceeded, my joist span tables all start at 2x6.

    1. matt2021 | | #3

      Thanks! The slab is structural. And I agree (from what I read) on 2x4 being an unusual size for floor joists; yet, they can be used, for smaller spans (6'-7') OR when additional support is given every, say, 5'. My concern is more about whether supporting the joists WITH FOAM would be considered acceptable. I trust the assembly will be quite solid; but I wonder whether it will be objected to, on the grounds that it's not in the code.

      The alternative would be to use 2x6 joists, and have less foam under them. Maybe I'll go for that. Yet, the two issues will still remain: 1) is having joists sit on rigid foam common enough now not to raise objections even if the prescriptive code does not mention that? 2) Would compensating for the 1"-1.5" slope by adding more rigid foam, and then spray foam to keep everything together, going to be considered acceptable?

  2. plumb_bob | | #2

    I do not work with the IBC, but I very much doubt you will find any prescriptive methods of supporting structural framing members on foam.
    A better method may be to place the foam over the slab and then have 2x4s on the flat fully supported by the foam, then sheathing on top. Then no concentrated loads would be created.

    1. matt2021 | | #4

      Actually, I am not sure what is meant, precisely, by "concentrated loads." I was hoping that adding pieces of rigid foam and spray foam wherever it is needed under the joists (needed because of the slope) would distribute the load along the whole length of each joists, not concentrating the load in just a couple of spots.

      About your suggestion: are you suggesting to add even more rigid foam than what I had planned, between the tiled floor and the 2x4s on the flat? The floor that is being framed will need to be 7" high (from existing tiled floor to the bottom of the underfloor). So, if built up with foam, and then only add 2x4s on the flat, I'll need 5.5" of rigid foam. Also, would it be OK to have the 2x4s just fastened (and how) at the two ends? With joists, placed on the edge, I could have used joist hangers.

  3. DC_Contrarian | | #5

    You could make an argument that the entire assembly is just the floor finishing material. Don't call them joists, call them sleepers.

    What I would do is run the foam parallel to the floor, then taper the joists to make the floor level. It's easier to do and the joist is better supported. I would size the joists so that at the highest point of the floor the joist has zero thickness.

    Let's say you want the floor to support 50 PSF. At 16" spacing that means the joists are supporting 67 pounds per linear foot, or 5.5 pounds per linear inch, or just under 4 PSI. So your 25 PSI foam is ample. If you don't have uniform support you'll get spots where the load is concentrated and the foam is deformed, in time your floor will sag.

    If the slope of the floor is uniform, it's easier to run the joists/sleepers across the slope. That way each one is the same thickness for its entire length. It's easier to rip a straight line than an angle.

    1. matt2021 | | #6

      Thank you, DC_Contrarian! Let me go in order, to make sure I understand, and make the best of, your suggestions:

      "You could make an argument that the entire assembly is just the floor finishing material. Don't call them joists, call them sleepers."

      Unfortunately, I called them joists in the permit request, though I don't know whether that matters. In any event, are thinking of them being on the flat then?

      "What I would do is run the foam parallel to the floor, then taper the joists to make the floor level. It's easier to do and the joist is better supported. I would size the joists so that at the highest point of the floor the joist has zero thickness.
      If the slope of the floor is uniform, it's easier to run the joists/sleepers across the slope. That way each one is the same thickness for its entire length. It's easier to rip a straight line than an angle."

      My contractor would like to avoid ripping all the joists. I am quite confident that the slope is uniform. So, running the joists/sleepers in the other direction would require much less and easier ripping. Indeed, if using sleepers, it's unclear how many of them would be needed. If aiming for 0" at the highest points, It almost seems that we would be moving towards a joist-less floor, one made entirely of foam. Am I getting that right? If, instead, we go for only a layer of foam and regular joists, then ripping them along a straight line would indeed be simpler. However, in that direction, the span is 23', past what on can do with 2x6s. So, again, we are back to the question of whether their standing on a layer of foam would give enough support (and that seems definitely the case) AND be recognized as doing so.

      "Let's say you want the floor to support 50 PSF. At 16" spacing that means the joists are supporting 67 pounds per linear foot, or 5.5 pounds per linear inch, or just under 4 PSI. So your 25 PSI foam is ample. If you don't have uniform support you'll get spots where the load is concentrated and the foam is deformed, in time your floor will sag."

      So, having a board of 2" Foamular 150 next to one of Foamular 250 is dangerous?

      Also, I am really not knowledgeable enough about compressive strength: what is a the compressive strength of a 1.5" Foamular 250 on top of a 2.5" Dow (again, 25 psi)? Does it stay the same at 25 psi, or does it increase with the total thickness of the foam layers?


      1. Patrick_OSullivan | | #7

        > My contractor would like to avoid ripping all the joists. I am quite confident that the slope is uniform.

        Find a new contractor. (I know, I know, easier said than done.)

        Ripping a uniform slope into these sleepers is far less fussy than trying to support them with bits and bobs of foam and wood.

        1. matt2021 | | #10

          Thanks, Patrick! My contractor is actually quite accommodating, and there is room for conversation. I am looking for the solution that would make more sense, and cause no issues with the inspections. The thought of using foam, including spray foam, was suggested by the spray foam person (who is very competent, and whose judgment I trust); I just fear the inspector might object to that approach.

          1. DC_Contrarian | | #13

            I think the question you should be asking is not whether the way you want to do it is code-compliant, but whether it's a good way or the best way to proceed.

            I think you need to abandon the idea of framing members supporting the floor. Instead, you need to think of a flat floor resting on the concrete slab. There are two complications: first, you want a continuous layer of insulation, and second, the slab isn't level.

            It's not uncommon to do a continuous layer of foam over concrete, we talk about that all the time here. If you put two layers of OSB over the foam, with joints overlapping, that are screwed to each other, you can have a floating assembly that is heavy and solid enough that it won't move (or almost floating, sometimes it helps to put a few Tapcons through into the concrete). With this kind of assembly the compression loads on the foam are so low that compression isn't a worry.

            The other issue is leveling the floor. This is accomplished with some sort of shimming. However, shims put high point loads on the surfaces that they bear on, you just can't shim the foam or use the foam for shims. What you need to do instead is to shim between the two layers of OSB. Alternately you can have a single layer of OSB and something resting on the foam with a large enough area that compression isn't a worry, That's what I was talking about in post #5.

            Alternately, you could have the support of the floor rest directly on the concrete, and not have continuous insulation. The customary way to do this is sleepers, which are pieces of wood which are supported along their entire length. Sleepers would be cut so that when they sit on the concrete their tops are level. Since they are supported there are no span limitations. Your subfloor can span 24" so you could space them every 24" so you wouldn't lose that much insulation value by not having it continuous.

      2. DC_Contrarian | | #9

        "However, in that direction, the span is 23', past what on can do with 2x6s."

        So the idea is the sleepers are supported along their entire length by foam and then the basement floor, so they aren't spanning anything. So it doesn't matter how long they are.

        Let's say the whole floor is 14.5' wide, the slope is constant, and the low end is 1.5" lower than the high end. So on the high end you've got the subfloor resting on the foam, and at the low end you've got a 1.5" sleeper -- a 2x4 on its side. In between are 10 other sleepers on 16" centers. Each one is 0.136" shorter than the preceding ones, slightly over 1/8". So the first one you rip to 1-3/8", the next one to 1-1/4". The middle one is 3/4", so could just use strips of plywood, it's easier than ripping narrow pieces. Plywood comes in 3/4", 5/8", 7/16", 3/8" and 1/4". You'd have to figure out how to get 1/8" for the last one. For the thicker ones you could also stack two pieces of plywood rather than ripping anything.

        It's somewhat common in basements to just put down subfloor over foam. The challenge is how to keep the subfloor from moving. If the subfloor is screwed to the sleepers it it will be one solid heavy piece that won't move. That's an argument for making the thinnest sleeper maybe 3/8" so there's something to screw into. So have your sleepers range from 3/8" to 2-1/8".

        1. matt2021 | | #11

          Thanks, DC_Contrarian!

          In fact, I am framing the floor, over an existing floor, in a porch-to-room conversion, not a basement. So, perhaps concerns about the subfloor moving are further mitigated by the fact that the subfloor will be screwed into the rim joists all round, on four sides.

          I worry that the floor might be bouncy or squeaky. Maybe I should go for two layers of plywood, plus the sleepers your mentioning.

          I am still puzzled about the foam's compression resistance: I have some Foamular 150 and 250; can they be used next each other, or is one section of the floor going to be weaker than the other in a troublesome way?

          1. Expert Member
            Michael Maines | | #21

            Foamular 150 is rated for 15 psi; Foamular 250 is rated for 25 psi. The ratings are for 10% deformation at the full load (i.e., 15 psi or 25 psi.) Foam deformation is fairly linear under load, meaning that a load on 15 psi foam will "sink in" about 1.67 times more than it would on 25 psi foam. (25/15 = 1.67). That's for short-term loading such as someone walking on the floor.

            There is another consideration: long-term creep, for sustained loads on the foam. The typical factor is 30% or 0.30; 15 psi foam can support a 5 psi sustained load; 25 psi foam can sustain an 8.3 psi load.

            Residential floors are usually designed for a 40 psf live load and a 5-10 psf dead load. In a situation like yours, 5 psf is probably fine, so call the total load 45 psf. If you have sleepers 16" o.c., that means that each liner foot of sleeper has a load of 45*(16/12) = 60 plf, or 5 pli. If the sleeper is on its 1.5" edge, that means the load on the foam is 5/1.5 or 3.3 psi, just fine for either 15 or 25 psi foam.

            I don't see the point of doubling the subfloor if you have fully supported sleepers. Standard 3/4" subfloor material (I use Advantech) does a good job of sharing the load between adjacent joists or sleepers.

            In case it's not clear, "pli" is pounds per linear inch, "psf" is pounds per square foot, etc.

  4. adrienne_in_nj | | #8

    Is this a single family home? If so, the IRC applies. It’s my understanding that the IBC only applies for structures not covered by the IRC.

    Is this a porch that you are enclosing or a garage that you are converting? I have a couple of small rooms in my house that are constructed with sleepers on top of a slab, but the slab is level. It’s an older home and this was the way it was originally constructed, so presumably it was approved at the time of construction. They’re constructed as follows:
    2x4 exterior walls on top of footings. 2x6 Rim joists attached to the inside of the 2x4’s. This creates a balloon framed type situation and air sealing must be thought through. 2x6 sleepers are attached to the rim joists with nails. The sleepers are just an inch or so above the slab, so there’s not enough room for joist hangers. There is about 1” of EPS under the sleepers (I didn’t measure the thickness but the sleepers are very close to the slab.). The sleepers are supported every 5’ or so (I didn’t measure) with blocking/shims. I’m unsure if there are footings of any kind under the blocking/shims but my guess would be no. You could construct something like this and the balloon framing would allow you to construct a level floor system without ripping the sleepers to account for the sloped floor. I think 2x4 floor joists would be a problem as there would be too much deflection. The larger the joist, the longer they can span without support. You could also look into some engineered joists that can span more than dimensional lumber without support. Why do you want to use 2x4’s? Is it because of a desire to get a certain amount of foam under the floor while keeping the room height, or to match the floor height with other floors in the house?

    1. matt2021 | | #12

      Thank you, Adrienne!

      It is a single-family home; so, it sounds like I got the wrong code reference. I still worry about receiving objections to anything that is not quite standard. Originally the inspect had indicated, as a standard way of doing things, sleepers against the existing floor, and joists sitting on them; yet, like you, I'd like to have foam under the joists, to mitigate thermal bridging.

      It a porch-to-room conversion, and indeed what you are describing is very close to what I was thinking of doing: rim joists secured to the posts on three side and to the house on the fourth side.

      From the existing floor to the bottom of the new subfloor I have 7" (that's determined by the height of the subfloor in the room nearby: I'd making the new floor flush with that of the other room). I'd like to fill those 7" with insulation, and I liked the idea of having rigid foam, then the joists, and fluffy insulation (or more rigid foam and then fluffy insulation) between the joists. (I am going to add a sketch to my post).

      2x4s would seem to be a good size to achieve what I want. Yet, I could also have only 1.5" of foam against the concrete, tiled floor, and use 2x6s. In both cases, however, there are the two issues: 1) how to compensate for the slope without ripping all the joists, and 2) whatever I do, to do things so as not to have issues with inspections.

      From the various responses I have received, it seems that maybe I could even just use sleepers, and even fairly thin ones. My permit application mentions joists though, and I don't know whether that will turn up being a problem. Also, I did like the idea of using joist hangers, so as to have a firmer floor. It seems that the foam, though, would give enough support. Indeed, as some have pointed out, perhaps there is no issue of how far the joists will span, as boards that rest on rigid foam are really not spanning for any length.

      I confess that I am having a hard time deciding between the different options.

  5. matt2021 | | #14

    @DC_Contrarian, Thanks for your helpful suggestions, including the ones in post #13!

    Absolutely, I want to have a good, sound approach, regardless of code considerations. My reference to code (as well as to what is customary) only has to do with making sure the township will not object (as that could have disastrous consequences). My building permit currently mentions joists (of size to be determined), 16 O.C., joist hangers. Small departures from that plan are very likely to be OK. Greater departures would require me to submit for a change, wait 30 days, and - if my approach departs from the code - possibly an engineer report. They want to inspect the framing before the insulation is installed. If I have a layer of insulation under the joists, that's close enough to what I have in the permit. But if I have a floating floor, made all of insulation, there will be very little, in terms of framing, for them to inspect: just the rim joists really.

    In any event, I'll draw several sketches, of different floor options, including the ones you suggest. You don't seem to have a preference between the option that uses two layers of OSB (or plywood?), with "shimming" strips in between, and the one with only one layer of OSB and the shims against the foam, it seems. But you definitely prefer the joistless, floating floor option. I really appreciate all your input! Whatever we go for, I'll post updates.

    1. DC_Contrarian | | #15

      If I were you I would ask the building inspector if instead of putting in joists he'd be OK if you put a floor over the existing concrete floor. Hard to imagine he'd object to that.

      Words have meanings. "Joists" mean structural members that span a horizontal distance, supported at their ends and possibly at midpoints. If they're supported at the midpoints, it has to be something structural -- which means there's a load path to a footing. If your floor is just regular poured concrete, it's not structural and it can't be used to support joists. So the joists have to span the entire room and be supported solely at their ends. For unsupported joists with a span of 14.5' you need at a minimum 2x8 Douglas fir on 12" centers. The way the code works is that there are certain prescribed ways of doing things, if you do it that way you're OK. You might be able to come up with an equivalently good way, but if it's not the prescribed way you need an engineer or architect to sign off on it.

      I'm not saying he will, but there's no reason in the world why the building official shouldn't sign off on a flat floor supported by the existing slab. It's not like this is something that can collapse on someone and kill them.

  6. gusfhb | | #16

    They are not joists as they have no span, and they carry no load. They are, to my mind superfluous. I wouldn't even call them sleepers. They are stiffeners for the plywood
    There is not structural reason that the floor would not pass muster without the 2x4s

    1. matt2021 | | #17

      Thanks, Gusfhb! I am still weighing options. And, to be honest, the matter keeps confusing me. It seems that a floor framed with “regular” joists and one with no joists at all would both be acceptable, and honestly I can’t decide.

  7. Deleted | | #18


  8. AntonioO | | #19

    Why not use a concrete leveler before putting down your sheets of foam? That would eliminate the need to use shims, spray foam as a gap filler, or scribed 2x4's, would it not?

    1. matt2021 | | #20

      AntonioO, Thanks! Indeed, I have been thinking on using concrete level (which I assume can be used on tiles?). I am not crazy about having the level material in contact with the wood, be that of the existing sill plate or of the new floor's rim joists; yet, perhaps that's not a real problem?

      In any event, I will discuss the possibility with the contractor, in just a couple of days. First, I need to make sure the slope is measured with precision, as it might turn up being less pronounced than I had been told.

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