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Community and Q&A

10 psi EPS foam vs. 25 psi XPS foam for a finished basement subfloor

dnnrobert | Posted in General Questions on


Currently having a house built in the Columbus, OH area. Planning on slowly finishing about half of the basement (around 1000 square feet) after we move in with the following methods:

Ceiling – Paint black with sprayer gun.
Floors – Cover with 1″ R-4 EPS foam insulation, 3/4″ tongue-and-groove OSB installed directly over with Tapcon screws into slab.
Walls – Cover with 2″ R-8 EPS foam insulation, 2×4 framing attached through sub floor, R-11 unfaced fiberglass between studs, finished with 1/2″ drywall.

Ceiling height before finishing will be about 8’8″ so height isn’t a major concern (but would rather not have to rebuild stairs or anything drastic as a result). Foundation will have Delta-MS waterproofing system plus exterior drain tiles.

My main concern is that the EPS foam carried by the local big box store is only rated 10 PSI. I considered 25 PSI XPS, but due to the environmental impact and cost, I’m wondering if it is a good idea. Will 10 PSI be enough under a 3/4″ OSB subfloor? It will not be supporting any structural walls, but I do intend to put a slate pool table in the basement.

Additionally, am I approaching this the right way in general? The goal of course is to have a mold-free, warm basement, but I will be on a limited budget. Is there anything I can do to cut costs while still maintaining a warm, dry basement? Thank you in advance for your help.

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  1. Expert Member
    BILL WICHERS | | #1

    You can get type-II EPS from insulation suppliers. It’s denser, better, easier to work with, and is available in the 25 PSI rating (and others). Don’t limit yourself to what is available at the box stores. You need to contact a commerical insulation supply house. You’re probably going to have a big enough order with your project that you might even save some money this way.


  2. Expert Member
    Dana Dorsett | | #2

    For the amount of foam your talking don't even consider box store pricing/quality- you have multiple foam vendors with new, reclaimed and factory seconds foam in your area. Run this search:

    These folks look particularly local and cheap:

    Type-VIII roofing EPS or Type-II EPS would be fine for the floor, cheaper and better than junky box-store Type-I goods.

    For the walls 2.5- 3" of reclaimed roofing polyiso strapped to with 1x furring through screwed to the foundation walls works, and is cheaper than a 2" box store EPS + R11 batt wall solution. Just be sure that the floor EPS runs under the cut bottom edge of the polyiso to keep the polyiso off the slab (where it could slowly wick moisture.)

  3. Jon_R | | #3

    And since you know approximately where the pool table will be, you could use higher density just in that area.

    Might be fine either way, but I've never see data to prove that. Accounting for creep and deformation, you want load much less than the nominal rating.

  4. Trevor_Lambert | | #4

    The main concern you have with a floor like this is local loading, and the ability of the sub floor to resist flexing. The amount of flex will depend on both the density of the foam below in combination with the sub floor's ability to spread out the load. I would use plywood in this application.

    What is the fastening schedule for the tapcons? If you can preload the foam, it will become much less of an issue. The 10psi spec is for a compression of 10% from nominal. It would require a lot more force to compress an additional 10%.

    Speaking of tapcons, I would recommend sleeve or wedge anchors instead. You can actually strip the concrete pretty easily with tapcons, and I've even snapped the heads off them before. If you ever have to pull the subfloor up again the sleeve anchors are more forgiving for reuse.

  5. Expert Member
    BILL WICHERS | | #5

    Thinking about the pool table, if you’re worried about localized floor loading, you could make discs of something like two layers of 3/4” plywood. A 6” diameter disk would have 28.27 square inches of area and would be able to support just shy of 707 pounds on 25 PSI rated foam. The discs would help spread out the point load created by the legs on your pool table. You could make nice looking discs and they’d be like architectural details while serving a functional purpose.


    1. Jon_R | | #22

      A 1/4" steel plate would have similar effect with much less height change. Don't use the nominal foam rating in support calculations (it's far too high when you consider acceptable deflection and creep).

  6. dnnrobert | | #6

    Thank you everyone for your responses!

    I will definitely shop around for better quality/priced insulation. I guess my main concern was transportation, as I don't have a good way of hauling that much foam. But I'm sure I can find an insulation supplier that will deliver to me.

    I like where you're going with the furring/thicker foam insulation idea, Dana. Seems like it would save a lot of lumber expense. The only places where I will need electrical outlets are on interior walls, so no wires or boxes to run. Can I just run vertical strapping and attach drywall directly to it? Or do I need to run both horizontal and vertical? I saw a This Old House video where they went both directions, but I don't really see the point unless I'm missing something.

    As for the Tapcons, I was thinking one every 2 feet or so around the perimeter of the subfloor panels. I used Tapcons to install a new toilet flange on a slab a couple years ago, so I had just planned on that due to familiarity. I am open to trying the anchors, although they seem a little pricey in comparison. I've also read a little bit about powder-actuated tools. Would a decent one be a good option for securing lumber through the concrete?

    I hadn't thought about using heavier-duty foam under the pool table or using pieces of plywood to spread the weight load. Those are both options I may consider to be on the safe side.

  7. Expert Member
    Dana Dorsett | | #7

    I did my basement with 3" roofing iso and 1x4s, hanging the wallboard on the furring. My furring happens to be horizontal, but vertical works too.

    The longest masonry screws that most box-stores carry would only be good for 2" or maybe 2.5" foam + 1x furring, but longer screws are available from web-stores if you can't find a local source. I doubt there's a powder actuated version appropriate for anything thicker than 1.5" foam.

  8. Expert Member
    BILL WICHERS | | #8

    Having used many (thousands) of powder-actuated fasteners in the past, I would NOT try using them for this application. Powder-actuated fasteners are not, shall we say, very controllable. With something soft like foam, I think you’d find almost every faster would just blow through and go way too deep. You’d also have a difficult time controlling the alignment, and if you’re not perfectly perpendicular to the hardest material in the stackup, you’re almost assured to blow out the hole.

    Personally, I’d use tapcons. I know some people don’t like them, but they’ve always worked well for me. Just make sure to drill the correct size hole. Sleeve or drop-in anchors are next best, but then you have the issue of hole alignment which will be a pain. Tapcons you can drill all the way through the stack of material, then screw in the fastener and be done. Use a good bit, SDS drill, and ideally, a nut driver and hex-head tapcon. My trick to use a can of canned air to blow out the dust in the hole after drilling so that a vacuum can get it clear is a huge time saver too, especially with deep holes.


  9. dnnrobert | | #9

    Thanks Dana and Bill for the fastener advice. If I can get some cheap, 3" thick foam delivered, I think I might go the furring route instead of framing out walls. I didn't have much problem with Tapcon screws when I used them, but the thought of using them for this large of a project is a bit daunting. I will be purchasing a hammer drill for the job. Last time I only needed to drive in 3 Tapcons, so I managed to use my regular drill.

    One other question I have -- the builder insists on including vinyl blanket insulation for the top 4 feet of the foundation, which I completely understand as they have to maintain a certain HERS score for their certification. I have already decided on tearing it down, as I don't like the idea of covering it up below grade. However, the non-common walls of the garage will not be insulated or drywalled, so I was thinking about using the insulation there. I've never worked with the stuff, so I have no idea what condition it will be in after I tear it down. Is it likely to be usable in this manner? I would hate to just throw it all out, both for the wastefulness and the hassle of doing so.

  10. Expert Member
    MALCOLM TAYLOR | | #10


    I find these a lot easier to use than Tapcons:

  11. Expert Member
    BILL WICHERS | | #11

    I highly recommend the Bosch SDS-plus hammer drills (these are the smaller ones). I loooove mine and use it all the time. Once you’ve gone over to SDS bits in a hammer drill you’ll never want to use anything else. They’re just so much better for this application. The chisel attachments are nice too, they just sort of vibrate through concrete. Saves tons of effort. The SDS carbide bits seem to hold up better too, it’s very noticeable. With the number of holes you have to drill, you’re going to want to use a good tool.

    You may be better off using two layers of foam instead of one 3” layer. Two layers will let you overlap seams for less air leakage, and thicknesses under 2” tend to be more common and also cheaper. Just something to think about, the R value will be the same either way.


    1. Jon_R | | #12

      I recommend a rotary hammer (perhaps Bill's intent) over a hammer drill. Makes drilling in concrete almost as fast as wood.

      1. Expert Member
        BILL WICHERS | | #13

        Yes, I meant rotary hammer. I tend to use the two terms interchangeably. The Bosch drills are selectable with a lever on the side.


    2. dnnrobert | | #17

      Hi Bill,

      I am reading up on rotary hammers. Is yours corded or cordless?

      1. Expert Member
        BILL WICHERS | | #19

        Mine is corded. I’m still a little suspicious of cordless tools for heavy duty stuff, and drilling holes in concrete is heavy duty (I frequently drill holes for 1/2” anchors).

        The most important thing is to get an SDS-plus chuck. It’s a kind of snap-in chuck and doesn’t loosen up as you go. Regular Jacobs chucks tend to loosen up if you’re drilling lots of holes with a rotary hammer, and then you get to practice your swear words. I’ll never go back to a non-SDS drill for masonry.


  12. STEPHEN SHEEHY | | #14

    Is it too late to put foam under the slab? And 1" seems kind of skimpy .

    1. Trevor_Lambert | | #15

      I missed the fact this was a new house, assuming it was a retrofit for some reason. Definitely, insulation below the slab makes much more sense. Then the density of the foam hardly even matters. And yes, 1" is on the very thin side.

      1. Expert Member
        Dana Dorsett | | #18

        I'd missed that too!

        Even using virgin-stock EPS it's financially rational to put 2" of EPS under the slab in a zone 5 climate, 4" if it's a radiant floor slab. Using dirt-cheap reclaimed roofing foam 3" is a rational starting point even if the slab isn't part of the heating system. Two layers of 2" (or whatever thickness is available as long as it adds up to at least 3") with the seams staggered between layers is the way to go.

        Putting at 2" of EPS on the slab edges to thermally isolate the slab from the foundation walls & footing makes sense too.

        1. Expert Member
          Michael Maines | | #21

          Dana, I'm curious why you prefer using two layers under a slab. I've used up to 12" EPS in single sheets (more like blocks when you get to that point) and as long as the substrate is properly prepared, I find using a single thickness to be easier.

          1. Expert Member
            Dana Dorsett | | #25

            >I'm curious why you prefer using two layers under a slab.

            If the EPS ever shrinks with age having multiple layers guarantees no temperature striping. Under a slab it's not as big an issue as on walls or roofs that see a much bigger delta-T, but it's not much additional cost to do it in two layers instead of one.

    2. dnnrobert | | #16

      I will ask if it can be done at our pre-construction meeting. The first time I asked, I was told it's not included, but hopefully it is something they would be willing to do.

      1. Expert Member
        BILL WICHERS | | #20

        It’s minimal labor to install the foam. The foam weighs very little, and since it’ll mostly go in as full sheets there isn’t even very much cutting. Your costs should mostly be materials. Even if it’s a small change order, it’s worth doing. You’ll never get another chance to put insulation UNDER the slab.


  13. Jon_R | | #23

    IMO, you should also ask them to lap the under-slab poly a few inches up the walls. The idea is that should there ever be any water running down the wall, it gets directed to below the slab.

  14. STEPHEN SHEEHY | | #24

    Putting the foam under the slab will probably turn out to be cheaper than putting it above the slab and adding osb on top. I'd use 3-4. " You will have lots of flooring options once you've eliminated the condensation issue created by a cold slab. You can leave concrete as your finished floor. Or put tile, carpet, wood, etc.

  15. dnnrobert | | #26

    Hey Folks,

    Thanks again for all the great advice. I have another question. I have been reading about the blanket insulation that will be included on the top 4 feet of our basement walls, and originally was considering leaving it in the unfinished half of the basement. However, I have read that these can trap moisture and create mold problems. I won't have the budget to finish the entire basement, at least not right away. Would I be better off stripping the insulation and leaving the walls bare? How much affect will this have on the temperature and heating bills? The rim joists will be insulated with unfaced fiberglass from what I've been told/seen in the builder's model home, which I will be leaving in place regardless. Thanks!

    1. dnnrobert | | #27

      *Bump* Any thoughts on the above question! Thanks!

      1. Jon_R | | #28

        Wait and measure will be more reliable than any guess.

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