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Musings of an Energy Nerd

Installing Rigid Foam Above a Concrete Slab

You can sandwich rigid foam between an existing concrete slab and new plywood or OSB subflooring

If your basement slab lacks sub-slab foam, you'll have to install the insulation above the slab. There are several ways to install subflooring above the rigid foam: you can use either one or two layers of plywood or OSB, and the subflooring can either be fastened to the concrete with long Tapcon screws or it can simply float above the foam. If you choose to have a floating subfloor, it's best to install two layers and screw them together.

If you’re building a new cold-climate home with a basement, you’ll probably want to install a continuous horizontal layer of rigid foam under the basement slab. Even though the rigid foam won’t save enough energy to justify its cost, it’s worth installing for another reason: it will reduce summertime condensation and mold accumulation, and will therefore help your basement smell better. (To learn more about sub-slab foam, see “All About Basements.”)

What if you’re living in an older house that has an uninsulated basement slab? If you want the benefits of floor insulation, you’ll have to install the rigid foam on top of your existing slab.

Before we discuss slab insulation, it’s worth reviewing the three basic mechanisms that can make a basement slab damp:

If you install carpeting on an uninsulated slab, you can end up with moldy carpeting. The cold slab is a condensing surface, and the indoor air (especially during the summer) provides the moisture. That’s why you want to install rigid foam on your basement slab before you install carpeting.

Here are the basic steps to installing rigid foam above an existing slab:

Ideally, your basement slab was installed over a layer of 6 mil polyethylene. If you’re not sure whether your slab has poly underneath, you can always drill a hole through your slab in an inconspicuous area of your basement. If the diameter of the hole is at least 3/4 inch, you should be able to tell (with your finger, a probe, or a flashlight) whether there is any poly under the slab.

If there’s no poly under your slab, or you’re worried that there may not be any poly under your slab, it’s probably a good idea to install a layer of 6-mil poly between the…

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27 Comments

  1. Brian Bailey | | #1

    Martin,

    You've apparently read my mind, once again, by publishing an article on a project already on the docket for my 40-year-old house. Or maybe it's just that my house has the usual suite of "normal" issues. Either way, thank you!

    I have a question regarding your mention of using interior poly for the basement floor in an interior-foam retrofit: should the poly extend up the walls and, if so, how far? In my situation, in would certainly be possible to run the poly up the walls as high as the foam will go, which is to the top of the concrete.

    I plan on using spray foam to insulate the top surface of the concrete wall and around the rim joist (between floor joist bays). That will be after air-sealing the rim joist area to the best of my ability. The floor joists rest on top of the concrete - they are not embedded.

    Any comments or thoughts are greatly appreciated!

  2. User avater GBA Editor
    Martin Holladay | | #2

    Brian,
    You wrote, "I plan on using spray foam to insulate the top surface of the concrete wall."

    I'm not sure what you mean by "the top surface." There are two uncertainties:

    1. Are you using open-cell spray foam or closed-cell spray foam?

    2. Are you only insulating the top few feet of the wall, or are you insulating the entire wall, from the slab up to the top of the wall?

    If you are insulating the wall on the interior, from the slab all the way to the top, with closed-cell spray foam, you don't need any polyethylene. The closed-cell spray foam will be a vapor barrier, a water barrier, an air barrier, and insulation.

    For more information, see "How to Insulate a Basement Wall."

    1. Brian Bailey | | #3

      My thought is to use rigid foam (EPS) for both the walls and the floor, and running from the slab all the way up to the top of the walls.

      By "top surface" I mean the exposed horizontal surface of the wall, upon which the joists rest. It's here that I plan to used closed-cell spray foam, which will also run continuously to cover the rim joist etc.

      I will do the rigid foam work myself and let a local pro handle the spray foam.

      1. User avater GBA Editor
        Martin Holladay | | #4

        Brian,
        In my experience, a layer of EPS foam provides a perfectly adequate vapor retarder in this location, so you don't need polyethylene.

        However, polyethylene is a vapor barrier, while EPS is only a vapor retarder. So if you want to be absolutely sure that there isn't any water vapor transmission from the concrete to the interior, you can install poly between the concrete and the EPS if you want. I would call it optional.

        Remember, installing polyethylene on the wall makes it hard to use adhesive to attach the EPS to the concrete wall.

  3. User avater
    Reid Baldwin | | #5

    I am considering replacing basement flooring at my parent's house, which currently has carpet over concrete with no foam. Would I need to do anything special around existing partition walls?

    1. User avater GBA Editor
      Martin Holladay | | #7

      Reid,
      Q. "I am considering replacing basement flooring at my parent's house, which currently has carpet over concrete with no foam. Would I need to do anything special around existing partition walls?"

      A. What type of flooring will you be installing? Will you install carpeting again? If so, most experts advise that it's better to have an insulated slab under the carpeting. Then again, if they've had carpeting for years with no complaints, their basement may be dry enough (or warm enough) to avoid problems.

      I'm not sure I understand you question about the partitions. Clearly, no one is advising you to install carpeting under the partition bottom plates -- so you'll work around the partitions.

      1. Bradley Weingartner | | #25

        Honestly I think I'd be mildly concerned about the bottom plates on the partition walls. Current state, the carpet may be permeable enough to prevent moisture problems. Future state, If you suddenly have a material with low-perms over all of the slab except the bottom plates, you will see an increase in moisture at those plates. This could accelerate rot and mold problems in those partition walls. I believe this to be a fair concern.

        1. User avater GBA Editor
          Martin Holladay | | #26

          Bradley,
          When Reid posted his question, I wasn't sure what his concern was. It's possible that you have correctly identified his concern.

          Bottom plates installed directly on a concrete slab are supposed to be pressure-treated. If this partition has a pressure-treated bottom plate, as it should, I wouldn't worry.

          1. Bradley Weingartner | | #27

            Hi, I'm Brad and I think I figured how to update my username!

            I agree that if the bottom plates are pressure treated and there is a capillary break, Reid will be fine. Pressure treated on its own will not do much of anything to prevent excess moisture from getting into the wall though.

            Reid's parents may also be lucky and just have a dry basement. Of course, there are two kinds of basements - those that are wet and those that will be wet!

  4. Lukas Peter | | #6

    Thank you for the very timely and excellent article!
    Do you have any advice on how to proceed if the concrete slab is sloped? I am planning to convert a garage into an ADU (accessory dwelling unit). I hope to avoid the use of self leveling concrete for environmental reasons. Would a leveling of dry and clean sand, on top of the EPS and below the plywood, work?
    I was thinking of placing pressure treated and ripped sleepers at 8 feet on center on top of the eps to create guides to level the sand. These sleepers would align with the short edges of the first plywood layer placed on top. The two layers of plywood and sand would be floating on top of the insulation.

    1. User avater GBA Editor
      Martin Holladay | | #8

      Lukas,
      My first reaction to your question was to suggest self-leveling compound, until you wrote that you are trying to avoid it. I'm not sure why you don't want to use it.

      I'd be leery of trying your idea (sand on top of the EPS). I'd be more likely to use crushed stone (carefully leveled) under the EPS. Perhaps other GBA readers have more suggestions.

      1. Lukas Peter | | #12

        Thank you Martin. The sand idea comes from a remodel experience in a very old multistory building where I found sand under the wood plank flooring between the support beams. I suspect it was installed to provide sound isolation btw the floors.
        I am trying to use less concrete to reduce the carbon footprint where possible. I do recognize that this may not always be possible.

    2. Tyler Keniston | | #15

      I'm in a similar situation.

      Would a foam subtractive method be totally crazy i wonder? I.e. level some rails to ride a sled with some foam eating tool on it. I could only see this being feasible if there was a tool properly suited that could do it quickly enough and collect dust.

      edit: for slightly unlevel, not many inches of course. I also realized that it'd be quite a waste of foam. I suppose you could use some different layers of 1-2 " foam to build up prior where its the lowest to minimize eating tons of foam.

      1. User avater GBA Editor
        Martin Holladay | | #16

        Tyler,
        Q. "Would a foam subtractive method be totally crazy, I wonder?"

        A. In my opinion -- yes, it would. Use floor leveling compound or crushed stone.

  5. User avater
    Peter Engle | | #9

    Roofing insulation manufacturers can provide customized, sloped insulation boards. Both EPS and polyiso should be available. This will be virgin material as it is manufactured custom for the job, but it would work, and probably be significantly cheaper than self-leveling compound. An added bonus would be that the insulation value would increase as you get closer to the primary exterior wall.

    If you're looking for a concrete (pun intended) solution, self-leveling compound would still not be the choice. These products are very expensive, and you would need a lot of it to fill the typical 3"-6" slope of a garage floor. Lightweight (gypsum) concrete would be a much better solution, or even just concrete grout - that's concrete without the bigger aggregate. Either one can be ordered loose enough to easily rake and trowel flat. You would want it to taper to a minimum of 1" thickness at the thin edge.

    1. Lukas Peter | | #13

      Thank you Peter. I’ll price the tapered insulation and compare it to the concrete leveling per your suggestion. The amount of cement used for this type of non-structural concrete would be very low and therefore have a relatively low carbon footprint.

  6. Kohta Ueno | | #10

    Nice column, Martin!

    Install one or two layers of tongue-and-groove OSB or plywood on top of the rigid foam. If you install just one layer, you’ll need to fasten the OSB or plywood to the concrete slab with long Tapcon screws that extend through the rigid foam.

    This is the method a friend and I did in his basement. We were very happy with the results--per your other correspondents, it results in a very solid-feeling floor, with minimal "bounce." If there are "bouncy" spots, you can just add another Tapcon.

    I concur with the recommendation for adding a countersink to the Tapcon hole. I've generally found that Tapcons do not "pull" the same way as wood screws, so the fastener will not countersink on its own into plywood or OSB. So countersinks are required to keep the fastener heads flush.

    In the example shown below, we used 1" XPS (due to head height restrictions) and taped seams. My friend omitted the polyethylene.

    If you want a utilitarian basement floor rather than a finished space, you might consider installing a cementitious tile underlayment panel (something like Durock) directly on top of the rigid foam insulation. This type of panel is heavy and flat enough to stay in place by gravity, without any fasteners, and can be used as “industrial-look” flooring.

    This is the method I used in my basement--trying to have dry tool storage and workshop space; it went together very quickly and easily. Taped the 2" XPS seams, and put the Durock on top--XPS foam recovered from a dumpster, yeah! I won't be roller skating around down there, so there's no need for a high durability material. I definitely recommend cement backer board (Durock, Wonderboard) over fiber cement tile backer (Hardie)--the latter is "stiff" enough that a point load will "curl" up the other part of the board. The "floppy" Durock conforms to uneven surfaces. One issue with this system is that the edges of Durock are not durable relative to impact. So if you have a condition where the edge is exposed (e.g., door threshold between insulated and uninsulated slab), it needs some protection. That's another item on my project list. :/

    FYI--the foam + cement board retrofit is what we used in the basement of the Utica NY retrofit you wrote about (https://lakesideca.info/article/the-high-cost-of-deep-energy-retrofits).

    1. User avater GBA Editor
      Martin Holladay | | #17

      Thanks, Kohta. I appreciate the tips and the photos.

  7. Kohta Ueno | | #11

    One more thing--I'd recommend EPS and XPS over polyisocyanurate in this on-slab application. This is admittedly old (BETT 1982) data, but it indicates that polyiso loses a lot more of its R-value due to moisture uptake than XPS or EPS (in buried conditions). Of course, there are those other studies out there that says EPS gains much less moisture than XPS.... paid for by the EPS industry. Of course, the EPS density matters as well.

  8. User avater
    Peter Engle | | #14

    Kohta,

    While I agree that polyiso might not be the best choice for this application, I wouldn't really call these conditions anything like underground with soil contact. Unless your basement is constantly wet, the foam is going to see dry conditions nearly all the time. I would think the moisture uptake (and R-value reduction) would be small.

    Seems like a good idea for grad student work.

  9. Kohta Ueno | | #18

    Hi Peter. My thought is that in the Northeast, there are lots of older housing stock with poorly poured basement slabs. I often refer to them as borderline dirty concrete/conrete-y dirt. :) So of course, no vapor control underneath. That was the slab condition at the two projects that were the source of the photos above.

    Therefore, the conditions at the slab-to-foam interface are likely to rise to 100% RH for most of the year--so it is an area where you want a material that is resistant to moisture uptake. I agree that this is not as challenging as burying foam in dirt, but I still would recommend EPS/XPS over polyiso here.

  10. User avater
    Peter Engle | | #19

    Point taken. The wetter the conditions, the less optimal polyiso performance is going to be. I just wanted to point out that it shouldn't be ruled out in all cases, because there are plenty of slabs that are dry, but uninsulated where polyiso might be a good choice.

  11. Stephen Mager | | #20

    I've got a split level home built in 1979 in upstate NY (zone 5). Kind of funny, at the time, they built the mid-level section with a vented crawlspace, with common interior doors for access to the crawl space. Combine that with fiberglass insulation between the floor joists, and spotty rigid insulation installed below the floor joists in the crawl space and no insulation on the foundation walls or floor... it's not at all ideal. I'm looking to seal up the vents in the crawlspace, add some air movement from the conditioned space into the crawl space and add rigid insulation to the walls and floors. Are there any issues leaving the rigid foam exposed on the walls and floors of the crawlspace area?

    Assuming I utilize Dow XPS insulation, I believe for crawl spaces, I can install without a thermal barrier or ignition barrier. The link provides an ICC-ES Evaluation Report:

    1. User avater GBA Editor
      Martin Holladay | | #22

      Stephen,
      Q. "Are there any issues leaving the rigid foam exposed on the walls and floors of the crawlspace area?"

      A. As far as I know, the code requirements for covering rigid foam with an ignition barrier or a thermal barrier are the same as the code requirements for these barriers over spray foam. For more information on the issue, see "Thermal Barriers and Ignition Barriers for Spray Foam."

      A thermal barrier is more stringent than an ignition barrier. Covering foam in a crawl space with the less stringent barrier (an ignition barrier) is only possible if it meets the following criteria: if the crawl space is only accessed for repairs or maintenance; if there is no easy access to the space; and if it isn’t used for storage.

      As far as I know, XPS needs to be covered with either a thermal barrier or an ignition barrier for fire safety. But if you have any doubts, you should contact your local building department to find out how local inspectors interpret the code in your jurisdiction.

      1. Stephen Mager | | #24

        appreciate the reference to the other article - very helpful.

  12. Stephen Hood | | #21

    Hi Martin,

    Thanks for the timely article. I'm currently building a home in North Louisiana (Climate Zone 3). We have a slab on grade foundation (post tension stressed). However, for a room of the house we are using as a home theater, we dug down 2 feet. So, it's not a full basement...just a partial sunken room. The intention is to build a staggered seating platform to have 2 rows of seating. So, the rear portion of the room will be built up on two layers of 2x12s to bring it level with the rest of the 1st floor in the house. However, the front portion of the room (about half) will remain at grade level in the room (2 feet lower than the rest of the first floor). We have installed a French drain around the base of the exterior walls on this room and applied water sealing compound and dimpled mat. On the inside of the walls, I have applied a layer of 3/4" EPS foam. Under the concrete slab, we used a 10 mil sheet of poly.

    Now, my question is what should I do on the floor? We were planning to install a 1/2" rubber underlayment (similar to horse mat material) glued to the concrete. On top of that we would glue a layer of 3/4" Advantech paneling (this is a sound isolating technique prescribed by the home theater designer). Then, we would install carpet on top of the Advantech (with the typical carpet pad) in the front half of the room. In the rear half, we would build the stage/riser platform on top of the Advantech.

    However, reading your article, I'm wondering if I should put some EPS foam down on the concrete before putting in the rubber mat underlayment. And would I really need 2" of EPS being in Zone 3 where it is predominantly warm...but can be very rainy/wet? Could I get away with something like 1/2" or 3/4"? Or should I just stick with the original plan and use a special glue that will also serve as a water barrier?

    Thanks in advance!

  13. User avater GBA Editor
    Martin Holladay | | #23

    Stephen,
    In your climate zone, you probably don't need rigid foam for this floor. It's a judgment call. For more information on this issue, see "Determining Sub-Slab Rigid Foam Thickness."

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