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Can I use this foam under concrete?

mikeysp | Posted in General Questions on

Hi. I am building a post frame shop and will put in radiant heat. When I purchased my property it had a whole bunch of the foam panels as shown in the pics. Enough for under the slab. It appears to be some culled 2″x2’x4′ vinyl panels. It has a 3 or 4mm thick skin on each side and is foam in the middle of the sandwich. Ity seams pretty firm. I am trying to identify the type of foam as I hear that Polyiso is not good under concrete. I am also trying to figure out the PSI, as I also read that high density is very important, so the foam supports the slab and is not smashed. 

Any counsel on how to proceed would be greatly appreciated. Obviously, if I cannot figure this out, I will have to flip the bill and go from free to a few grand for the foam. I prefer free very much 🙂

Thank you in advance.


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  1. onslow | | #1


    If you can walk on them near the edges without leaving foot prints, then likely they can take the process of a crew pouring and leveling the slab. Once the concrete is solid the per sq. inch or foot is pretty small. Depending on where you are, ordering recycled foam is a great way to get full size material. I got my load of 4x8' foam from a recycler in Denver, but many places now handle the foam taken off of big commercial buildings. I got extra lucky in that the 3" XPS (oh no!) was definitely firmer than the stuff at HD, probably 25psi grade. Thats 3600 pounds per sq. ft. if it scales. The toughness was of great value given how many times and how many people stomped around on it while setting the mesh and embedded pex. The 2x4 foot size will make navigating around on top of the panels a bit dicey, especially with the skin of vinyl/pvc. Maybe taping will help. Corralling them while pouring might be an issue if post frame shop means something like a pole barn. How are you handling the perimeter? Any edge foam?

    I am puzzled by what the original purpose was for the panels. Was there ever an insulated shed like a milk house? Anyway, more to the point would be the thickness of the foam you have, which does appear to be a urethane or polyiso. Hard to calculate actual R value without know for sure which it is. Safest to assume R5 if they are older. As for being under a slab, water intrusion is the issue and if you have graded properly and put down 3" of WASHED stone to ensure no water can migrate upward, then I would probably gamble as well. Others may/will disagree.

    If you are in a CZ 5-6 location, I would urge going for 3" minimum and 4" if you can get cheap 2" 4x8 stock. Ground temps are like endless 50F days, so you can do the calculation for losses to ground. The edges in winter are be where the highest delta T will be found, so plan accordingly. Some will say the ground will warm up under the slab over time, but I would ask who paid to warm it up.

    I first put down the foam sheets and then covered that with fiber embedded plastic film. (internet #203635222 at HD about $130.) When I set my pex for future possible need, I first laid out the pattern of tubes for three (mostly) matched length circuits. The pumping volumes are better handled that way. I had the advantage of 3" foam, so I used landscaping staples to hold the pex down first then had the crews put 6" box mesh wire on top. Then I lashed the pex to the underside of the mesh with zip ties, un-stapling as I went. Two good reasons for this.

    If you wait and tie it to the top of the mesh, the pex will be that much closer to the surface. If the pex bows between tie points, and it will, the risk of being just under the surface is that much greater. My cement guy warned me about a job that ended poorly when the client insisted on keeping the pex close to the top for better warmth. Hairline cracks developed along many sections of pex path where only 1/2" or so of concrete covered it. I did the same even in my garage with a 5" pour.

    While it may be arguable that setting the pex deeper is going to increase losses to ground, I would maintain that the trade-off is trivial compared to cracking the slab. Besides, if the foam under is a bit thicker, the losses to ground will be far slower than the heat loss to the side you want to be warm.

    Think hard about where to bring the loop ends up and how you will attach the pumps and whatnot. It may be worth boxing out a small port in the slab for the loop ends. I ran the pex through 1" PVC conduit elbows that I strapped to the wall with conduit clamps. Lots of different ways to make the bends stay put. Pressure test them and check for hold at pressure for at least a day. Pray the pour goes smoothly and nobody hooks the pex while lifting the mesh (which was supposed to be set on cradles) with rakes.

    If you want to be able to set anchors in the floor for any machinery, then be very rigorous about the spacing and placement of the pex. Make a story tape to go with any photos. I have lots of photos and no story tape to measure of my presumed lolly column reference points. Oops.

  2. mikeysp | | #2

    Roger, thank you very much! I am in zone 4a (Nashille area); so 2" should do for me. I did wonder about shop floor and any fuel, acetone, etc... spill and what that will mean as some amount gets to the foam will it create voids and settling? Has this been a problem in any mechanic shops? I would not think enough would get through, but not sure. I am planning to place foam between the bottom girt on the post frame (pole barn) and the slab. I will try the walk on the foam test. I will peel off one of the vinyl layers and test it. I dod not know what it was from. The previous owner actually built a garage from it. I mean he cut out a little foam and overlapped the vinykl and used small screws to hold it together like a shingle building. 8 ft spans on the walls and 2ft on the roof. Shed roof. I just thought it would be something to recycle. I have a MASSIVE amount of polyiso that I am using for walls and roof on the house I am building, but I had nothing for under the slab. I am scrounging materials for cheapest house/debt free.


  3. Expert Member
    AKOS TOTH | | #3

    It is most likely polyurethane foam, looks like stuff intended for walk in freezers. Some are pretty good but some do absorb water. I would cut a piece, measure the weight and submerge it in water for a couple of weeks. If it gains weight, it might not be best bellow slab.

  4. Expert Member
    BILL WICHERS | | #4

    It looks like polyiso to me, but’s it difficult to be sure from those pics. Polyiso is not suitable for underslab insulation.

    If you have ANY DOUBT AT ALL if this is useable, DO NOT use it under your slab. There are no easy or cheap ways to fix problems with things under a slab, so it’s really important to do it right the first time with this kind of project.

    If you want to keep costs down, EPS is probably your cheapest option, and it’s readily available from commercial insulation suppliers. Type II EPS is a little more expensive, but it’s a LOT easier to work with so I’d recommend using type II.


  5. onslow | | #5


    I looked into the EPS vs XPS battle long ago and it is still alive. EPS will absorb water into the interstitial bits between those little pellets that shed all over. The water will go in and come out with similar ease. XPS will absorb water more slowly and release it very slowly according to some sources. I have not seen it and the bits I use outdoors have faired well despite four seasons and four years. Polyiso and polyurethane are considered closed cell but both will absorb water.

    So the key question is - where is the water that will be absorbed coming from and where will it go if given opportunity. Under a slab, you should not be seeing bulk water anywhere close to the foam being used for insulation. That is why I said WASHED stone not compacted gravel or pit run. Gravel or other fills can hold and transport enough moisture to become a problem. You must account for ground water and runoff competently and not live with a soggy sponge under or all around your building. Keeping the under slab foam out of water is key regardless of type.

    If you insist on dunking your foam in water, all will take on some. This will be true of foam on the outsides of foundations where runoff and ground water will meet your wall. If the water comes and goes fairly quickly, then EPS will have some advantage. IF it persists and the footing drain is not up to the task, then XPS may over many years remain more compromised than the EPS. Just take care of the water.

    Zephyr 7 is quite right not to use Polyiso in this vertical underground context. I would contend that under a slab it should be survivable if placed and protected properly. I would use EPS at the perimeter if you want a buffer that will behave better than Polyiso. Polyiso is used on roofs extensively and leaks will cause loss of insulation value. EPS used on roofs would have the same problem. The water going in will have a hard time coming out in most roof lay-ups. It is a calculated risk to use Polyiso vertically underground and on roofs. That said, it is your budget.

    I used recycled XPS under my slab and completely around my foundation. The foundation foam is sealed in Grace asphaltum sheeting all the way down and over the footing. The footing drain is next to the footing where it belongs, not on top of the footing. This makes the pipe set well below my slab height. The drains are fabric wrapped then embedded in washed stone, which is fabric wrapped. Ground graded accordingly and rain runoff directed away 30'. So far so good.

    One insulation that I have not heard much about underground is Rockwool. I am baffled how something so transparent to water vapor would not be equally able to absorb water in below grade applications. I have seen postings of people intending to insulate foundations with it, but I question the success if the soil is at all moist or subject to enough ground water that a footing drain would have water to drain. If anyone has practical info I would love to see it.

    The following is a somewhat random collection of links where various pitches are made for the different foams. You can peruse them and decide who is overplaying their case and what level of risk you wish to take on. I really wish I could find the report I once read on foam removed from a foundation in MN after 15 yrs. You will also likely find conflicting opinions much as I did.

    I have a wood shop so I can't answer the solvents question beyond "don't spill" its dangerous. Be very careful of fumes and electrical sparks or gas water heaters if you have them in the shop. Also fine dust of many types. Bondo dust might pose the same risk of explosive combustion as flour or grain dust. More than few elevators have blown from sparks in dust filled grain silos.

    I have spilled some gas and diesel in the garage which was sealed with something that will soften if I don't wipe it up fast. I will ask someone with an auto shop if they do special stuff for their spills. I think one of the local dealer shops went all in and epoxied the entire shop floor. Maybe the kits they sell at HD would be sufficient.

  6. mikeysp | | #6

    Great. Thank you for all the advice. I just submerged a 4 gram cube of the mystery foam with a 2 gram cube of pink fomular. It is in a plastic food container with the lid, so they are both submerged completely. I will see what happens. I think I will do a second sample set floating in a pail of water, just for additional data to look at. I will post results tomorrow, unless more than a day is needed? I would think with the slight hydraulic pressure of being submerged, it will give an additional boost to the water to make entry into the foam.

    I did jump on a square foot of the foam and it did not budge. Actually snapped in one spot, but not a dent at all. Much more dense than the pink fomular I have from Home Depot. Also, turns out the 3/16 skins on each side of the foam is a reinforced fiber composite of some sort. I will post some pics when I see how the water submersion test goes.


    1. Expert Member
      BILL WICHERS | | #7

      Wait at least 3-4 days as it’s a slow process.

      I would still recommend NOT using mystery foam in an underslab application where any potential issues could be such a big problem down the road with no easy solutions.


  7. mikeysp | | #8

    This is great. Thank you all. The current test includes XPS (pink), Polyiso, and mystery foam. Of note: the weight of the mystery foam is about double that of my polyiso. Also note: I removed the skins and plastic from all the foams, so it is foam alone. I have a bucket with floating foams and a plastic container and cover with submerged foam. Bill, you are right to warn me. I will see if I can find some used XPS or High Density EPS for a good price. I have not ruled out using this foam. High risk investments are risky, and it can be a big payoff or complete loss. I walk forward with caution, while looking on facebook and craigslist for foam. Anyone know of a source in TN, AL, KY, AR? I live near Nashville, but I have a large trailer and can drive several hours if the cost benefit is there. My time is not near as valuable as my limited dollar bills. We are doing EVEYTHING cash out of pocket. Debt-Never again is my motto :)


  8. mikeysp | | #9

    I pulled the foam, dried the outside with a towel and weighed it. Then put it back in for another day. After 22 hours these are the results:

    Submersed samples
    Pink foam gained 50% weight
    Polyiso gained 100% weight
    Mystery gained 0% weight

    Floating samples:
    Pink gained 0%
    Polyiso gained 100%
    Mystery gained 0%

    this is not very accurate. I am using a kitchen food scale which is pretty decent. However, I have a 100 grain jewelers scale. If I can find it, I am going to redo this test with much better cube size and weight consistencies. I did dry off the outside of residual water. The pink foam does have more surface area as it is two 1/2" thick pieces while the poly and mystery are 2" thick. I will continue to post on it as they are back in the water after weighing.

  9. Expert Member
    BILL WICHERS | | #10

    There is a large foam reclaimer in Chicago, and another in Maryland (I think Baltimore). The one in Maryland has shipping points all over, so they might have something near you. There is another place in Indiana, but I don’t think they have much besides polyiso at the moment. That one is I can’t remember the other two, but they advertise in craigslist pretty much constantly.


  10. mikeysp | | #11

    Thought I would finish up with my numbers. Even though it was not accurate enough. It has prompted me to do the test again with my more accurate scale when I find it. I will also copy a test I saw of dyed water to see how deep it gets over several days.

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