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Used 3″ polyiso insulation under slabs?

Svig | Posted in Energy Efficiency and Durability on

I am building a pretty good house next summer. I have an opportunity to purchase used 3″ faced polyiso (asking $25 a sheet). I am wondering if this would be a good fit for under my rat slab in my unvented crawl space and my attached garage slab. Both house and garage will have poured ICF walls in the foundation. Zone 7 northern MN. Looking to save a little money and buying used foam of course is a better environmental fit.

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  1. GBA Editor
    Martin Holladay | | #1

    Buying used rigid foam is a great idea. But you want to buy used EPS or used XPS, not polyiso, for use under a slab. Polyiso absorbs too much water to be used in contact with the ground.

  2. Expert Member
    Dana Dorsett | | #2

    What he said- NEVER under a slab, but fine on the INTERIOR side of a foundation wall, if you keep the cut-edge of the polyiso off the slab.

    There is usually reclaimed roofing EPS around for cheap if you look for it. EPS under slabs has a long and successful history. There is sometimes reclaimed XPS too. From a performance point of view assume re-used XPS to be R4.2/inch, never R5, though some of the pre-Montreal Protocol goods blown with CFCs can still perform at a higher R.

  3. Svig | | #3

    I thought as much, thanks. Rarely see any reclaimed eps or xps here in MN.

  4. Irishjake | | #4


    Check with for EPS, XPS or Iso.
    They can ship it to you, and often times can even source the material near you. I bought a whole ton of it for my project and saved thousands.

  5. Svig | | #5

    Thanks Brad, I will check with them.

  6. Expert Member
    Dana Dorsett | | #6

    Insulation Depot has changed their name to Nationwide Foam (but still headquartered at 703 Waverly, Framingham MA )

  7. Svig | | #7

    Thanks Dana, price has dropped to $9 a sheet for three inch polyiso, My crawl space floor will be a foot or so above my footings, and I will be well drained with a sump that I hope never gets any water in it. If I put, say 1" xps, then the polyiso, would that be acceptable? Just trying to save a little money.

  8. charlie_sullivan | | #8

    Ooh, $9 for 3" sounds irresistible. But I still wouldn't use it below the slab--it might be OK in your configuration, but you are putting it where it really can't be replaced if it fails. How about two layers of exterior foam over a 2x4 wall?

  9. RD3Sunworks | | #9


    In northern MN, and 3 in. polyiso at $9/sheet, I'm with Charlie. If the polyiso is in good shape, I would be buying it right now. My house in MN would be two layers of 3 in. polyiso over 2x6 studs. I had no problem attaching to studs through 5 inches of foam, and I know that 6 inches has been done before. At that price, your (foam) price for two layers of 3-in.polyiso would be much cheaper than the price I paid for two layers of 2-inch polyiso.

  10. Irishjake | | #10


    That IS a very good price for poly-iso, if it's in good condition. Does that include shipping? Did you get that price from Nationwide Insulation? Just never, never, never use the poly-iso under a slab. You can get reclaimed EPS for less than $9/sheet, so why use poly-iso and chance it. If it gets wet - which it mind as well just throw out your money. The XPS or EPS wouldn't stop water migration or vapor from getting into the poly-iso.

  11. Chaubenee | | #11

    If you are doing a vented roof, insulating the attic with cellulose I would also be inclined to put that up on my 2nd floor ceiling with taped seams, use furring strips, and attach my sheet rock below it, to get that R20 there before you place your attic cellulose at R40 or whatever you want. That is if you have nine foot ceilings upstairs and don't mind losing a few inches of ceiling height. Face the foil side towards the sheet rock. That is a lot of R for 9$...

  12. PAUL KUENN | | #12

    Good day Steve!

    I'd give this guy in Madison WI a call and make an offer:

    and keep watching Craig's list. Two layers of 2" would work great as you can cover the seams. There's always good deals for EPS a day's drive away.

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