GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Where to Put Polyiso for Slab

14a_Builder | Posted in Green Building Techniques on

I have seen posts for and against Poly ISO under slab and want to get some feedback on a alt design technique.

Our project, is a monolithic slab pour with radiant, single story, with a walkout lower level. We would like to maintain the exposed concrete floors. Located in CT zone 5

Under slab options- confusion –

I am reading that the 6-15 mill poly barrier should be BELOW the 4″ of compacted stone so that the water in the concrete can dissipate and cure faster, makes sense.

But, I am also reading that the poly barrier should be installed on top of the stone below the insulation.

And, I am seeing videos where it is installed ABOVE the insulation. So I am assuming the insulation is placed directly of top of the stone. This certainly does not aid in the water discipation from the concrete.

Now, in regards to types of insulation –

It seems the jury says uses only XPS 1.5″ pr better under the slab. But there are many posts favoring Poly ISO under slab. Now the XPS seems to be reccomended because it does not absorb water and some say the poly iso does. Neither is cheap, but there seems to be alot recycled / 2nds of poly iso available ( and better R value) making it cheaper than XPS. So what if we were to install a poly layer on top of the stone, then the poly iso, the another poly barrier layer (in effect sandwiching the ISO) on top and hen pour the slab? Wouldn’t this eleviate the problem with the ISO getting wet and giving us a better Rvalue under slab and save a few dollars?

Exterior of slab can 1″+ xps and then flashed for edge protection

Interior foundation walls can also use the 2″ iso fastened directly to walls, should there be an air gap between the wall studs?

What say you?


GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    1. Polyisocyanurate should never be used under a slab because it can absorb moisture. In this location, use either XPS or high-density EPS (Type IX).

    2. The polyethylene should always be installed directly under the concrete (in other words, above the rigid foam). While concrete contractors (especially those who add too much water to their mix because they are more interested in helping the concrete flow into place than they are in delivering strong concrete) like installations that allow water to "bleed" into a layer of sand or crushed stone, the only advantage is to the contractor, who is in a hurry for beer time and who wants to get his power trowel on the slab as quickly as possible.

    Here's more information: Concrete Floor Problems.

  2. Expert Member
    Dana Dorsett | | #2

    Depending on the load constraints & thickness of the slab you can often get away with 1.5lb. EPS (Type-II) rather than 2lb EPS (Type-IX) but going lower density than 1.5lbs isn't going to meet code in most apps. You can park your Hummer (or even a small bulldozer) on a 4" slab without deforming 1.5lb EPS, or cracking the slab, but if the slab is supporting the house, not just the floor you'll need higher density goods. Type-II EPS is rated for 15psi, Type-IX is rated for 25psi, but for a residential slab there are no loads that would come anywhere near a deforming pressure with the weight distributed by 4" of concrete. A 1-2" rat-slab could have cracking issues with either type of EPS under a 50 gallon hot water heater (450lbs+ ) supported with 3-4 stubby feet, but not with a 3-4" slab.

    NEVER use a hygroscopic foam like iso in a completely-buried app. Iso can be used on the interior of sub-grade walls as long as the facers are rated <1 indeed.

  3. wjrobinson | | #3

    Building permits, stamped plans, reviewed and approved by your permit office. Where I work, you can't just dream up a plan from an armchair and an internet connection.

    If you are building for yourself and no permits required then do your thing. You still have to stop most winter cold at the edge and under the footers. Footers and foam is sharp pencil time.

  4. 14a_Builder | | #4

    Thanks for the quick feedback Our project is definitely NOT arm chair click and design!

    I am a very out of the box thinker and always open to any new design alternative.

    To clarify, the poly iOS I am referring to is made by Firestone. It is a CLOSED cell, 20+ psi rated product with fiberglass on both sides.

    The slab will also be 4000psi rated with mesh, rebar and PEX.

    So I understand the water absorption factor of a OPEN cell foam and the degrading R value over time of such product, this does not seem to a factor here.

    Maybe I'm missing something, but I certainly thought that isolating the poly iOS between 2 layers of 6-10 poly barrier would offer the protection from the elements

  5. wjrobinson | | #5

    Chris, post 3 is my opinion complete. Your project, your decision next.

  6. GBA Editor
    Martin Holladay | | #6

    I think you are talking about polyisocyanurate (sometime shortened to polyiso). It is not spelled "poly ISO." I have no reason to believe that polyiso from Firestone has different physical properties from polyiso made by other manufacturers.

    You don't want to use polyiso under a slab because it can absorb water.

    Wrapping a piece of polyiso with polyethylene won't keep it dry. Try this experiment: wrap up a paperback book and a box of Cheerios with polyethylene, and bury the bundle for a year. Dig it up after a year and tell me what the paperback book looks like, and how the Cheerios taste.

  7. wjrobinson | | #7

    Martin, now you've done it. Smashed my funny bone over your last post! Love it. Save. Much better than all my years of splainin how lobster traps work.

  8. DubixCube | | #8

    I'm a bit late to the party in reading these comments, (about 9 years) but I will say that the comments made about polyiso here are only true with specific brands of polyiso. The polyiso foam itself doesn’t absorb any measurable amount of water- a negligible amount at best. However, there are a couple of well-known brands of polyiso that embed glass fiber within the foam itself (referred to in ASTM as "Type 1, Class 2" foam) Glass wicks water, and thus those particular brands/types of polyiso will appear to absorb the water, and could be an issue in below grade and under slab applications.

    Most manufacturers of polyiso do not embed glass fiber in their foam (Type 1, Class 1), so the blanket answer made above that "Polyisocyanurate should never be used under a slab because it can absorb moisture" is only true when speaking about particular brands of polyiso. Outside of that- polyiso is a good/ great rigid insulation choice in this application. So, the answer should be "Depends on the brand and type of polyiso you are using"

    Polyiso should ABSOLUTELY be considered in this application. Higher R-value performance with less material thickness, and equivalent to better moisture absorption (or lack of) than other foams mentioned in these comments.

    1. charlie_sullivan | | #9

      Your information may not be correct. This data sheet says that Class 1 is without glass fibers and Class 2 is with, backwards of what you said. And both have the same moisture absorption number listed.

      Do you have a source for your claims?

      But it's kind of a moot point anyway because for below grade, there's not much reason to care about R-per-inch. What you want is high R-per-dollar, and EPS wins there.

  9. DubixCube | | #10

    Hey Charlie- good catch! I did mix up the Class. I’ll adjust my comment accordingly.

    I disagree about it being a moot point. I think some builders would find benefit to thinner insulation below their pours... But I do agree on your price point. EPS is cheap. Though I question if "cheap" should be the only determining factor in what people use. If you were to put a piece of EPS in the middle of a water-tight container, and seal the edges of the foam, then fill one side with water... it would take about 10 minutes max before someone realized that price shouldn’t be the only consideration.

    That being said, the price point per R-Value of the rigid foams are not that vastly different from each other when you compare the dollar per “R”, rather than the dollar per inch.

    1. Expert Member
      MALCOLM TAYLOR | | #11


      I'm left trying to think up a situation where the depth of sub-slab foam is an issue?

    2. charlie_sullivan | | #12

      If the behavior of EPS in the presence of water worries you, you should be even more worried about the behavior of polyiso. You haven't provided any support for your minority view that it's OK as long as it doesn't have glass fibers.

      1. DubixCube | | #15

        please see attachment below that shows one brand of polyiso tested to the appropriate ASTM absorption standards.

    3. Expert Member
      BILL WICHERS | | #13

      Where are you getting this info? Polyiso wicks water due to it's cell structure, not "glass fibers". I can tell you that glass doesn't wick water, either. Glass doesn't care if it gets wet, and doesn't absorb water. Glass fibers are sometimes used as reinforcement in various products, and they don't themselves change the properties of the material in regards to moisture absorption.

      Polyiso isn't usually rated for compressive strength, either, whereas EPS and XPS both are (or have readily available variants that are). Compressive strength is important for under slab use.

      EPS will release that water if it can. This is why you still provide drainage under insulated slabs, usually with the use of clean stone. Polyiso is MUCH slower to release any absorbed water.

      Unless there is a specific type of polyiso being manufacturer that has undergone lab tested and is specifically rated for underslab use (as EPS and XPS often are), then it SHOULD NOT be considered for such use.


      1. DubixCube | | #14

        heres a clip from a TER report for Rmax products that shows it testing, and passing, the required ASTM absorption standards.

        Also- I'm not sure where you hear that polyiso isnt rated for compressive strength. All polyiso is graded for compressive strength, and often that compressive strength is in the design requirements when it's specified. (generally 20-25 psi)

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |