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

2″ rigid foam perimeter

user-1145045326 | Posted in General Questions on

Hello all!!  I have a slab addition project.  I have covered the entire area with 2″ rigid foam all the way to the foundation walls. The foundation walls (about 3′-4′ high) are also vertically insulated with 2″ rigid foam to the footing.  (extruded polystyrene)
I am getting ready to cover the 2″ rigid XPS  slab with white poly plastic 6 mil. and over the foundation walls. After that I need a short vertical piece of  2″ rigid foam on the perimeter sitting on the plastic and have to “snap concrete grade lines” on it.  I can not snap lines on the plastic sheeting. The pour stops 1 1/8″ from top of foundation wall. The short vertical 2″ rigid foam (about 5″ high) will not be covered in plastic in order to be protected from the concrete with superplasticizer. Is there a 2″ rigid foam with a protective barrier laminated on to it so the wet concrete can lay against it and I can snap grade lines on it?  The concrete foundation wall has a 3″ ledge all the way around. 2″ of rigid foam (vertically) and 1″ left for slab to rest on. I have bolted 2″ aluminum angle iron on the house basement side for slab to rest on. After pour I will cut off excess plastic. Rehash in order:
 2″ rigid foam & foundation wall covered in plastic  and 5″ vertical rigid foam on perimeter. Thank 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. Expert Member
    BILL WICHERS | | #1

    I'm assuming you're using a chalk line, and the chalk won't stick to poly sheeting, and probably won't stick to XPS very well either. My guess is that EPS is the only commonly available rigid foam type that chalk might stick to, but I've never tried it.

    I think the Sharpie marker is probably your friend here -- just run it along the "chalk" line and use it to mark the poly sheet or XPS. It's a bit more labor this way, but you save a lot on material, so you should come out ahead. I'd probably go with either a Super Sharpie (big and fat version of the regular Sharpie), or one of the Sharpie Magnums (even bigger and fatter version). Make sure to have several markers on hand if you do this -- the tips don't hold up very well marking construction materials.


    1. user-1145045326 | | #3

      I can get a line of some kind on the rigid foam. That is not a problem. I am concerned about the wet concrete up against the XPS. Looking for 2" xps with some sort of lamination to resist the wet concrete. I do not want the rigid foam to get wet and stay wet from the concrete.

      1. Expert Member
        Michael Maines | | #4

        There is no problem with having concrete against XPS. It doesn't absorb water and it is completely inert to concrete's alkalinity.

        1. user-1145045326 | | #6

          Okay then why put the 6 mil over the rigid foam as Joe L. explains in Slab Happy?

          1. Expert Member
            Michael Maines | | #8

            Because the water in the concrete can get below the foam and cause the foam to float up.

          2. Expert Member
            MALCOLM TAYLOR | | #10


            And that bleed water can stay trapped there, taking months to dry. But that isn't inside the XPS, it's below it.

  2. Expert Member
    Michael Maines | | #2

    I would put the 6-mil poly behind the vertical foam. Red or black chalk will stick to XPS. Blue or white chalk tends to flake off. Tajima sells an ink line but that shouldn't be necessary in your case.

    1. user-1145045326 | | #9

      Mike, thanks for your attention but Joe L. makes the point if XPS gets wet from concrete it will stay wet with the plastic on the other side of the foam. It is trapped. It is only water resistent, not water proof. I am only trying to find out if there is a rigid foam that is laminated and is even more water resistent than the usual. ???

  3. Expert Member

    User ...326,

    Be careful. Aluminum reacts with concrete and should not be in direct contact.

    1. user-1145045326 | | #7

      Thanks. I will butyl tape the angle.

      1. Expert Member
        BILL WICHERS | | #13

        Aluminum will also react with steel fasteners. What you want to do is use stainless steel fasteners and tape between the aluminum extrusion and the concrete, using a heavy tape that won't be easily punctured (Orbit's #53550 tape is a 20 mil thick tape usually use for pipe protection that will work well here).

        If you're question you still need answered is about "some kind of rigid foam with a protective barrier", you could try R-tech's EPS, which has a poly facer laminated to it. I don't think you really need that here though, the XPS should be fine by itself.


    2. user-1145045326 | | #11

      Come to think of it the XPS is sitting on the angle. The concrete will not be in direct contact with the aluminum. Does anybody have an answer to my question? I joined up to find things out.

      1. Expert Member
        MALCOLM TAYLOR | | #12


        Haven't you already been given your answers?

        - You don't need to protect the XPS from the concrete. That was based on a misunderstanding of Joe L's caution about sub-slab poly.
        - Tajima sells chalk that will stick to anything, or you can use a marker.

        - Didn't you bolt the aluminum angle to a concrete wall without any intervening material?

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |