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

Polyiso on Basement Wall as Thermal Break for Slab?

DennisDipswitch | Posted in General Questions on

Am I able to attach polyiso to a basement or crawlspace foundation wall,and then pour the concrete floor slab against it,or is a different material needed for the ground contact necessity?

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

    Ideally, you have a capillary break between your footing and your concrete wall, so your proposed detail should be fine.

    However, if your site is damp, or if you are uncertain whether you foundation contractor remembered the capillary break, or if you are the type of person who sleeps better at night when you know that you have planned for every eventuality, you could use vertical XPS at the perimeter of the slab, and polyiso from the slab to the top of the wall.

  2. DennisDipswitch | | #2

    How damp is damp?This foundation is built above a very sandy well drained soil,with no real moisture problems,other than bordering a flood plain(sea water) ,so the potential problems would only be evident with a historical event.

    And how moisture sensitive is polyiso? Is it a mositure wicking concern,or would the material be destroyed if exposed to a broken pipe in a basement? Is there some kind of edge protection detailing needed to protect it as a thermal break for a slab?

  3. GBA Editor
    Martin Holladay | | #3

    There are many unknowns here, but I do know the answer to your question. You are worried, so install the XPS already (just at the perimeter of the slab). Above the slab, install polyiso.

  4. DennisDipswitch | | #4

    Thank you for the counseling,Sir.

  5. Expert Member
    Dana Dorsett | | #5

    What Martin said, though you could also use EPS rather than XPS for the slab-contact material, and be somewhat greener due to the 99.5% lower-impact blowing agents used.

    A one-off flooding event that dissipates quickly isn't a problem for iso, but sitting with the edge in a puddle for days/weeks/months is. If you submerge the stuff it will eventual saturate the submerged portion even wicking a bit beyond the water level, and would take months & months (years?) to dry. But it has to be in contact with the water for a an extended period of time. Were it super-sensitive to intermittent short term moisture it would never pass muster as roofing insulation, yet that's it's most-common application.

    If there's a high-tide mark on the walls from prior flooding to refer to, use EPS up to that point, with a bit more. If you're really concerned about the storm-surge flood hanging in there for awhile, make it an all-EPS show. EPS is regularly used in marine environments for everything from dock-floats to lobster-pot buoys- it can take saltwater for years.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |