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

Perimeter Slab insulation around monolithic pour

joenorm | Posted in General Questions on

I have read most articles and Q&A on this topic. I still don’t quite get the typical detailing of rigid foam around a monolithic slab. 

If I fully support my walls over the footing there is 2″ of rigid foam outside the plane of the wall at the foundation. Would I use a step flashing here? How do I protect the buried rigid foam from bugs? Or do I just cross my fingers and hope for the best?

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
    Michael Maines | | #1

    Joe, there are a few options. You can cover the projecting insulation with a cap flashing and/or a water table--a trim board that sometimes has a sloped board on top. Or if you're using 2x6 framing, you can overhang it, as long as most of the plate is supported. Or you can use exterior insulation on the above-grade walls. Or you can use double-stud walls that overhang the foundation insulation. There are probably more options but those are the ones that come to mind.

  2. environsllc | | #2

    Hey Joe, we use that approach often when building ADUs in Portland Oregon. We install 3" of XPS around the perimeter and then have 10' sheets of flashing made to wrap around there perimeter. The height of the vertical face of the metal varies depending on distance to grade. The metal takes 90 over the face of the foam and has a 5 degree angle over the~3" and then goes up the face of the sheathing a few inches. Our WRB then laps over the metal. We don't have an issue with bugs eating the foam here (at least not one that I have discovered).

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |