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

Borate Treated XPS needed if installing outside stemwall & partially below grade?

paula_builds | Posted in Green Products and Materials on

I have a conditioned crawlspace (new construction) so I’m looking at what insulation product to put outside my stemwalls.  I’m planning on asphalt damp-proofing under the foamboard (on the stemwall) and a drainage surface (dimpleboard) over the foamboard on the uphill side before backfill.  Where the foamboard isn’t backfilled against, I’ll have to cover it with something.  My drafting professional suggested a “cementitious board”.

I’m supposed to have R15, so my default choice is probably 3 inches of the pink foam board.  However there’s the foil backed rigid insulation also (polyisocyanurate) which apparently is more expensive.

Climate:  I’m on the wet side of Oregon, on a nicely sloped hillside.  Apparently termites aren’t too bad here.


1.  I recall hearing that insects or mice or something likes to eat the pink board.  Should I look for a borate treated product?  or will covering the insulation with flashing/cementitious board be adequate?

2.  I see that the foam backed insulation has a higher r value and higher cost.  Is there a difference in how attractive these two are for insects or mice?
(The foil backed insulation is not recommended for below grade.)

3.  What ideas are there for covering the foam board insulation on the downhill side of the house where there isn’t much backfill to cover the stemwall (just the minimum 12 inches above footing).  Would the normal thing be this cemetitious board (??) with stucco?  or just a parge coat applied directly onto the insulation?

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. paula_builds | | #1

    I'm reading this and getting a LOT of possible options for my question 3.

  2. Expert Member
    BILL WICHERS | | #2

    You CANNOT use polyisocyanurate (polyiso) insulation in an application like this where it will be buried in the ground. No point in considering it any further.

    You can use XPS and EPS here, or rigid mineral wool, although I’d go with either XPS or EPS myself. Insects can build nests inside either type of foam board if it’s moist, as far as I know there isn’t an advantage to one type or the other in regards to critter resistance.

    Is this foam board going to be exposed above grade much? If it’s not, then the visual appearance of whatever you use to protect it won’t really matter.


  3. paula_builds | | #3

    Glad to know that polyiso won't work below grade.

    The foam board won't have much exposure on the uphill side, but will be exposed for about 3 feet on the downhill side, so I do have to figure something out. about half the house has a deck which will partially conceal the stemwall, but I will still need to protect the insulation from insects & animals chewing...

  4. seabornman | | #4

    I've been very happy with a cementitious coating (I used Tuff II purchased at HD) over a self-stick
    mesh (Styro brand). So far (two years) it looks the same and has withstood weed-eater abuse. You can extend it below grade. I didn't do anything to protect XPS down to footing - just tried to keep big rocks (which are prevalent here) away from the foam.

  5. Expert Member
    Michael Maines | | #5

    XPS (blue, pink or green foam board) has blowing agents that are extremely potent greenhouse gasses. Unless you can get it used, please don't use XPS. Plus insects love tunneling in it. I spec borate-treated EPS, which in 15 psi or higher densities works well below grade. I second Joel's suggestion--I spec Styro Industries' Tuff II coating with self-stick fiberglass mesh. For even more protection you can install expanded metal lathe instead of the fiberglass.

    1. paula_builds | | #6

      I am really happy to know about the greenhouse gases. I hope I can locate the borate EPS.
      Thank you for the information...

      1. Expert Member
        Michael Maines | | #7

        Paula, the brand we have access to in New England is also available near the west coast, though not super close to you. It would still worth inquiring about shipping costs: There may be other brands that include borate treatment; I'm not sure.

        1. paula_builds | | #8

          Hi Michael,
          I have a further question. If I am able to use spray foam insulation in the house walls (between studs) what is the greenhouse gas concern for that product?

  6. Mark_Fine_Homebuilding_Techeditor | | #9

    Hello paula_builds,
    My name is Mark Petersen. I'm an editor at Fine Homebuilding. I would like to use this Q&A in our Ask the Expert department in the magazine. Would you be alright with that? Please send your reply to [email protected]

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |