GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X 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

Can I put polyiso on both sides of a CMU wall?

michaelbluejay | Posted in Green Building Techniques on

I have a completely above-grade single-story house with exterior walls made out of 6″ CMU (hollow/unfilled) in Climate Zone 2, no inside framing, you go inside and you see the same CMU blocks that you saw from the outside.

Well, that is until I installed 3/4″ polyiso on the inside a few years ago.  Now I’d like to install more insulation, so I planned to install 1.5″ to 2″ of polyiso + furring (rainscreen) + siding on the outside, but now I’m wondering if insulating the exterior and making a CMU sandwich between two impermeable polyiso layers is a mistake since I read somewhere that masonry is supposed to breathe to dry out.  I don’t know how the wall is gonna get wet when it’s completely covered, and I don’t know why wet masonry would even be a problem because it can’t rot like wood, but anyway, is this a problem, and if so, what’s the worst that can happen?

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.

Replies

  1. michaelbluejay | | #1

    Well, I need to move forward on my project, and the materials are already purchased (and half installed before I realized I might be making a mistake), so I'm proceeding with the installation. If a problem develops, I presume the solution will be to remove the polyiso from one side of the wall, so I'd remove it from the inside, because that's only 3/4" of insulation while the outside is 1.5".

    But I'm still wondering, what problem might I experience with this kind of vapor sandwich? Rotting my top plates since that's the only direction any moisture can go?

  2. gusfhb | | #2

    Well, i don't think the concrete is going to care. Plenty of 80 year old block foundations sitting in the mud. I am in zone 5 and have a few spots where the formerly uninsulated poured concrete ended up with foam on both sides. No problem so far...

  3. Expert Member
    BILL WICHERS | | #3

    Masonry doesn't need to breathe, and doesn't need to dry out. Concrete doesn't care if it is wet. the only issue you could potentially have would be moisture wicking up higher in the wall and wetting framing above, but that's probably pretty unlikely in this specific situation that you have. If you're really worried, just get fiber faced roofing polyiso which is a little bit vapor open and will allow some drying and don't worry about it.

    Bill

    1. michaelbluejay | | #4

      Great, thank you for the replies, guys! And, by sheer coincidence, my polyiso *does* happen to be the fiber-faced roofing stuff. Whew.

      I should mention that there's 4-6" of exposed concrete slab below the CMU, so maybe that will promote any drying that needs to happen.

Log in or create an account to post an answer.

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |