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

Vapor Retarder for Below-Grade Cinder-Block Wall

X_NavyFC | Posted in Green Building Techniques on

I’m going to be remodeling the lower level of a bi-level house in climate zone 5 (Northern Illinois).

I’m confused on the correct way to do a vapor barrier on a cinder block wall that is approx 42” below grade as is common in the bi-levels.

If I used 2” XPS rigid foam against the blocks and then framed my wall in front and filled stud bays with Rockwool, is a vapor barrier needed ?

I’m thinking the XPS would block vapor at the block so that would cause me to not need an air/vapor barrier such as Certainteed’s Smart Membrane?

If I decided to use Rockwool’s ComfortBoard 80 against the block instead of XPS (since bugs like to burrow in foam), how would I approach an air/vapor barrier in that circumstance?

Thanks in advanced!!

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. user-2310254 | | #1


    Be sure to read Martin's basement article: Note that it calls for R-15 for your climate.

    You don't want to use air permeable insulation such as Rockwool against the wall. More than anything, you want to confirm that there are no bulk water issues before you start. You might want to check your local Craigslist to see if reclaimed is available since it's a lot greener than new XPS. (And it may be cheaper as well.)

    1. X_NavyFC | | #3


      Thanks for the reply. I wasn’t able to read the full article (not a member).

      When you say R-15, I assume you are meaning total wall insulation and not just the rigid board insulation on the block. Is that correct?

      I’m surprised Rockwool is air permeable since they use mineral wool as fire stopping but never really thought about it.

      Thanks again!

      1. user-2310254 | | #5


        R-15 is the total, but you should check your local code requirements since they might be different. It's my understanding that the foam layer needs to be thick enough to prevent condensation forming on the wall. Martin indicates that's at least R-5, but you can use thicker foam.

        1. X_NavyFC | | #6

          Thanks Steve.
          So assume that I end up going with foil faced polyiso 2” and then rockwool in the cavities.

          Would I need to use the MemBrain air/vapor barrier in that scenario?

        2. X_NavyFC | | #9

          Ok, thanks for clarifying

  2. charlie_sullivan | | #2

    To follow up on the comment about XPS, there is no building material worse for the climate than regular XPS. Fortunately, there are many alternatives: EPS, GPS, polyiso, reclaimed foam, and the new Owens Corning "NGX" foam. Any of those is vastly better than XPS on climate impact, and most will give you more R value per dollar spent so you end up with better performance as well.

  3. X_NavyFC | | #4


    Thanks for taking the time to reply.

    I wonder why any articles or videos on the subject of basement insulation always seem to be done using xps or closed cell spray foam along the cement or block? Would these be cheaper than some of the alternatives that you listed?

    I was thinking the ComfortBoard 80 would have been a suitable substitute for xps but now Steve has me rethinking that.

    I know in my circumstance it also comes down to availability. I’ll have to check around and see which of the alternatives that you mentioned are available and how they compare to performance and price.

    1. charlie_sullivan | | #14

      Why do articles usually use XPS? Probably mostly because it's been the most readily available in home centers. And because awareness of its extreme climate impact has been limited until recently. Also, some low-density EPS is pretty crumbly and a little annoying to work with, but you can get EPS in the same density as XPS at which point it's not so hard to work with.

  4. Jon_R | | #7

    The latest from Building Science. Note the cautions about air and vapor (around 1 perm is best) permeability.

    1. X_NavyFC | | #10

      Thanks for linking this article, it has alot of good info. Basically what I get from it is that I need a low or no permeable rigid foam insulation along the block, unfaced fiberglass or Rockwool in the stud cavities, and no air/vapor barrier over that assembly.

      Since Certainteed’s MemBrain appears to allow vapor to pass through to the warm side, I think it’s use would be ok but I don’t see a need for it at all. Thanks

      1. Jon_R | | #13

        I think Joe L is clear about recommending "approximately 1 perm". This isn't a maximum or a minimum, so don't do near zero perms or many perms.

  5. Expert Member
    Akos | | #8

    ComfortBoard 80 is vapor open, it should not be used for interior insulation in a basement. For interior basement insulation you want a foam layer that is less than 1 perm (EPS/GPS/NGX/plyiso closed cell SPF are all fine).

    1. X_NavyFC | | #11

      Thanks for the reply and the information. I’m glad I asked about this before using the ComfortBoard in my project and causing a problem!

    2. Deleted | | #12


Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |