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

Interior insulation for an existing cmu wall with Eifs on the extrerior

johnarch1 | Posted in GBA Pro Help on

Hi Everyone,

I have a question about insulating and existing above  grade cmu wall.
We are in NJ – climate zone 5

The existing cmu wall has Eifs on the exterior with 3″ of rigid insulation
( 3″ = R15 +/-)

On the interior there is existing 1″ furring strips with no insulation in between. At a couple of location we have full 2×4 walls on the interior with aprox 1″ air gap between the studs and the existing cmu walls.

From a code point of view we already meet code: mass wall insulation requirements is 13 exterior / 17 interior

I am struggling to figure out if it makes sense to add additional insulation on the interior and if so what type. I do not want to cause a moisture issue in the future.

Here are my questions:  

1- Would adding 1″ of rigid foam between the existing furring strips make sense. We would not be able to do this on all the exterior walls as some have existing gyp. bd. to remain on the furring strips.  

On the walls which we can add rigid insulation would the rigid insulation need to be air sealed? How would we do this.

Would we tape the joint between the rigid and the furring strip.
Is air sealing in our condition critical or is it ok to just add the rigid insulation between the furring strips with not taping.

2. On the cmu walls that have 2×4 wall in the interior would adding fiberglass batt insulation be acceptable in our condition. I know that his is normally not recommended but I am wondering if this is ok considering that we have Eifs with rigid insulation on the exterior.

Would we want to keep the Kraft face / vapor barrier or would it be better to insulate with no faced batt insulation.

Any advice or insights into the above would be greatly appreciated. We want to maximize the insulation but do not want to risk creating a moisture / mold issue in the future.

Thanks, John

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. luke_p | | #1

    Do you know if the top of the walls are air sealed? If the wall tops are not air tight, then your EFIS isn't going to be performing well since the CMU are hollow you will have a direct open pathway for cold air to fall down inside the walls from the attic (or parapet as in the case on my house)
    Are all of the exterior walls CMU with EFIS or are there also traditionally wood framed exterior walls that join the CMU walls. I ask since the interface would be another area you'd want to make sure you are not getting air leakage into the hollow cores. Same at all penetrations like doors and windows.

  2. johnarch1 | | #2

    The cmu is 12" and is filled solid. Yes another part of the existing house is standard wood frame construction.

  3. johnarch1 | | #3

    John here again.

    I would really appreciate if the experts could please comment on my questions above. Thanks. John

  4. Expert Member
    PETER G ENGLE PE | | #4

    With 3" of rigid insulation on the outside of a CMU wall, you are already in pretty good shape, as you have found, meeting current code-level insulation values. Additional insulation would not provide much bang for your bucks, even if the walls are already open and exposed. Adding another 1" of rigid between the existing furring strips might provide a bit better performance, but you would probably not notice the difference inside. The EIFS is generally a good air barrier, though the newer water-managed EIFS has a small air gap that can allow some air leakage from top and bottom. I would perform blower door testing before spending any significant effort or expense in making the walls more airtight. I would not recommend fiberglass or other fluffy insulation on the interior. Even with the 3" of exterior insulation, EIFS can leak into the walls. Small leaks will migrate through the CMU and dissipate over time, but damp spots can happen and fluffy insulation does not like dampness.

    Your money would probably be better spent chasing air leakage through the attic floor and adding insulation to the attic. A whole-house energy survey can help you make these decisions.

  5. johnarch1 | | #5

    Got it. Thanks. John

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |