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Interior Insulation for Cinder-Block Wall

M N | Posted in General Questions on

I have a small commercial building an existing 10″ cinder block wall with almost no R-value in zone 7A.
It is not possible to insulate the wall from the outside because it sits right on the property line so insulate it from the inside is a must.
I am planning to put 4″ EPS directly against the block wall  with adhesive and then 1×3 furring, for electrical allowance then drywall.
Questions:
1. Would 4″ EPS work as vapour barrier and I don’t need to use any kind of poly between drywall and furring?
2. Is any treatment required for the cinder block wall to protect moisture trapped between EPS and the block wall?

Thank you very much in advance.

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Replies

  1. Expert Member
    Zephyr7 | | #1

    4" of EPS is in class II vapor retarder territory. You shouldn't need anything else unless your local code requires it. Note that if you use type I EPS, you're not going to get a great bond to the wall with glue (due to the relative fragility of the type I EPS material), so you'll need to be careful until you have the furring strips up which will ensure the EPS stays tight to the wall (you're going to through-screw those furring strips to the block wall behind the EPS, correct?).

    If this is an above-grade wall, which it sounds like it is, then you shouldn't need to do anything to the block wall itself. If it's a below-grade wall, then you might want to consider something like dimple mat IF you've had issues with water coming through the wall in the past (in which case that dimple mat would also need to tie into a perimeter drain to really add value).

    Bill

  2. M N | | #2

    Bill, thank you for the prompt response.

    The wall is above grade, since the building is ~70years old, there wasn't any insulation applied.

    I might have to put 2x4 stud wall with batt insulation in cavity behind 4" EPS to meet zone 7A prescriptive R-Value (r-27). I asked a contractor I know and was told that we will need to put poly as required by code here.

    Now a more complicated question. At the front of the building, for the look, I want to put EIFS stucco on the outside (since the front wall is not on the property line) and then 2x4 studs filled with batt insulation on the inside face of the block wall. I attached a sketch showing where I want to put the vapor barrier layer at. Do you have any comment?

    Miles N.

  3. Expert Member
    Dana Dorsett | | #3

    >"1. Would 4″ EPS work as vapour barrier and I don’t need to use any kind of poly between drywall and furring?"

    A CMU block wall is highly moisture tolerant and fairly vapor open (unless painted with a low-perm paint) and does NOT need to be protected from interior side moisture drives with a vapor barrier (unlike wood sheathed construction.)

    At 4" EPS that's at least 1.25lbs per cubic foot density (Type VII) is sufficiently low permeance that it would meet the NBC code definition of "vapour barrier", and would be fine even if there were susceptible wood on the exterior side of the insulation, and would be protective enough against freeze/thaw spalling even if the exterior is painted.

    >"Now a more complicated question. At the front of the building, for the look, I want to put EIFS stucco on the outside (since the front wall is not on the property line) and then 2x4 studs filled with batt insulation on the inside face of the block wall. I attached a sketch showing where I want to put the vapor barrier layer at. Do you have any comment?"

    Even though the CMU and EIFS are moisture tolerant, the 2x4 studs are not. It would be better if there were some sort of capillary break between the stud edges & CMU. If the EIFS is a low-permeance type you DEFINITELY want the poly sheeting on the interior side, even though that creates a moisture trap.

    The more robust approach would be a "cavity wall", with a half inch to inch of air between the studwall & masonry, with vents in the vertical mortar to the exterior every couple of blocks at both the bottom and top courses 0f block. The exterior side of the batts would need some sort of air barrier to meet it's tested R-value performance, but something vapor-open such as housewrap works just fine, despite being awkward or impossible to install after the studs are already up. With a cavity wall it's still going worth it to install interior side poly in your climate zone, but even if it were skipped it would still do OK with just standard interior latex paint on gyprock in most applications as long as your exterior side air barrier was vapor open.

    1. M N | | #4

      Dana, great insights and knowledge from you.

      Looks like I could go ahead with the side wall. For the front wall, I'll need to discuss with the contractor to see if the air gap with air barrier is possible to do.
      This will be a long run since I have to consider the cost and get a permit from the city.

      I will get back with updates.

      Miles N.

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