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Basement Wall Insulation Question

mcervinojr | Posted in General Questions on

I’m looking for some advice on how to properly insulate my basement walls in Passaic County, NJ.  Right on the border of zone #4 and #5.  Basement will eventually be finished and conditioned w/ a mini split HVAC unit.

The house is 12 years old, basement is CMU block, painted with 2 coats of Drylok.  French drains around w/ floating slab & sump pump.  Walls are dry w/ the exception of one very small leak that is due to a gutter which I will fix in the spring.

I’m getting a lot of conflicting information online and from my architect on how to properly insulate.  Currently the basement is framed w/ 2×4″s w/ a 1″ gap between the CMU and backside of the stud wall.  Architect detail shows to install R13 Rockwool between the studs then green sheetrock but leaving a 1″ gap between the back of the Rockwool and CMU.

However, I see multiple instances of online resources saying that XPS foamboard should be installed against foundation, then 2×4’s and cavities filled w/ batts.  Obviously that would pose a problem for me at this point and I would have to rip out the framing to redo it.

I spoke w/ the Rockwool technical department and they recommended that if I cannot remove the studs to install the rigid XPS against the foundation, next best course of action would be to press the Rockwool directly against the CMU; then install a smart membrane like Certainteed Membrain on the inside of the wall prior to installing the sheetrock.

Ideally I’d like to be able to get this done without removing the framing but advice would be appreciated.


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  1. Expert Member


    You need a layer of impermeable insulation against the concrete to stop moist air condensing on the cold inside surface. The techs at Rockwool are right - the best solution is to use foam board for that layer, but if you don't want to remove the framing, the only good alternative is spray foam. This blog explains much better than I have what is in play:

    1. Deleted | | #20


  2. mcervinojr | | #2

    Appreciate the quick response. I'm not too keen on spray foam. I'll see if there's a way to sneak some rigid board in there.

    Also, if rigid foam is used against the foundation, why is Rockwool necessary and not just typical fiberglass batts?

    What about the membrane?

    1. Expert Member
      MALCOLM TAYLOR | | #3


      Basement walls that rely on good interior air-sealing, a variable-perm membrane, and use permeable insulation ( like Rockwool suggested as an alternative), sometimes work and sometimes don't. If you weren't planning of finishing the space that might be a risk worth taking, as you could assess how much moisture and mold you were getting quite easily, and remediate if necessary. For a finished basement I think you would be best sticking with the assemblies Martin suggests.

  3. mcervinojr | | #4

    Malcolm, thank you.

    I think I can squeeze 1" EPS boards in. Would that be sufficient? The EPS seems more flexible than XPS. Is one better than another?

    Also, I'm still not clear if a smart membrane should be used on the studs w/ Rockwool if I get the EPS against the concrete.


    1. Expert Member
      MALCOLM TAYLOR | | #5


      The article I linked to calls for at least R-5 of foam against the wall in zones 4 & 5 (and remember it needs to be sealed between sheets and at the edges), so I would choose the type of foam largely based on that. 1" of EPS is about R-4, XPS about R-5, and Polyiso around R-6.

      1. mcervinojr | | #6

        Thank You!

        If I get this foam against the wall, would I still need a smart membrane over the Rockwool, or can I install green board drywall directly on the studs?

        1. Expert Member
          MALCOLM TAYLOR | | #7


          No need to any membrane, just the drywall.

  4. mcervinojr | | #8

    Got the EPS in the gap, however I have an area at the top of the foundation where it steps back for the sill. How is this handled?

    1. Expert Member
      MALCOLM TAYLOR | | #9


      See this section from the article I linked to.

  5. mcervinojr | | #10


    Last question. Does the batt in the cavity get pressed directly against the EPS ? I would assume there should be no gap

    1. Expert Member
      MALCOLM TAYLOR | | #11


      It looks like the batts will completely fill the cavity and sit against the foam, but yes, it's best to avoid a gap between the two types of insulation if possible.

  6. Expert Member
    ARMANDO COBO | | #12

    I guess code books are just a sugestion...

    In CZ 4, on the 2018 & 2021 IRC Table N1102.1.2 you need R10ci OR R13 cavity minimum.

    In CZ 5, on the 2018 & 2021 IRC Table N1102.1.2 you need R15ci OR R19 cavity minimum OR R13+R5ci

    1. Expert Member
      MALCOLM TAYLOR | | #13


      Right, which was what I linked to in post #1, and, and what my advice in post #5 was based on.
      The wall has ended up with R-4 ci and R-13 batts.

  7. Ryan_SLC | | #14

    Rockwool PDF instructions for residential basement and crawl space clearly shows that ComfortBoard/Batt is affixed directly to the concrete wall, studs are built touching the ComfortBoard, and R Rockwool fills bays. No air gaps in any space.

    By Rockwool's instructions, Rockwool is directly touching the concrete and framing all in one Rockwool product with no gap. Rockwool also says it does not wick moisture.

    Instructions can be found by Googling: ROCKWOOL Comfortbatt - Residential Installation Guide
    Also the shorter version here:

    1. Expert Member
      MALCOLM TAYLOR | | #15


      mcervinojr has installed foam, not rock wool.

      The question is: when you have a small space between the framing and the continuous insulation that will result in the batt insulation having a gap on one side or the other, which one do you choose, and does it matter? Given it's small gap, and rock wool isn't very susceptible to wind-washing, I think the effect of the gap on either side is in the weeds.

      1. Ryan_SLC | | #16

        Rockwool Comfortbatt/Board rigid comes in #8 1/2" and 1" thicknesses. Pretty cheap too for the 1" 2x4 boards at R 4.

        The assumption is it has to be foam or the called for 1" gap. I was just saying with their own mention of Rockwool, Rockwool install documentation exactly meets their situation. All you'd need to do to meet Rockwool Comfortbatt/Board 60/80 install instructions in that described stud wall is cut 2x4 Comfortbatt board to slide it in the gap. Slide in between that 1" gap, fill the bays with R13 or R15 Rockwool and you've fully met the Rockwool install instructions for basement walls.

        1. Patrick_OSullivan | | #17

          A lot of people here are not comfortable recommending permeable insulation (e.g. Rockwool), even in board form, against a basement wall. In other words, there is skepticism about the suitability/resilience of Rockwool's installation recommendation.

          1. Ryan_SLC | | #18

            I get it.

            But I also don't see any testing that Rockwool sponges or wicks moisture out.

            If we are making a false assumption here, there is no solution that plastic foam isn't the answer. I know I don't know. It would be odd that Rockwool has multiple instances of rotting stud assembly designs for home owners to destroy their homes' with. Not saying it's not possible, but...

          2. Expert Member
            Akos | | #22

            This is a bad idea. I though Rockwool has removed references of comfort board being used inside in a basement.

            When it comes to moisture and air movement there is no major difference between comfort board and comfort batt, both are just as permeable. Comfort board does a better job of controlling convective airflow but that isn't the issue here. We know batts are a risky basement insulation option, so this isn't any better just way more expensive to do.

            I have a place that is insulated like this and unless RH is kept in check (dehumidifier running full tilt from early strip to late fall) it starts to mold.

        2. Expert Member
          MALCOLM TAYLOR | | #19


          The assumption that it has to be foam has a good building science basis. As is explained in the link in post #1, you need a layer of well air-sealed impermeable insulation to stop moist interior air from coming in contact with the concrete and condensing. Using permeable insulation against the concrete introduces risk into the assembly. It may sometimes work, but it isn't one typically recommended - except by manufacturers trying to extend the situations in which they can sell their products.

          1. Ryan_SLC | | #21

            And I welcome the learning experience. Being a neophyte, I can only search this forum. I see tons of reference to Martin's article. One might call it circular.

            Martin is a field expert. I DO NOT challenge that. That said, I don't see the follow up testing. I don't see data behind the article.

            Rockwool ComfortBatt/Boardis designed for against concrete basement wall. Rockwool states it it is vapor open, but does not wick. Shouldn't data show this. Or a study. Or an example?

            I am not challenging. I just can't find the details behind it. all.

          2. Expert Member
            MALCOLM TAYLOR | | #24


            You don't need field testing of easily understood fundamental elements of how moisture works in assemblies. If you are asking for evidence that basement walls with batt insulation have mold and condensation issues, it's a widespread problem - and it's what has lead to the advice in Martin's article. No one stopped using permeable insulation against concrete walls for the fun of it.

            I'm not sure how useful this will be as it has already been explained in this discussion several times, as it was in Martin's blog, and numerous BSC articles but here goes:

            You are misunderstanding the issue. This has nothing to do with wicking, and it is the insulation being vapour-open that is the problem. By being permeable, rock wool allows warm humid air to reach the cold concrete, where it condenses. That now wet interface experiences mold and decay of nearby wood. To stop that happening you need an continuous layer of insulation against the interior face of the concrete that is a) vapour-closed so the moist air can not get to the concrete, and b) thick en0ugh so that the interior face of the foam stays warm enough so it doesn't to become a condensing surface itself.

          3. Ryan_SLC | | #25

            So why not install it as Rockwool instructions say, with a vapor barrier.

            Is the vapor always present because of the concrete always being "wet"?

            I mean, I am at a loss on the comment that someone is surprised Rockwool says this. They say it on about every document they have on their website that Batt/Board is good for direct basement/craw space wall. Taking it outside, how could Rockwool ever be an exterior insulation. Honest question. Are people just accepting furring strips are being saturated daily by their rock wool components?

          4. Expert Member
            MALCOLM TAYLOR | | #26


            Did you read the BSC article I linked to?

            I'm sorry but I'm going to bow out of this discussion. You are repeatedly argumentative about things you don't have any understanding of, show no real inclination to get that knowledge, and appear advice resistant.

          5. Ryan_SLC | | #27

            Sorry. Not trying to be. Rockwool shows it listed multiple times on their website.

            I get what you are saying, but I've not been pointed to data but an idea.

            Thanks Malcolm for the help on everything. Don't assume I'm being argumentative, I'm trying to see where it's not just theoretical but, yep, Comfortbatt will mold out a basement wall when attached to concrete as Rockwool instructions say.


  8. Ryan_SLC | | #23

    Your second picture, picture alone, shows you aren't meeting fire block code.

    If it's kicked out 1 inch, like basement walls tend to be, it might be good time to check the walls are fire blocked for horizontal spread and the top plate is block for vertical to ceiling spread.

    If not, you could make the mistake of jamming foam into a place it cannot go.

    1. mcervinojr | | #28

      How do you gather that from the photo? There is a continuous strip of plywood at the top of the wall installed for vertical fire block. Theres also pieces spaced every 8 feet or so behind the 2x4’s for horizontal fire block.

      Next question for you all is how do you handle batts where there is a step back in the foundation?

      1. Ryan_SLC | | #29

        Horizontal is the what isn't seen in the picture.

        There needs to a block at the horizontal where the wall and ceiling connects so the ceiling and wall. You can't see it in the pictures you have and the 1 inch kick out means it's slight more difficult than simply the top plate serving as the block in the wall.

        If you can put your rigid foam up that wall and hit the ceiling, you know they don't have the fire block for the wall.

        1. Expert Member
          MALCOLM TAYLOR | | #30


          The horizontal fire blocking is clearly visible just as mcervinojr described it.

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