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Community and Q&A

Insulating Foundation Walls with Fiberglass Batts

GROUNDUP | Posted in General Questions on

Hello- I know that with fiberglass batts its recommended to leave a gap between the foundation wall and the fiberglass (or not use it at all). Roxul recommends that the batts be directly against the foundation with no gaps. They say the gaps will cause a convention loop and condensation/mold. My house does have a gap between the batts and the wall and wondering if i should expect issues. Its a new home.

Would love to hear from anyone who has used this product on their foundation wall. We are climate zone 5. Thank you!

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  1. GBA Editor
    Kiley Jacques | | #1

    As you know “fluffy” insulation like fiberglass batts, mineral wool batts, and cellulose is air-permeable. When this type of insulation is installed in direct contact with concrete, the moisture in the interior air condenses against the cold concrete surface, leading to mold and rot. That’s why many high-performance designers and builders advise against installing fluffy insulation against a basement wall. I'll be curious to learn if the experts here think the gap will head off that problem.

    1. GROUNDUP | | #4

      Hi there. I would agree with what you are saying but roxul specifically emailed me back stating that they do not recommend any air gap at all. Roxul direct against concrete. Not sure for climate 5 if they would require a vapor barrier or not though.

  2. plumb_bob | | #2

    It is simply not recommended to have fiberglass insulation used to insulate a concrete foundation wall, for the reasons mentioned above. I have seen soaking wet batts in brand new construction when used in this application.
    Even with the gap, where is the trapped moisture supposed to go? Not through the poly...

    1. GROUNDUP | | #3

      So most of my basement is roxul. However there is one wall they used fiber glass but with a vapor barrier up against the concrete. So the assembly is concrete, poly, fiberglass batts and sheet rock. Do you see an issue with the fiberglass with this assembly?

      1. charlie_sullivan | | #6

        Yes, the issue is that the poly will be colder than the room, air, for most if not all of the year. If it's enough colder to be colder than the dew point in the room, there will be condensation on the fiberglass side of the poly. And even when it's not quite that bad, the cold poly will result in very slow drying of any moisture that has condensed there.

  3. plumb_bob | | #5

    The vapour barrier should be on the warm side. If you open it up and feel the back of the batts I would guess they are wet.

    1. GROUNDUP | | #7

      I feel like there is so much conflicting info out there. Building science corp says not to have it on the inside bc the assembly can't dry. I'll open it and see!

      1. Expert Member
        MALCOLM TAYLOR | | #8


        The advice Kiley, Charlie and plumb_bob have given you doesn't conflict with what Building Science suggests. None of them are recommending using batt insulation with a vapour barrier either on the inside, or against the concrete. You need to use an impermeable insulation like foam board against the concrete to stop interior moisture form getting to the cold surface of the foundstion wall. Once that's in you can use batts inside that, but no interior vapour barrier.

        1. GROUNDUP | | #9

          Ok thank you. Do you think my basement dehumidifer will help stop issues from this? It's u fortunate because it's a completely finished basement that's sounds like it needs to be gutted and redone.

          1. Expert Member
            MALCOLM TAYLOR | | #10


            Sometimes it works. It was the standard detail for a couple of decades. People stopped using it because enough basement walls experienced moisture or mold problems that it seemed like a better solution should be found. But not all the houses built that way have problems. I'd be inclined to just keep an eye on it and see how it does before committing to ripping everything out.

  4. the74impala | | #11

    Do you have any exterior foam insulation? My 1993 house in zone 7 had paper backed fiberglass, spf bottom plates, and drywall. After gutting it,, he only areas that had problems were the bulk water intrusion areas. I am not putting it back that way, but I would not gut it unless you have other significant problems.

    1. GROUNDUP | | #12

      Thank you. We have had other problems but this potential one came to my attention recently. We have insulation on most of the exterior where the roxul batts are. The wall with the fiberglass is actually an exterior /Interior wall. It's a foundation wall but there is a slab on grade garage adjacent if that makes sense. I used glas roc instead of sheetrock there so hopefully that helps. I'll cut open a section and check

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