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Community and Q&A

Safe to put foil-faced polyiso insulation on basement wall without a capillary break on footing?

BuildingAHome | Posted in Energy Efficiency and Durability on

I have footings that were installed without a capillary break to the walls. I was planning on putting 1″ of foil-faced polyiso against the walls followed by a stud-bay with rockwool inside for my finished basement based on the advice I have seen on this site. Do I need to worry about the foam absorbing moisture from the concrete due to the lack of capillary break? Was then thinking well maybe I could put polyethylene against the concrete before the foam is up, but I see that Martin recommends against that. Any advice here? I can’t find any information about how much moisture potentially will actually get wicked through the footing to the walls. Thanks!

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  1. Expert Member


    Outside the rarefied world of GBA, capillary breaks are a very niche technique. Neither the widely used assembly you are suggesting, or Martin's advice, are contingent there being a capillary break below the stem-walls to work, and I don't think the absence of a capillary break necessitates adding poly to the wall.

  2. Expert Member
    BILL WICHERS | | #2

    The foil facer on the polyiso is already a vapor barrier, and will act to protect the foam. I don't think you have anything to worry about there.

    If you wall is consistently dry (as in you never see dark areas where the wall is damp), then you're probably reasonably safe without a capillary break at the top of the wall under the rim joist, but adding a vapor barrier against the wall does increase the risk of moisture issues in the rim joist area somewhat. I think that risk is pretty low though if you've always had a dry wall.

    I'd put all your R value in the polyiso and skip the rockwool in the studwall.


    1. BuildingAHome | | #3

      Thank you for the response! It is under construction, we just missed the capillary break detail. The TUFF n DRI system + 2 5/8" exterior drain board is being used so hopefully this properly waterproofs the walls. Follow up question, since the polyiso will be a vapor barrier, why is that okay but putting polyethylene sheeting against the concrete (with foam over it) not okay as I have read on this site?

      1. Expert Member
        MALCOLM TAYLOR | | #4


        The presence of moisture and mold on basement walls depends on air being able to move from the interior to the concrete. That's why the recommendation for foam, and that the foam needs to be carefully sealed at the joints and perimeter. I suspect that if you could figure out a way to seal the perimeter of the poly, and also the foam over it, that the poly would not cause any problems.

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