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Community and Q&A

Sill/band-joist: open or closed cell?

user-1135248 | Posted in Green Building Techniques on

I’m planning a DER that includes external polyiso installed down to
the foundation wall, thus covering the sill and band-joist. The
contractor has specified closed-cell sprayfoam on the *inside*
of the sill and joist area to help air-seal and further insulate
that area. The sill is directly on top of the cinderblock wall
without any capillary break that I can discern; it’s sufficiently
above grade that most moisture problems that might create
don’t seem to be an issue. However, I’m concerned that since
there is invariably a little bit of moisture drive upward from
the block wall into the sill, that entombing the whole area
in closed-cell sprayfoam on the inside and the foil-face polyiso
on the outside would create a moisture trap. We’re not jacking
up the house to insert a capillary break, so I’ve asked to change
the sprayfoam to open-cell like Icynene or whatever, thinking
that the higher vapor-permeability thereof will allow much better
drying toward the inside where I have better control of the
humidity anyway.

I tried bouncing this off the BSC folks but got no reply to the email.


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  1. GBA Editor
    Martin Holladay | | #1

    I understand the logic of your decision to switch to open-cell foam, and it can't hurt to go that way. However, I'm not sure whether the would be enough drying potential through the Icynene to make much of a difference. This is one of those areas where the final decision is a judgment call.

    Regardless of what type of foam you use, it wouldn't hurt to poke an awl through the foam every 5 years to be sure that your rim joist isn't getting punky.

  2. user-1135248 | | #2

    That's interesting, thanks. It begs another question, which
    would be ... are there good strategies for sprayfoam installations
    for periodically checking up on what's underneath them? You
    can't exactly pull aside a batt to look. While an "icepick
    test" might leave a minimal breach of the foam layer behind,
    how many little holes could that add up to taken over many areas
    and several rounds of testing. Or could there be some clever
    way to build an "inspection hatch" sort of setup in key places
    during foaming? Even just a carefully cut wedge through it,
    down to the substrate surface but which could be removed and
    replaced to maintain the seal, might be something to think about.


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