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Community and Q&A

Band joist insulation (first floor ceiling / second floor floor)

kyva1929 | Posted in Energy Efficiency and Durability on


I know that the best approach to insulate band joist would be using spray foam / foam board + spray foam sealing the perimeter. However, the builder has already put in fiberglass insulation along the band/rim without air sealing – would this cause condensation problem? The building is in zone 5.

Thank you for your help.

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Would this cause a condensation problem?"

    A. It might. However, for a variety of reasons, the rim joist between the first and second floors is much less susceptible to moisture problems than a rim joist in the basement, so you probably don't have to worry.

    The main reasons that basement rim joists are the most problematic are the following:

    (a) Basements tend to be more damp than upstairs rooms, and

    (b) When fiberglass insulation is installed near basement rim joists, the insulation is often left exposed, while the fiberglass insulation installed on other floors is usually behind drywall.

    That said, if I were building a new house, I would install rigid foam or mineral wool on the exterior side of the wall sheathing. This solves the "cold rim joist" problem.

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