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Insulating second floor band joists from above

user-671582 | Posted in Green Building Techniques on

A another question led to discussion of band joist insulation, but I have more specific questions that I think warrant a new topic.

I’d like to insulate the second floor band joist in our 1980’s colonial house. We’re replace the upstairs floors and I think it makes the most sense to insulate from the inside, upstairs floor (as opposed to from the outside or first floor ceiling).

That leaves two options: dense pack cellulose using the ‘grain bag’ method from holes cut in the floor, or a foam approach enabled by cutting away the subfloor near the perimeter.

The cellulose approach seems straightforward, but I worry about insulating that area without air sealing. I understand that cellulose resists airflow, but isn’t a strict air barrier. The ‘How to Install Cellulose Insulation’ article on this site suggests that insulating the rim joist with cellulose is routine, but it doesn’t address the air sealing issue. Is this a risky practice?

Insulating with foam addresses the air sealing issue, but is more labor intensive. Does cutting the subfloor around the perimeter of the house weaken the structure at all? I was thinking of cutting a 2-foot wide strip (or 16″ when parallel to the joists) so that the cut portion isn’t too narrow and doesn’t flex. Is there a better method or any pitfall to this idea?

Thanks.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Jeffrey,
    Done properly, either approach will work.

    I imagine that the spray foam approach would do a slightly better job of reducing air infiltration. But the difference between the two methods is relatively small, so you should probably make your decision based on factors other than thermal performance.

    Cutting back the subfloor and then patching the subfloor later won't cause any structural problems.

  2. user-671582 | | #2

    Thanks, Martin. The foam approach is slightly more diy friendly, but I'll compare costs with professional dense packing.

  3. Expert Member
    MALCOLM TAYLOR | | #3

    Jeffrey,
    A small caveat to Martin's advice: Cutting back the subfloor where the joists are perpendicular to the exterior wall is fairly straightforward. Once the foam is installed, you simply block at both edges of the cut and replace the plywood. Cutting where the joists are parallel is a bit more involved. The plywood is helping to support the portion of the bottom plates that is not directly under the rim joists. You can't effectively provide support for the removed subfloor by blocking, as there is nothing solid to attach the blocking to.
    Further, unless your house as built on a 16" module, there is a good chance that the last joist will fall close to the exterior wall. Perhaps too close to give access to the rim joist to install the foam.
    It's not a job I'd look forward to.

  4. GBA Editor
    Martin Holladay | | #4

    Malcolm,
    Excellent points. Thanks for modifying my advice.

  5. user-671582 | | #5

    Malcolm,

    Thanks for the very detailed description of what's involved - that's exactly what I was looking for. I think your insight tips the scales in favor of a dense-pack cellulose approach. It sounds like spending extra time and effort on air sealing other parts of the assembly is a better use of time.

    Thanks again!

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