I'm just beginning a basement finishing project, so I expect this is my first of many posts.

Right now I'm wondering about whether it would be worthwhile to add insulation to my rim joists. I live in Minnesota and the rim joists that I have are an engineered product that has a layer of foam insulation in the middle--rated R-8. My question is whether I can get away with just throwing in some extra fiberglass batting. I know this is generally not a good idea for the rim joist area, but I thought since there's already foam insulation that that would mean that I already have a pretty good air seal in place. Would the warning against using batting in this scenario still apply? I want to use batting because it seems like it would be quite a bit easier than the alternatives (except the alternative of doing nothing).

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    I my experience, it's mostly a question of whether you have plumbing in an outside wall, where burying it in insulation (so it's not warmed by the basement air) might let it get cold enough to freeze. If the builder or architect kept the plumbing to inside walls, I don't see a problem with adding more insulation (other than it being potential rodent habitat.) – Ecnerwal Jun 4 at 3:30
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    @Ecnerwal That is good for an answer, the only thing I would add is do not use batts with a vapor barrier. That would potentially trap moisture between the foam and vapor barrier. I feel it would only be a small chance that would happen, but I am following the rule of no insulation between 2 vapor barriers. – Jack Jun 4 at 4:18

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