I am currently insulating my rim joists with rigid foam board, then spraying great stuff around the edges. However, there is one rim joist that will be too difficult to get the foam board into because of a steel I-beam. Would it be a bad idea to spray just this one rim joist with a lot of great stuff?

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Only if you get the foam everywhere but where you want it.

If you can't see the location very well, in addition to being difficult to reach, you may not get a "perfect" application of foam, but an imperfect application will still give you more insulation than no application at all.

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