I'm planning on sealing/insulating my rim joists with a combination of extruded foam boards and one-part SPF (likely great-stuff fireblock or similar). My questions are two-fold:
- I plan on making fairly snug XPS blocks and sealing the perimeter of each block with either caulking or a foam bead. Foam is obviously a bit quicker. Has anyone had issues with this system and had to use caulking due to seasonal expansion/contraction (I'm in a moderate climate)?
- I'd love to use polyiso for its fire-rating, but can't source the 2" anywhere near me. Can I use a skim (1/2") of polyiso on top of the XPS and get similar fire resistance or does a fire-rated spray covering make more sense. The basement is partially finished, hence the desire for fire rating.

Dan