How elastomeric is mastic? Is it flexible if a joint moves or would it need to crack in order to yield to movement?
Seems foam isn't elastomeric at all either. I used it on a PVC plumbing penetration through a wood top plate in deep winter and when the pipe expanded and contracted over the season it separated. I had some remaining flexible silicone caulk from a different project so I covered the gap, not sure if will be permanently sealed or not. Either way even with it cracked, it still will be better than the fiberglass filled 1/2" gap it used to be.
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