Using Polyiso to Insulate Top Floor Ceiling -- What about roof leaks?

I'm redoing a bathroom on the top floor of my house. It's in a dormer, so the other side of the bathroom ceiling is the roof. If I insulate the ceiling with polyisocyanurate, and given that this material is closed cell, I'm thinking this will be a problem in that I won't be able to notice roof leaks, as the water will be diverted elsewhere into the house. Maybe this is a silly concern, but in fact, I'm redoing the bathroom because of a roof leak that caved in the ceiling, so it's top of mind right now.

Question: should I avoid using polyisocyanurate to insulate an exterior facing ceiling because it'll prevent me from noticing leaks sooner than later?