We have a 1954 brick chimney that has a metal liner inside and is only used by the natural gas hot water tank. It's had fiberglass right up against if for guessing 20+ years.
I'm air sealing the attic, it comes right up and through the attic with a couple inches gap between the framing and the brick.
I've read online about using sheet metal and high temperature caulk to close this air gap, and to keep all insulation away except (some say) rock wool.
Is this really necessary and/or required in my situation? This isn't a wood burning chimney, it only exhausts 1 50 gallon hot water tank through a metal flue inside the brick.
I'd like to just put rigid foam and stick it in place with Great Stuff and then insulate right up to the chimney. Most of my google searches have led me to info on insulating between the metal flue and the chimney, that's not what I'm looking for.