I want something that will shrink against the pipe/seam and make an
airtight seal.
Sounds good, but that might make it almost impossible to later take
the pipes apart as the inevitable day arrives when they need to be
replaced. Unless you have some expensive ones that don't rust.
I am far from a metallurgist but it seems to me that metal expands
when it heats. What you desire may be alchemy. Do you own a pointy hat?
Anyone price the double- or triple-walled pipes recently?
My foggy upstairs is thinking that five or seven small self-tapping
screws spaced around the joint would keep it pretty tight.