Don't use expandable foam on a flue pipe, it is a fire hazard. Most
rigid fiberglass pipe insulation is good for 800 degrees F. (according
to the data sheets the last time I bought some). There is other pipe
insulation that can go higher in temperature, but why pay for that?
Typically your flue gas temperature will not be over about 300 degrees
F. It depends on how much heat is extracted in the furnace and how
much house air is mixed in at the draft diverter if the furnace has
one. Using sheet metal to block the gap between the flue pipe and the
nearest wood will help block air moving from the house into the attic.
You could also install "B" vent for a gas appliance. It typically does
not need clearance between the flue and adjacent combustible materials.
The B-vent in insulated flue pipe.

Stretch