I have my furnace exhaust coming up from the basement to my attic.
Currently the B-vent comes into my attic floor from the second floor
ceiling with a good-sized triangular cutout around it, sorta like this,
looking from the top:

(If you aren't viewing in a fixed font, this won't make sense,
picture a triangle with a circle inside)
/\
/ \
/ \
/ ___ \
/ / \ \
/ ; ; \
/ \___/ \
--------------

A significant draft comes out this triangle into my unheated attic.
I'd like to seal it up but am unsure how to proceed.

If memory serves, no flammable material can be within 1" of a B-vent.
So, could any nonflammable insulation be used to cover the hole and
butt up against the pipe? Are there any other issues?


Thanks,

Max