Does this insulation need to be clear of the furnace exhaust vent?
In this state double or single wall pipe must be at least 4 inches from combustible.
i believe the standards are 1" clearance for B vent and 6" for single wall vent.
of course, as Charley pointed out, state or local requirements may reign supreme.
CC has it right for the minimum clearance from combustables. 1" for B vent, 6" for C vent (single wall), BUT… the clearance for single wall CAN be reduced to 1" if a heat shield (sheet metal) is installed between the vent and the combustable surface. But to be sure, check local regulations.