thimble clearance

my thimble has about 1.5 inches from the outside of the insulated vent pipe to the round sheetmetal wall. They say go 3 inches to insulation.
does that mean that I cant put fiberglass insulation around the sheetmetal round... I have to go an additional 1.5 inches clear around the outer ring..??
Does Home Depot sell a TOTALLY non combustible insulation that can be formed around round objects..??thanks

What is the brand and model of the wall thimble and the pipe that you are using?

Eric

duravent big E breckwell I think from the instructions that you can wrap the round in fiberglass.... that gives you a 1.5 inch air clearance and the pipe itself doesnt even get that hot