> I am going to insulate the upper ,"corrigated" portion of the exhaust vent 
> .I think I can do this by packing in rock wool(if I could find it) or 
> fiberglass insulation like found in houses.

Fiberglass ought to be fine. "Rock wool" is probably overkill. Fiberglass 
(being made out of glass fibers) won't burn-- just be sure there's no 
backing material like paper or something attached.