Fraze,
If you google (re-insulating Hardy) You will find a post on Outdoor Wood Furnace Info that has Bull's post about using the Iso Insulation board. I read were another guy used 2" thick Tank Wrap for his OWB and said that it is cool to the touch. The Tank Wrap is made of Mineral Wool so it can withstand high temps. I'm going to check with some local insulation companies and tell them what I am looking to insulate and see if they can offer anything. My local Hardy dealer said it was $150 for the H2 kit and probably $100 in shipping.