Brian, I don't recall any single engined GA aircraft I've ever flown or worked on having thermal or fire insulation on the cabin side of the firewall. I don't believe that's what the FAA expects, nor is it necessary IMHO. If you have fire coming past the f/wall, you have bigger problems than insulation. That, to me, is where a handy fire extinguisher comes in. At least you have a fighting chance then. FWIW.