Fiberglass, at least what that bus manufacturers use, is crap. It retains water, dirt, mold, etc. all of which contribute to corrosion from the inside, while not really retaining heat. If you look at some of the other shots, particularly where Marge is at work pulling the interior rear cap off, you can see that in the insulation. I want it out of the bus, replaced with rigid foamboard so that heating and cooling costs are minimal. Plus, I want to get at the seams and recaulk them just to be on the safe side, and then prime and spray with undercoating, the interior of the roof for a watertight fit.