kmead
Old enough to know better
I beg to differ, when offered as insulation it doesn’t hold water as it is hydrophobic. In this application the binder ensures it won’t absorb and hold water.I wouldnt use rock wool since it can hold tremendeus of water (you use it in hydroponic growing as well, just because it adsorbs water)
Some heat resistant closed cell insulation is the best I think.
The underside would be better served by one of the newer insulation packages developed for modern cars versus any fibrous insulation in my mind.