I did the 40mm poystyrene on top of my bus floor as the underside is kinda cluttered, but just layered another ply layer on top... works well for me.
Id say its cheap enough that if a couple bits broke off over time (more likely ground/sticks scraping than wind in my opinion) you'd easily/cheaply be able to replace them, and only a hard layer like ply would protect them from the bottom anyway?
Fire is only a hazard if you cover a heating element like the exhaust or bad wiring anyway, so silver foil wouldn't help there either?
8.5m 1990 Dennis Dart bus, 6L turbo -4 pax
- 24v system: 2x 104ah AGM, 20a PWM controller 500W solar; 1200w puresine inverter
120L grey/fresh water, 75L compressor fridge, gas Califont, Diesel MK85 heater [awaiting install]