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: 100ah LifePo4 @24v +130ah AGM(2x12v), 40A Traceran controller, 780W solar; 1200w & 300W puresine inverters
120L grey/fresh water, 255L Samsung fridge, LPG Califont, 5KW Chinese Diesel Heater