Having been through a bus fire --- I think you might be better off to spend
the money on an automatic fire extinquisher system. Once the magnesium
ignites, it is tough to extinquish. The fire dept. put 8000 gal of water on my
bus.  The problem with adding insulation is that you might be increasing the
risk of fire if you decrease the already minimal air space over the engine,
this may increase the temperatures.  Also, as we all know, adding weight will
slow the bus down.