According to zyberengel :
Have you tried Aluminium foil its cheaper and more practical
fire resistant and easier to install it is something you could buy from
a grocery store

And has _zero_ insulation value. Except inosofar as it may seal
air leaks. He's trying to insulate it, not improve its fire resistance.

R means retardant

R is insulation factor. Not fire retarding rating.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.