Cheap insulation. Is there such a thing?

I think we have hit the "Magic Triangle".
Pick any two. Can't have all three.



If it has to be fire resistant, that rules out cardboard and scrap bubble wrap, etc.

The only thing that comes to mind is foil backed poly.
It's only R 4

Which reminds me to go to Lowes Depot this AM and get a fire bottle for the car.