Years ago I remember seeing a ceiling material that was basically a type of foam rubber. It had a textured sort of surface, that had a semi-glossy white painted surface, and was designed to be glued to the ceiling. I believe I saw it used in mobile homes. Seems to me it would make a light, good insulating material for the ceiling of a TTT and would eliminate the need for a wood ceiling layer, and thereby reduce weight and cost.
Anyone remember the stuff and possibly if and where it is available?