HIGH insulation value per inch for ceiling or walls = 26
PolyIso insulation 4" thick 4x8 of Polyisocyanurate foam insulation board-- nearly impossible to acquire this material at low volume. Mostly have to buy in pallet volumes.
Only 3 sheets still available of PolyIso at $60/ sheet, 4 ft x 8 ft x 4 inches thick
I can instruct in best cutting methods for thick PolyIso. . .