Insulated with Dows Thermax hardboard insulation. It is composed as the same foam sprayed out of the big rigs but its UNIFORMLY 1,55 inches so you get exactly what you pay for. It also comes with both sides with radiant barrier to block the UV rays, And it is fire resistant. I personally through a piece into a bonfire and the aluminum radiant barrier melted off but the board never burned up….AT ALL. It was the only thing left after the fire!