Abstract: An improved roof insulation system and method is disclosed for a building having a sidewall and an upper supporting surface. The insulation system includes a first substantially rigid insulation disposed about the periphery of the upper surface of the building. A second flexible insulation is disposed within the central area defined by the first rigid insulation thereby insulating the entire upper supporting surface of the building. A flexible waterproof covering is located upon the first and second insulations with the first insulation supporting the periphery of the waterproof covering and with the second insulation supporting the central area of the waterproof covering. Flashing is secured by mechanical fasteners to seal the terminal ends of the waterproof covering to the building about the periphery of the upper supporting surfaces thereby producing a waterproof insulating roof system.