Abstract: A hoarding assembly having a non-combustible innermost portion is sealed against the cladding of a hot surface, such as that of a large boiler, to create an envelope. Outside air may enter the otherwise airtight envelope through controlled inlets in the hoarding assembly. The boiler is insulated with a layer of asbestos that is to be replaced. A vertically extending first array of supply air inlets is provided in one end wall of the hoarding assembly and a vertically extending second array of suction outlets is provided in the other end wall. A fan assembly injects cool air through the first array and a negative air machine assembly withdraws air through the second array. The placement of the inlets and outlets results in a wall of air moving parallel to the boiler wall and having its centerline spaced from the cladding, typically about 1 to 3 feet from it. With the foregoing operating, the sheet metal cladding is removed from the boiler. A narrow vertical strip of the asbestos is then removed.