Abstract: An insulated furnace, method of manufacture and purging operation which includes a plurality of interconnected modular panels of fibrous material forming a furnace chamber, a heat source for the furnace chamber and a furnace shell surrounding the fibrous material panels so as to form a space between the shell and the fibrous material panels and to provide a thermal break of greater than 10.degree. F. such that the furnace can be rapidly heated and water vapor and air can be rapidly purged from the furnace. An insulation panel is formed of at least one block of fibrous material in a molded form which includes mixed fibers and a frame member to which the at least one block of material is secured.