A high efficiency air filter assembly (100) for high temperature applications including: a filter core (102) including a unitary sheet of air-permeable filter media folded in accordion fashion to form a plurality of side-by-side pleats with zig-zag edges on two opposite sides and flat panels on the other two sides, and a spacer member between opposing walls of successive pleats, a box-like metal frame (108) enclosing said filter core on four sides in air tight engagement to define an air flow opening extending between upstream and downstream sides of said assembly, wherein the zig-zag edges of the unitary sheet of air-permeable filter media are sealingly embedded in a solid inorganic sealant composition (118) in the frame, and wherein the unitary sheet of air-permeable filter media preferably consists of a binder-free glass or mineral fiber filter media. Use of the high efficiency air filter assembly in high temperature applications wherein the filter assembly is repeatedly or continuously heated to a temperature in the range of 200-500° C.