What is disclosed is a method for making a gastight multiple-walled panel of extruded resin from an integrally extruded hollow panel comprising two parallel, plane outer walls having supporting partitions disposed between them and defining long hollow cells with open end faces, which method comprises heating the hollow panel at least in the area of the open end faces of said hollow cells until the resin is in the thermoplastic state, and then heat sealing the two outer walls in the thermoplastic area thereof in a gastight manner either to each other or to a tape of a further thermoplastic material covering the end faces, the seal so produced being held for at least 1 minute at a temperature ranging from 10.degree. C. beneath the Vicat softening point to 20.degree. C. above the Vicat softening point of the resin.