A thermally insulating lining for the housing of a heater system for a Stirling, or hot air engine ensures that only safe temperature will be reached on the outside of the heater system. The insulating lining is arranged in two parts. An insulating wall is made up of individually adjacentely placed, replaceable insulating elements in the form of ceramic tiles, and/or ceramic tubes, and an intermediate space between the tiles and the outer wall. The intermediate space is separated, if desired, from the insulating elements by a ceramic paper. The intermediate space is packed with ceramic fiber batts, or loose, lump ceramic insulating material.