Abstract: Thermally insulating panels are formed at opposite sides with finishing edge strips about which sheet metal facings of the panels engage, the strips having ribs and grooves to allow mortise and tenon interconnection of the panels. A fold between a groove and a rib of the respective panel receives a metal insert which provides structural support and interengaging flanges of the insert, which preferably is composed of high density PVC, can interlock for effective sealing.