Abstract: A composite interlocking siding panel includes an elongate sheet metal or vinyl outer panel having a nailing strip along one longitudinal edge and mating interlocking means extending along each longitudinal edge. An elongate boardlike member of insulating material is bonded to the back of the outer panel. The insulating member is so configured that it may be of any selected thickness without interfering with the installation of the composite panel on a building wall. The insulating member includes a projection extending along one longitudinal edge which projects beyond the nailing strip and is configured, when installed, to snugly fit within an undercut recess in the mating longitudinal edge of a like interlocked panel with the rear surfaces of the insulating members lying in a common plane in their finally installed position.