An insulated, one-side finished, building panel light enough to be handled by no more than two people during installation. The panel has an inner, relatively thick, insulation base and an outer, relatively thin, finish layer. The finish layer comprises a stucco-like material securely adhered to one surface of the insulation base. The panel has outer and inner surfaces and top and bottom edges, with the outer surface of the panel formed by the outer surface of the finish layer. The top and bottom sides of the finish layer form part of the top and bottom edges of the panel. The siding panel is provided with interlocking means for securely interlocking the top and bottom edges of adjacent siding panels together. The outer surface of the panel has finished groove forming sections at both the top and bottom edges. The groove forming sections of adjacent panels form a finished groove with the joint between the panels in the groove.