A prefabricated insulated building panel comprising: a sheet of rigid thermally insulating material having opposite first and second sides and opposite first and second ends collectively delimiting a first face and a second face of the sheet facing in opposite directions and collectively defining a periphery of the sheet of rigid insulating material; an inner structural layer connected to the first face of the rigid thermally insulating material for carrying load exerted on the panel; the rigid thermally insulating material defining in the second face thereof a plurality of grooves each having a base recessed from the second face of the rigid thermally insulating material; the grooves each extending from a location on the second face of the rigid thermally insulating material to the periphery of the sheet so as to be open at an end of the respective groove which terminates at the periphery of the sheet; composite cementitious material bonded to the second face of the rigid thermally insulating material to provide a cured cementitious outer layer with a thickness measured from the second face of the rigid thermally insulating material to an outer face of the outer layer such that the cured cementitious layer is supported at the second face of the rigid insulating material by bonding action with the rigid thermally insulating material; characterised in that the composite cementitious material covering the grooves so as to define circumferentially enclosed channels which are closed opposite the bases of the grooves to define paths for fluid flow from locations within the periphery of the panel to an outside of the panel; and the composite cementitious material wrapping about outer edges of the grooves formed between the second face of the rigid thermally insulating material and sidewalls of the grooves which extend from the second face to the respective base such that the composite cementitious material extends into the grooves so that the channels each are collectively defined by the composite cementitious material spanning from one of the sidewalls of the respective groove to the other, the base of the groove, and a portion of each one of the sidewalls of the groove. a sheet of rigid thermally insulating material having opposite first and second sides and opposite first and second ends collectively delimiting a first face and a second face of the sheet facing in opposite directions and collectively defining a periphery of the sheet of rigid insulating material; an inner structural layer connected to the first face of the rigid thermally insulating material for carrying load exerted on the panel; the rigid thermally insulating material defining in the second face thereof a plurality of grooves each having a base recessed from the second face of the rigid thermally insulating material; the grooves each extending from a location on the second face of the rigid thermally insulating material to the periphery of the sheet so as to be open at an end of the respective groove which terminates at the periphery of the sheet; composite cementitious material bonded to the second face of the rigid thermally insulating material to provide a cured cementitious outer layer with a thickness measured from the second face of the rigid thermally insulating material to an outer face of the outer layer such that the cured cementitious layer is supported at the second face of the rigid insulating material by bonding action with the rigid thermally insulating material; characterised in that the composite cementitious material covering the grooves so as to define circumferentially enclosed channels which are closed opposite the bases of the grooves to define paths for fluid flow from locations within the periphery of the panel to an outside of the panel; and the composite cementitious material wrapping about outer edges of the grooves formed between the second face of the rigid thermally insulating material and sidewalls of the grooves which extend from the second face to the respective base such that the composite cementitious material extends into the grooves so that the channels each are collectively defined by the composite cementitious material spanning from one of the sidewalls of the respective groove to the other, the base of the groove, and a portion of each one of the sidewalls of the groove.