A sleeve system for mounting onto an opening in a wall, comprising: an elongated, generally cylindrical open-ended sleeve with an outer surface and an inner surface and an outer diameter; first and second mounting plate parts which each comprise a base and which mate together and secure to each other to form a mounting plate; wherein the mounting plate formed by the mated and secured mounting plate parts comprises: a mounting plate base formed by mating together the base of the first mounting plate part and the base of the second mounting plate part; an annular collar which extends from the base of the mounting plate and defines an opening in the mounting plate, the annular collar being arranged so that when the first and second mounting plate parts are mated together and secured to each other, at least portions of the annular collar press against portions of the outer surface of the sleeve and thereby securely hold the sleeve in the opening of the mounting plate; and, wherein each mounting plate part comprises at least one tongue arranged so that when the mounting plate parts are mated together the at least one tongue of each mounting plate part is in overlapping relation with at least a portion of the mounting plate base that is provided by the base of the other mounting plate part, such that the tongue is in overlapping relation with at least a portion of a seam between the mated bases of the first and second mounting plate parts and so that the tongue obstructs a pathway through the mounting plate base through which gases might penetrate; and wherein each mounting plate part further comprises at least one fastening tab that protrudes outwardly from the base of the mounting plate part, each fastening tab being arranged to be secured to a fastening tab of the other mounting plate part so as to secure the pair of mounting plate parts together to form the mounting plate and each fastening tab being located radially outward from each tongue when the pair of mounting plates are secured together. an elongated, generally cylindrical open-ended sleeve with an outer surface and an inner surface and an outer diameter; first and second mounting plate parts which each comprise a base and which mate together and secure to each other to form a mounting plate; wherein the mounting plate formed by the mated and secured mounting plate parts comprises: a mounting plate base formed by mating together the base of the first mounting plate part and the base of the second mounting plate part; an annular collar which extends from the base of the mounting plate and defines an opening in the mounting plate, the annular collar being arranged so that when the first and second mounting plate parts are mated together and secured to each other, at least portions of the annular collar press against portions of the outer surface of the sleeve and thereby securely hold the sleeve in the opening of the mounting plate; and, wherein each mounting plate part comprises at least one tongue arranged so that when the mounting plate parts are mated together the at least one tongue of each mounting plate part is in overlapping relation with at least a portion of the mounting plate base that is provided by the base of the other mounting plate part, such that the tongue is in overlapping relation with at least a portion of a seam between the mated bases of the first and second mounting plate parts and so that the tongue obstructs a pathway through the mounting plate base through which gases might penetrate; and wherein each mounting plate part further comprises at least one fastening tab that protrudes outwardly from the base of the mounting plate part, each fastening tab being arranged to be secured to a fastening tab of the other mounting plate part so as to secure the pair of mounting plate parts together to form the mounting plate and each fastening tab being located radially outward from each tongue when the pair of mounting plates are secured together. a mounting plate base formed by mating together the base of the first mounting plate part and the base of the second mounting plate part; an annular collar which extends from the base of the mounting plate and defines an opening in the mounting plate, the annular collar being arranged so that when the first and second mounting plate parts are mated together and secured to each other, at least portions of the annular collar press against portions of the outer surface of the sleeve and thereby securely hold the sleeve in the opening of the mounting plate;