A method for manufacturing an opening panel of a door, for a door intended to equip an opening in a building between an exterior volume and an interior volume, said opening panel comprising: - a peripheral frame formed of two lateral posts (51, 52), a bottom transom and a top transom, said posts (51, 52) and transoms (53, 54) each comprising an external side face (51a, 52a, 53a, 54a), an internal side face (51b, 52b, 53b, 54b), an internal front face (51c, 52c, 53c, 54c) intended to be directed toward said interior volume of the building and an external front face (51d, 52d, 53d, 54d) intended to be directed towards said exterior volume of the building, said internal side faces (51b, 52b, 53b, 54b) of said posts (51, 52) and transoms (53, 54) forming together an internal edge that delimits a central housing, - an added insulating structure filling at least partially said central housing, - an internal facing closing said central housing on the side intended to be directed toward said interior volume of the building, - an external facing closing said central housing on the side intended to be directed toward said exterior volume of the building, characterized in that said manufacturing method comprises the operations consisting in: (a) preparing said peripheral frame, (b) providing a press-type installation comprising a horizontal press support and a press cover, (c) placing said peripheral frame on said horizontal press support, said central housing being closed on the side of said horizontal press support, and comprising an upper housing opening, (d) applying a liquid or semi-liquid mixture of insulating expandable foaming material into said central housing, through said upper housing opening, (e) operating said press cover so as to apply it against said peripheral frame and to close said upper housing opening, (f) allowing the insulating expandable foaming material to expand in said central housing, (g) after full expansion of said insulating expandable foaming material, operating said press cover to separate it from said peripheral frame, (h) extracting the peripheral frame /insulating foam assembly from said press. - a peripheral frame formed of two lateral posts (51, 52), a bottom transom and a top transom, - an added insulating structure filling at least partially said central housing, - an internal facing closing said central housing on the side intended to be directed toward said interior volume of the building, - an external facing closing said central housing on the side intended to be directed toward said exterior volume of the building, (a) preparing said peripheral frame, (b) providing a press-type installation comprising a horizontal press support and a press cover, (c) placing said peripheral frame on said horizontal press support, said central housing being closed on the side of said horizontal press support, and comprising an upper housing opening, (d) applying a liquid or semi-liquid mixture of insulating expandable foaming material into said central housing, through said upper housing opening, (e) operating said press cover so as to apply it against said peripheral frame and to close said upper housing opening, (f) allowing the insulating expandable foaming material to expand in said central housing, (g) after full expansion of said insulating expandable foaming material, operating said press cover to separate it from said peripheral frame, (h) extracting the peripheral frame /insulating foam assembly from said press.