Abstract: The invention relates to a structural element for the manufacture of casings, wall disks, boxes and such articles comprising a shell made of thermosetting plastic and preferably reinforced to give it shape stability by reinforcement members which, more particularly, are formed by reinforcement grids composed of iron bars welded to one another and are fully enclosed by the plastic. On account of the enclosed air chambers, these structural elements have good thermal insulation properties. They are exceptionally well suited, for example, for greenhouses, plant boxes and the like.