A core element is provided, having the shape of a block with a first surface, a second surface situated opposite the first surface, mutually facing longitudinal side faces, and mutually facing transverse side faces. The first surface is provided with the first groove that is a continuous groove which extends parallel to the longitudinal side faces. The longitudinal side faces are provided with the side wall grooves, the side wall grooves being continuous side wall grooves which extend between the first surface and the second surface and are separated from the first groove in the first surface.