Abstract: An extruded, modular panel unit, made of at least two sheet-like major surfaces inter-connected and spaced apart by a plurality of ribs dividing the space delimited by the major surfaces into a plurality of subspaces, and at least two pairs of spaced-apart, ridge-like joining projections extending in direction of extrusion, the two pairs of projections being located at one edge of the panel units. The joining projections rise from an outside face of one and the same major surface and point in the same direction, and in assembly the outside surfaces of the outer projections of the pairs of projections are in substantially direct contact with similar surfaces of coplanarly adjacent panels.