Abstract: When producing insulating units, in particular building units, plaster of Paris was previously poured into the mold, bloated polystyrene pearls being prekneaded into said plaster. Plaster of Paris has also been poured into molds, wherein a lining of an aggregate of fused, bloated polystyrene pearls were prearranged. The bloated polystyrene pearls according to the invention are first densely packed in the mold, whereupon plaster of Paris, cement or another hydraulic binding material is filled into the mold through a net or a perforated plate, said net or perforated plate keeping the pearls densely packed during the casting so as to ensure that they do not reposition as a consequence of their buoyancy in the cast, liquid binding material.