Abstract: A building block including at least one internal cavity is insulated by molding an insulating layer from expandable thermoplastic polymer particles, such as beaded polystyrene, to one or more interior surfaces of the cavity. The molding operation is carried out by positioning a male plug member within the internal cavity of the building block with at least one peripheral surface of the plug member spaced from at least one interior surface of the cavity to provide a peripheral compartment into which the polymer particles are directed for molding the layer. Thereafter, steam, under pressure, is injected into the peripheral compartment to cause the particles to expand and fuse together into an efficient insulating layer that is expanded into close conformity with the interior surface(s) of the cavity. During the steam injecting operation a positive inwardly directed external force is applied to external surfaces of at least two opposed walls of the block to counteract internal forces generated by the steam.