Process of making lightweight, rigid polyurethane foam

In a process of generating a polyurethane foam by forming a mixture comprising isocyanate and polyol reactants, catalyst, and blowing agent, which mixture reacts exothermically to yield a rigid polyurethane foam, the IMPROVEMENT wherein there is included in the mixture expandable polystyrene beads made of a polystyrene that has a softening point at or below the maximum temperature reached during the exothermic reaction, whereby the heat of the polyurethane-forming reaction causes the beads to expand and sinter, while trapped in a foaming matrix of polyurethane, creating gas-filled pockets in the foam that are lined with the polystyrene of which the beads are formed.