Volcanic tuff was added as a filler (up to 40% by mass) to raw materials used for prepg. of rigid polyurethane-polyisocyanurate foams. The foams were studied for apparent d., brittleness, compressive strength, stability of linear dimensions, change of vol., mass loss, combustion residue, 02 index, absorbability of water, Vicat softening point, thermal properties and it spectra. Addn. of the tuff resulted in a change of foam cell form, an increase in their mech. strength, heat resistance and thermal cond. and in a decrease in the foam inflammability.