Rigid polyurethane (PU) foams were modified by addn. of expanded graphite (plate filler) and cenospheres (spherical filler) to study the effect of the fillers shape and content on the cell structure and apparent d., compressive strength, friability, H2O absorption, coeff. of thermal cond. and dimensional stability of the resulting porous material. The highest cell nucleation capability was obsd. for the spherical filler introduced into the PU system up to 5% by mass and for the plate filler introduced above 7% by mass. The addn. of expandable graphite (1% by mass) and cenospheres (up to 5% by mass) resulted in a decrease in thermal cond. and H2O absorption. The PU foams showed good dimensional stability. The addn. of spherical filler (up to 10% by mass) resulted in an increase of the mech. strength of the PU foams.