Polymeric diphenylmethane-4,4'-diisocyanate and rapeseed oil-derived bio-polyols were used for prepn. of rigid polyurethane-polyisocyanurate foams with isocyanate index 150-250 in presence of a catalyst, blowing, surface-active and flame-retarding agents. The thermal stability of the foams increased while their flammability decreased with the increasing isocyanate index. Use of the bio-polyols resulted in increasing the 02 index up to above 21%.