Disclosed is a method for manufacturing an isocyanurate polyurethane-modified foam, called “PUIR,” including: a) providing an oil or oil mixture at least 50% by weight of the fatty acids having a carbon chain of C18 or more, the fatty acids having an overall iodine number Iiodine of at least 100 g of I2/100 g; b) epoxidating at least 50% of the double bonds in the oil to form the corresponding epoxides; and c) directly reacting, in situ, the epoxides that are obtained above with isocyanate or diisocyanate groups for obtaining corresponding oxazolidone derivatives, without passing through the intermediate formation of polyols obtained from epoxides, in the presence of at least one suitable catalyst and at least one expanding agent, so as to obtain the isocyanurate polyurethane-modified foam “PUIR.” Also disclosed are products, such as rigid insulation material, in particular as a material or panels of rigid insulation for roofing.