Process for producing a rigid polyurethane-polyisocyanurate foam C comprising the steps of reacting a polyester comprising carboxyl end groups with an epoxide of general formula, where R1 represents alkyl or aryl, by obeying a molar ratio of epoxy groups to carboxyl end groups at between 0.8:1 and 50:1, to obtain a polyester polyol having secondary hydroxyl end groups A1a which has a functionality of 1.8 to 6.5, and a hydroxyl number of 15 mg KOH/g to 500 mg KOH/g; foaming at isocyanate indexes computed as (moles of isocyanate groups / moles of isocyanate-reactive groups) 100 of 180 to 400, (i) a polyol component A1 comprising A1a with (ii) an isocyanate component B comprising a) at least one isocyanate B1 selected from the group consisting of: tolylene diisocyanate, diphenylmethane diisocyanate, polyphenylene polymethylene polyisocyanate, xylylene diisocyanate, naphthylene diisocyanate, hexamethylene diisocyanate, diisocyanatodicyclohexylmethane and isophorone diisocyanate, or b) an isocyanate-terminated prepolymer B2 prepared from at least a polyisocyanate B1 and an isocyanate-reactive compound, or c) mixtures of B1 and B2, in the presence of (iii) at least one blowing agent A2, and (iv) at least one catalyst A3, except that carbanionic catalysts shall be excluded. reacting a polyester comprising carboxyl end groups with an epoxide of general formula, where R1 represents alkyl or aryl, by obeying a molar ratio of epoxy groups to carboxyl end groups at between 0.8:1 and 50:1, to obtain a polyester polyol having secondary hydroxyl end groups A1a which has a functionality of 1.8 to 6.5, and a hydroxyl number of 15 mg KOH/g to 500 mg KOH/g; foaming at isocyanate indexes computed as (moles of isocyanate groups / moles of isocyanate-reactive groups) 100 of 180 to 400, (i) a polyol component A1 comprising A1a with (ii) an isocyanate component B comprising a) at least one isocyanate B1 selected from the group consisting of: tolylene diisocyanate, diphenylmethane diisocyanate, polyphenylene polymethylene polyisocyanate, xylylene diisocyanate, naphthylene diisocyanate, hexamethylene diisocyanate, diisocyanatodicyclohexylmethane and isophorone diisocyanate, or b) an isocyanate-terminated prepolymer B2 prepared from at least a polyisocyanate B1 and an isocyanate-reactive compound, or c) mixtures of B1 and B2, in the presence of (iii) at least one blowing agent A2, and (iv) at least one catalyst A3, except that carbanionic catalysts shall be excluded. (i) a polyol component A1 comprising A1a with (ii) an isocyanate component B comprising a) at least one isocyanate B1 selected from the group consisting of: tolylene diisocyanate, diphenylmethane diisocyanate, polyphenylene polymethylene polyisocyanate, xylylene diisocyanate, naphthylene diisocyanate, hexamethylene diisocyanate, diisocyanatodicyclohexylmethane and isophorone diisocyanate, or b) an isocyanate-terminated prepolymer B2 prepared from at least a polyisocyanate B1 and an isocyanate-reactive compound, or c) mixtures of B1 and B2, in the presence of a) at least one isocyanate B1 selected from the group consisting of: tolylene diisocyanate, diphenylmethane diisocyanate, polyphenylene polymethylene polyisocyanate, xylylene diisocyanate, naphthylene diisocyanate, hexamethylene diisocyanate, diisocyanatodicyclohexylmethane and isophorone diisocyanate, or tolylene diisocyanate, diphenylmethane diisocyanate, polyphenylene polymethylene polyisocyanate, xylylene diisocyanate, naphthylene diisocyanate, hexamethylene diisocyanate, diisocyanatodicyclohexylmethane and isophorone diisocyanate, or b) an isocyanate-terminated prepolymer B2 prepared from at least a polyisocyanate B1 and an isocyanate-reactive compound, or c) mixtures of B1 and B2, (iii) at least one blowing agent A2, and (iv) at least one catalyst A3, except that carbanionic catalysts shall be excluded.