The present invention relates to polyester polyols obtainable or obtained by esterification of 10 to 70 mol % of at least one compound from the group consisting of terephthalic acid (TPA), dimethyl terephthalate (DMT), polyethylene terephthalate (PET), phthalic anhydride (PA), phthalic acid and isophthalic acid, 0.1 to 30 mol % of one or more fatty acids and/or fatty acid derivatives, 10 to 70 mol % of one or more aliphatic or cycloaliphatic diols having 2 to 18 carbon atoms or alkoxylates thereof, 5 to 70 mol % of a polyether polyol prepared by alkoxylating an aromatic starter molecule having a functionality of not less than 2, and of 0 to 70 mol % of a tri- or polyol other than the polyether polyol, all based on the total amount of the components used, wherein the amounts used of the components add up to 100 mol %. The present invention further relates to a process for producing rigid polyurethane or polyisocyanurate foams which comprises reacting an isocyanate component with a polyol component (PK) comprising a polyester polyol of the present invention and further components, to the polyol component as such and also to the rigid polyurethane or polyisocyanurate foams obtainable or obtained by a process of the present invention. The present invention also relates to the method of using a polyester polyol (P1) of the present invention in the manufacture of rigid polyurethane foams or rigid polyisocyanurate foams.