The present invention relates to a method for producing a polyester polyol having a concentration of ether groups in the range from 9.0 mol/kg of polyester polyol to 22 mol/kg polyester polyol, characterized in that (i) in a first step (A) isophthalic acid, optionally in the form of a C1-C4 alkyl ester, and/or terephthalic acid, optionally in the form of a C1-C4 alkyl ester, is reacted with (B) oligoethylene glycol of the formula H—(OCH2CH2)n—OH having a numerical average number of oxyethylene groups n in the range from 3.0 to 9.0, in the presence of at least one catalyst selected from the group made up of tin(II) salts, bismuth(II) salts, and titanium tetraalkoxylates, at a temperature in the range from 160° C. to 240° C., and at a pressure in the range from 1 to 1013 mbar, for a duration in the range from 7 to 100 hours, and (ii) in a second step the reaction mixture resulting from step (i) is reacted with (C) phthalic acid and/or phthalic acid hydride, the polyester polyols obtained according to said method, and to the use thereof for producing PUR/PIR hard foam materials