(EN) The invention relates to the production and use of polyester polyols, formed from at least one carboxylic acid hydride and diethylene glycol, wherein a specialized reaction control substantially suppresses the formation of 1.4-dioxan from diethylene glycol. The invention also relates to the polyesterpolyol obtainable according to this method, to a method for producing a polyurethane (PUR) or polyisocyanurate foam (PIR) material and to metal composite elements containing the PUR or PIR foam according to the invention.