Process for producing rigid polyurethane/polyisocyanurate (PUR/PIR) foams comprising the step of i) reacting a reaction mixture containing A) a polyisocyanate component and B) an isocyanate-reactive component comprising B1) a polyol component, B2) a catalyst component, B3) optionally auxiliary and additive substances, and C) a physical blowing agent, characterized in that the component B) of the reaction mixture contains - 50.0% to 90.0% by weight of at least one polyester polyol and/or polyether ester polyol B1.a) having a hydroxyl number in the range from 80 mg KOH/g to 290 mg KOH/g determined according to DIN 53240, - 1.0% to 20.0% by weight of at least one polyether polyol B1.b) having a hydroxyl number in the range from 300 mg KOH/g to 600 mg KOH/g determined according to DIN 53240, - 0.0% to 5.0% by weight of low molecular weight isocyanate-reactive compounds B1.c) having a molar mass M n of less than 400 g/mol, - 1.0% to 30.0% by weight of at least one flame retardant B.3), - 0.1% to 4.0% by weight of potassium formate B2.a) and - 0.1% to 3.0% by weight of aminic components B2.b) wherein the reported % by weight values are in each case based on all components of the isocyanate-reactive composition B), and the component A) contains a mixture of diphenylmethane 4,4'-diisocyanate with its isomeric and higher-functional homologs, and in that the reaction mixture contains less than 0.2% by weight of formic acid and has an isocyanate index of ≥ 150 to ≤ 450, and with the proviso that the aminic component B2.b) and the reaction mixture reacted in step i) contain no aminic compounds of formula R 1 N(CH 3)(CH 2 CH 2 OR 2), wherein R 1 represents CH 3, CH 2 -CH 2 -N(CH 3) 2 or CH 2 -CH 2 OH, and R 2 represents H, CH 2 -CH 2 OH or CH 2 -CH 2 N(CH 3) 2. A) a polyisocyanate component and B) an isocyanate-reactive component comprising B1) a polyol component, B2) a catalyst component, B3) optionally auxiliary and additive substances, and B1) a polyol component, B2) a catalyst component, B3) optionally auxiliary and additive substances, and C) a physical blowing agent, - 50.0% to 90.0% by weight of at least one polyester polyol and/or polyether ester polyol B1.a) having a hydroxyl number in the range from 80 mg KOH/g to 290 mg KOH/g determined according to DIN 53240, - 1.0% to 20.0% by weight of at least one polyether polyol B1.b) having a hydroxyl number in the range from 300 mg KOH/g to 600 mg KOH/g determined according to DIN 53240, - 0.0% to 5.0% by weight of low molecular weight isocyanate-reactive compounds B1.c) having a molar mass M n of less than 400 g/mol, - 1.0% to 30.0% by weight of at least one flame retardant B.3), - 0.1% to 4.0% by weight of potassium formate B2.a) and - 0.1% to 3.0% by weight of aminic components B2.b) wherein the reported % by weight values are in each case based on all components of the isocyanate-reactive composition B), and the component A) contains a mixture of diphenylmethane 4,4'-diisocyanate with its isomeric and higher-functional homologs, and in that the reaction mixture contains less than 0.2% by weight of formic acid and has an isocyanate index of ≥ 150 to ≤ 450, and with the proviso that the aminic component B2.b) and the reaction mixture reacted in step i) contain no aminic compounds of formula R 1 N(CH 3)(CH 2 CH 2 OR 2), wherein R 1 represents CH 3, CH 2 -CH 2 -N(CH 3) 2 or CH 2 -CH 2 OH, and R 2 represents H, CH 2 -CH 2 OH or CH 2 -CH 2 N(CH 3) 2.