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 catalyst component contains potassium formate B2.a), and 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, and - 1.0% to 10.0% by weight of at least one polyol component B1.b) selected from the group consisting of polyether polyols, polycarbonate polyols and polyether-polycarbonate polyols having a hydroxyl number in the range from 300 mg KOH/g to 600 mg KOH/g determined according to DIN 53240, and - optionally further isocyanate-reactive components B1.c), wherein the reported % by weight values are in each case based on the total weight of the composition B), and in that the reaction mixture from step i) contains less than 0.3% by weight of water and less than 0.2% by weight of formic acid, and wherein the reaction mixture from step i) has an isocyanate index of ≥ 150 to 600. 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, in that the catalyst component contains potassium formate B2.a), and 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, and - 1.0% to 10.0% by weight of at least one polyol component B1.b) selected from the group consisting of polyether polyols, polycarbonate polyols and polyether-polycarbonate polyols having a hydroxyl number in the range from 300 mg KOH/g to 600 mg KOH/g determined according to DIN 53240, and - optionally further isocyanate-reactive components B1.c), - 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, and - 1.0% to 10.0% by weight of at least one polyol component B1.b) selected from the group consisting of polyether polyols, polycarbonate polyols and polyether-polycarbonate polyols having a hydroxyl number in the range from 300 mg KOH/g to 600 mg KOH/g determined according to DIN 53240, and - optionally further isocyanate-reactive components B1.c), wherein the reported % by weight values are in each case based on the total weight of the composition B), and in that the reaction mixture from step i) contains less than 0.3% by weight of water and less than 0.2% by weight of formic acid, and wherein the reaction mixture from step i) has an isocyanate index of ≥ 150 to 600.