Process for producing rigid PUR/PIR foams having an apparent density of 30-90 kg/m 3 according to ISO 845:2006, preferably 30-70 kg/m 3, an open-cell content of > 90% according to ISO 4590:2002, preferably ≥ 94%, and having an average cell diameter of < 50 μm according to optical microscopy evaluation, comprising the steps of: i) producing the inventive reaction mixture R) containing a polyol formulation P) containing - an isocyanate-reactive component A) comprising at least one polyol component A1) having a functionality f of > 2.5 which is selected from the group consisting of polyether polyols, polyester polyols, polycarbonate polyols, polyether polycarbonate polyols and polyether ester polyols, - a catalyst component D) at least containing a catalytically active compound D1) having Zerewitinoff-active hydrogens, - assistant and additive substances E) at least comprising a cell-opening compound, wherein the proportion of all primary OH functions present in the isocyanate-reactive component A) based on the total number of terminal OH functions in the component A) is at least 30%, a polyisocyanate component B) and CO 2 in the supercritical state as blowing agent component C), ii) introducing the foam-forming reaction mixture R) into a mold, iii) foaming the reaction mixture R) and iv) demolding the rigid PUR/PIR foam, wherein step i) and optionally ii) are carried out under supercritical conditions, and wherein step i) is carried out in a high-pressure mixing head, and wherein in step ii) the foam-forming reaction mixture R) composed of component A), blowing agent C) and polyisocyanate component B) is introduced into a sealed mold, wherein the counterpressure in the mold during injection is 2-90 bar. i) producing the inventive reaction mixture R) containing a polyol formulation P) containing - an isocyanate-reactive component A) comprising at least one polyol component A1) having a functionality f of > 2.5 which is selected from the group consisting of polyether polyols, polyester polyols, polycarbonate polyols, polyether polycarbonate polyols and polyether ester polyols, - a catalyst component D) at least containing a catalytically active compound D1) having Zerewitinoff-active hydrogens, - assistant and additive substances E) at least comprising a cell-opening compound, wherein the proportion of all primary OH functions present in the isocyanate-reactive component A) based on the total number of terminal OH functions in the component A) is at least 30%, a polyisocyanate component B) and CO 2 in the supercritical state as blowing agent component C), a polyol formulation P) containing - an isocyanate-reactive component A) comprising at least one polyol component A1) having a functionality f of > 2.5 which is selected from the group consisting of polyether polyols, polyester polyols, polycarbonate polyols, polyether polycarbonate polyols and polyether ester polyols, - a catalyst component D) at least containing a catalytically active compound D1) having Zerewitinoff-active hydrogens, - assistant and additive substances E) at least comprising a cell-opening compound, wherein the proportion of all primary OH functions present in the isocyanate-reactive component A) based on the total number of terminal OH functions in the component A) is at least 30%, - an isocyanate-reactive component A) comprising at least one polyol component A1) having a functionality f of > 2.5 which is selected from the group consisting of polyether polyols, polyester polyols, polycarbonate polyols, polyether polycarbonate polyols and polyether ester polyols, - a catalyst component D) at least containing a catalytically active compound D1) having Zerewitinoff-active hydrogens, - assistant and additive substances E) at least comprising a cell-opening compound, a polyisocyanate component B) and CO 2 in the supercritical state as blowing agent component C), ii) introducing the foam-forming reaction mixture R) into a mold, iii) foaming the reaction mixture R) and iv) demolding the rigid PUR/PIR foam, and wherein step i) is carried out in a high-pressure mixing head, and wherein in step ii) the foam-forming reaction mixture R) composed of component A), blowing agent C) and polyisocyanate component B) is introduced into a sealed mold, wherein the counterpressure in the mold during injection is 2-90 bar.