A process for producing rigid polyurethane foams or rigid polyisocyanurate foams is provided. The process contains the reaction of polyisocyanate, fatty acid modified polyetherpolyol, polyetherpolyol, optionally flame retardant, blowing agent, catalyst, and optionally further auxiliary and/or admixture agent, wherein the polyetherpolyol is obtained by a process containing reacting orthotolylenediamine and optionally further co-starters with alkylene oxide containing ethylene oxide wherein the ethylene oxide content is more than 20 wt %, and then reacting the reaction product with alkylene oxide containing propylene oxide wherein the 1,2-propylene oxide content is more than 20 wt %, in the presence of a catalyst.