Process for the production of rigid polyurethane foams

The invention relates to a process for the preparation of rigid polyurethane foams by reacting DOLLAR A a) polyisocyanates with DOLLAR A b) compounds having at least two isocyanate-reactive hydrogen atoms in the presence of DOLLAR A c) blowing agents, DOLLAR A characterized in that as compounds with at least two isocyanate-reactive hydrogen atoms b) a mixture of DOLLAR A b1) a polyether alcohol having a functionality of 4 and a hydroxyl number of 380 to 350 mg KOH / g and a viscosity of greater than 12000 mPa.s, producible by addition of ethylene oxide and or propylene oxide with TDA, DOLLAR A b2) a polyether alcohol having a functionality of 5 to 7.5 and a hydroxyl number of 380 to 480 mg KOH / g, preparable by addition of propylene oxide to sucrose and / or sorbitol, DOLLAR A b3) a Polyether alcohol having a functionality of 2 to 4 and a hydroxyl number of 140 to 250 mg KOH / g, b ar by addition of ethylene oxide and propylene oxide to TDA or of propylene oxide to 2-, 3- or 4-functional alcohols, TDA different amines or castor oil derivatives DOLLAR A is used.