Producing pu rigid foams

A process for producing polyurethane rigid foams, which comprises (a) reacting at least one polyisocyanate by using (b) at least one blowing agent with (c) three or more different compounds each having two or more isocyanate-reactive hydrogen atoms, wherein (i) one of the three or more different compounds each having two or more isocyanate-reactive hydrogen atoms is a polyether alcohol component obtainable by reacting a mixture of melamine and at least one further hydroxyl- and/or amino-functional compound having a functionality ranging from 2 to 8 with at least one alkylene oxide, and (ii) a further one of the three or more different compounds each having two or more isocyanate-reactive hydrogen atoms is a polyether alcohol component having a hydroxyl number of 300 to 600 mg KOH/g coupled with an average functionality of 4 to 8, and (iii) the third one of the three or more different compounds each having two or more isocyanate-reactive hydrogen atoms is a polyether alcohol and/or polyester alcohol component having a hydroxyl number of 100 to 350 mg KOH/g coupled with an average functionality of 2 to 5.