Process for the production of polyurethane rigid foams

A process for the production of polyurethane rigid foams by reaction of

• a) at least one polyisocyanate with
• b) compounds with at least two hydrogen atoms reactive with isocyanate groups, and
• c) at least one propellant,
• wherein for the compounds b) with at least two hydrogen atoms reactive with isocyanate groups a mixture of
• b1) at least one polyether alcohol with a hydroxyl number from 380 to 500 mg KOH/g, producible by addition of butylene oxide, optionally in combination with at least one further alkylene oxide, to at least one OH or NH functional starter compound with a functionality of 4 to 8 with the aid of a basic catalyst,
• b2) at least one polyether alcohol with a hydroxyl number from 360 to 450 mg KOH/g, producible by addition of an alkylene oxide to at least one aromatic or aliphatic amine, and
• b3) optionally at least one polyether alcohol with a hydroxyl number from 140 to 280 mg KOH/g, producible by addition of an alkylene oxide to at least one OH or NH functional starter compound,
• is used.