Thermally stable rigid foams based on isocyanate and having low brittleness and low thermal conductivity

A thermally stable rigid foam based on isocyanate comprising the reaction product of

• a) polyisocyanates with
• b) compounds containing at least two hydrogen atoms which are reactive toward isocyanate groups and,
• c) water, in the presence of
• d) physically acting blowing agents, and optionally,
• e) catalysts and auxiliaries and/or additives, wherein the component b) comprises
• bi) at least one polyether alcohol having a functionality of at least 1.5 and a hydroxyl number of from 10 to 100 mg KOH/g in an amount of from 20 to 60% by mass, based on b), and
• bii) at least one polyester alcohol in an amount of from 5 to 90% by mass, based on b), said at least one polyester alcohol having a hydroxyl number of from 200 to 600 mg KOH/g, and
• the water c) is present in an amount of at most 2% by mass, based on the sum of components b) to e), and the component d) comprises hydrocarbons and/or fluorine-containing hydrocarbons in an amount of from 5 to 30% by mass, based on the sum of the components b) to e).