The present invention relates to a rigid polyurethane foam comprising reinforcing materials which can be obtained by mixing (a) isocyanates which have a viscosity at 25° C. of less than 500 mPas with (b) compounds having groups which are reactive toward isocyanates, (c) blowing agents comprising water, (d) catalysts and, if appropriate, (e) further additives to form a reaction mixture and applying the reaction mixture to a reinforcing material, wherein the compounds (b) having groups which are reactive toward isocyanates comprise a polyetherol (b1) having a functionality of 4 or more and a viscosity at 25° C. of 10 000 mPas or less and a polyetherol (b2) having a functionality of 3 or less and a viscosity at 25° C. of 500 mPas or less. The present invention further relates to a process for producing such rigid polyurethane foams and the use of the rigid polyurethane foams for the insulation of liquefied natural gas tanks.