Water-blown rigid foams with improved mechanical properties at low temperatures

The present invention relates to a rigid polyurethane foam which can be obtained by mixing (a) isocyanates, (b) compounds having groups which are reactive toward isocyanates, (c) blowing agents comprising water, (d) catalysts and optionally (e) further additives to form a reaction mixture, applying the reaction mixture to a reinforcing material and curing the reaction mixture, where the isocyanates (a) have a viscosity of not more than 500 mPas at 25° C. and 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, a polyetherol (b2) having a functionality of 3.5 or less and a viscosity at 25° C. of 600 mPas or less, a polyesterol (b3) having a viscosity at 25° C. of 2000 mPas or less, chain extenders (b4) comprising at least 30% secondary OH groups and optionally a crosslinker (b5). 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.