A process for preparing polyurethane rigid foams from polyols and polyisocyanates as well as blowing agents and optionally foam auxiliary agents, characterized in that the polyurethane rigid foam is obtained by reactingA) a polyol component with on average at least 3 hydrogen atoms, containing1. 60 to 100% of polyethers and/or polyesters with at least 2 hydroxyl groups and a molecular weight of 250 to 1,500, which have a surface tension of 6 to 14 mN/m with respect to i-pentane and/or n-pentane as blowing agent,2. i-pentane and/or n-pentane as blowing agent,3. water and4. optionally auxiliary agents and additiveswithB) a polyisocyanate with an NCO-content of 20 to 48 wt. % which has a surface tension of 4.0 to 8 mN/m with respect to i-pentane or n-pentane as blowing agentis described.