A process for producing an open-cell rigid polyurethane foam involves reacting at least the components of at least one polyisocyanate, a polyol composition, and a cell opening agent. The polyol composition contains at least 30% to 75% by weight of at least one polyether polyol (b-1) having a functionality in the range of 5.0 to 7.9 and containing monomeric units derived from monosaccharides, oligosaccharides, polysaccharides, and/or sugar alcohols; >0% to 69.5% by weight of at least one polyether polyol (b-2) having an OH number in the range of >0 to 250 mg KOH/g; and at least 0.5% by weight of water. The isocyanate index is below 160 and the concentrations in % by weight for (b-1), (b-2), and water are based on the total amount of the components of the polyol composition.