Cavity blocks contain a rigid polyurethane foam having a density in the range of 30-70 g/L in at least one chamber, the rigid polyurethane foam being obtained by reacting at least one polyisocyanate a) with at least one compound having at least two isocyanate-reactive hydrogen atoms b) in the presence of at least one catalyst d) and using at least one blowing agent c), comprising at least one blowing agent ci), which only develops its blowing effect at the time of the reaction of a) with b), and at least one blowing agent cii), which has a boiling point which is below the temperature at which said components a) and b) are mixed, wherein the pressure buildup during the foaming reaction of the polyurethane remains below 20 kPa, and the invention also relates to the corresponding process for producing the cavity blocks.