Abstract: The invention provides a process for insulating a cavity in a built structure with a polyurethane rigid foam, by reaction of a) polyisocyanates with b) compounds having at least two isocyanate-reactive hydrogen atoms in the presence of c) blowing agents, wherein the reaction takes place in the cavity to be insulated, wherein the blowing agent c) comprises at least one blowing agent ci) which only develops its blowing effect at the stage of the reaction of a) with b), and a blowing agent cii) which has a boiling point which is below the temperature at which the components a) and b) are mixed.