The present invention relates to a process for producing a rigid polyurethane-polyisocyanurate foam C, comprising the step of reacting (i) an isocyanate-terminated prepolymer B with (ii) an activator component A comprising at least one trimerisation catalyst A1 and at least one blowing agent A3 in a reaction mixture to form a foam, characterised in that—there is used an isocyanate-terminated prepolymer B obtained from a reaction of an isocyanate B1 having a mean isocyanate functionality of from ≧2.3 to ≦2.9 with a polyol component B2, and—the activator component A comprises water as the blowing agent A3 in an amount of from ≧5 wt. % to ≦50 wt. %,—the isocyanate index in the reaction mixture is in a range of from ≧400 to ≦500, and—the isocyanate content of the prepolymer B is in a range of from ≧21 wt. % to ≦30 wt. %, based on the total mass of the prepolymer B, and—wherein in the reaction of the prepolymer B and the activator component A a conversion contribution to polyisocyanurate of ≦75% is achieved. Rigid foams C so produced have good flame retarding properties while at the same time having good insulating properties and stability properties. The present invention relates further to a rigid polyisocyanurate foam C produced by the process according to the invention, to the use of such a rigid polyisocyanurate foam C in the production of heat-insulating structural components, and to a heat-insulating structural component comprising such a rigid polyurethane-polyisocyanurate foam.