Improved rigid pur and pir foam

A method for preparing rigid polyisocyanurate (PIR) foams [rigid PIR foam, herein after] in which method the rigid PIR foam is prepared by reacting a composition (C) comprising:

• at least one isocyanate-reactive component having functional groups selected from hydroxyl, amine and thiol groups [isocyanate-reactive component (B1), herein after];
• at least one isocyanate component having an average functionality of less than 2.70 [isocyanate component (A1), herein after];
• at least one blowing agent [blowing agent (BA), herein after];
• with the proviso that the overall average functionality [Fn,avg(A), herein after] of all isocyanate components present in the composition (C) is less than 2.70;
• and
• wherein the composition (C) is characterized by an isocyanate index X in the range from 200<X≤400;
• and wherein the rigid PIR foams are produced between gas-tight facing sheets;
• and
• wherein the rigid PIR foam is characterized by a difference between the initial thermal conductivity value λini and the aged thermal conductivity value λaged (Δλ), of said rigid foam wherein:
• λ2<[6.49−(4.46*Fn,avg(A))−(0.02348*X)+(0.492*Fn,avg(A)*Fn,avg(A))+(0.01343*Fn,avg(A)*X)+0.3],
• wherein the initial thermal conductivity value λini and the aged thermal conductivity value λaged, expressed as mW/m·K at 10° C., are measured in accordance with ASTM C518 with a Fox 304 thermal conductivity instrument with a temperature gradient from 0° C. to 20° C., the initial thermal conductivity value λini is measured within 24 hours after production of the rigid PIR foam, the aged thermal conductivity value λaged is measured after ageing at 23° C./50% RH until the thermal conductivity difference between three consecutive weekly measurements is less than 0.05 mW/m·K.