Due to increased fire regulations in the building industry, urethane-modified isocyanurate foams (U-IR foams) have been widely used in place of flame retardant rigid urethane foams. However, commercial U-IR foams still have inherent defects in thermal stability, flammability, friability, and smoke density. This paper discusses a novel method for the preparation of amide-modified isocyanurate foams (A-IR foams) having excellent thermal stability and low friability. A-IR foams having higher thermal stability and higher flame retardation than those of U-IR foams were prepared using quasi-prepolymers prepared by the reaction of adipic acid with a polymeric MDI at about 70°C in tetrahydrofuran as solvent and at unusual COOH/NCO equivalent ratios of 1/30 to 1/10. After the reaction is completed, tetrahydrofuran was distilled out. In contrast, the one shot process using adipic acid as modifier resulted in inferior foams. As control, U-IR foams were prepared using prepolymers made by the reaction of 1,6-hexanediol with the same polymeric MDI. Both A-IR and U-IR foams were prepared using the prepolymers, HCFC-141b as blowing agent, and the same surfactants and catalysts. The thermal stability of the amide linkage is higher than that of the urethane linkage, and therefore, the resulting A-IR foams exhibited higher thermal stability than that of U-IR foams. The higher thermal stability of the A-IR foams resulted in higher temperature resistance, lower flammability and less smoke generation. In addition, A-IR foams exhibited lower friability even at high equivalent ratios. On the other hand, U-IR foams having the same NCO/OH equivalent ratios exhibited higher friability. Detailed studies in the relationship between chemical structure and properties were carried out. These studies included the effect of equivalent ratios on foam properties, e.g., thermal stability, friability, compressive strength, flammability, etc. The effect of catalysts on the reaction profile was also investigated. Comparison of thermal stability by means of TGA and of surface flammability by means of the Butler Chimney flammability test exhibited outstanding improvements in the A-IR foams in comparison with U-IR foams. In view of these data, it could be concluded that A-IR foams prepared by the quasiprepolymer process at high equivalent ratios exhibited superior properties. Accordingly, amide-modified polyisocyanurate foams could be advantageously used for high-rise building insulation, chemical plant insulation, residential insulation, etc. in place of urethane-modified polyisocyanurate foams.