Thermally stable foams based on polycyanurates of various chemical structure (derived from dicyanate ester of bisphenol A, DCBA, dicyanate ester of bisphenol E, DCBE, and oligo(3-methylene-1,5-phenylenecyanate), PT-15) were synthesized using foaming agents with different decomposition temperatures and gas release volumes during decomposition. By varying the monomer, the viscosity of the initial reaction mixture, the amount of foaming agent, or the molar mass of the surfactant, polymer foams with pore sizes ranging from ~455–580 µm to ~1–1.5 mm could be obtained (see Fig.1). Their morphology was evaluated using scanning electron microscopy. A highly porous structure was obtained when using the DCBE oligomer by heating at T ≈ 125°C for 1 hour with the addition of a complex catalyst, zinc (II) acetyl acetonate (AcAc)₂Zn with nonylphenol, and the foaming agent azodicarbonamide SBZ-2 in amounts of 1 and 2 wt.%. The average pore size in these samples was within the range of 60–220 µm and 200–800 µm, respectively. To assess their thermal insulation properties, the thermal conductivity (k) of the foam samples was determined using the transient plane source method. The k value decreased from 251 mW/m·K for the non-porous matrix to 71–128 mW/m·K for the porous foam samples, depending on the initial composition and the total porosity of the samples, which ranged from 31% to 81%. Mechanical testing of the thermostable polycyanurate foams showed sufficiently high values, with a compressive strength reaching ~49 MPa and a compressive modulus of ~835 MPa. The thermostable foams are promising for application as thermal insulation, particularly in the aerospace industry.