We developed shape-stabilized phase change composites (PCCs) for building materials with high heat-storage density by vacuum impregnation of the phase change material (PCM) octadecane into mesoporous SiO2. We examined the effects of the average pore size of mesoporous SiO2 on the melting point and latent heat of the PCCs. Cyclic tests of melting and freezing were performed to evaluate leakage and degradation of the PCCs. The thermophysical properties of the PCCs were measured by differential scanning calorimetry, and the following results were obtained: (1) The impregnation ratio of the composites was above 0.95; therefore, almost all pores were completely filled with PCM. (2) The melting point of the PCMs decreased with decreasing average pore diameter, and the melting point was established as a function of average pore diameter from the Gibbs-Thomson equation, taking into account the existence of a nonfreezing layer on the surface of the pore wall. (3) The octadecane/mesoporous SiO2 composites retained the full amount of impregnated PCM, even after cyclic heating and cooling. (C) 2015 Elsevier B.V. All rights reserved.