The purpose of this paper is to propose a material design method of lightweight cement-based composites that reduces the mass of building components and provides them with better insulation. To achieve this purpose, a number of special lightweight aggregates were tested. Furthermore, material design methods based on the packing density models and the void system model for lightweight porous mortar (LWPM) were applied. A series of experiments was carried out in order to investigate the mechanical properties and to examine the thermal conductivity of various types of lightweight mortar. The study identified that the adopted material design methods were effective in reducing the density of lightweight mortar and in obtaining sufficient insulation performance. Furthermore, compressive strength sufficient for required structural performance was also achieved in certain types of lightweight mortar. Finally, lightweight insulation block (LWIB) was developed by combining the packing density model and the void system model for achieving both of sufficient bearing capacity and high insulation performance.