Insulation properties of glass wool (GW) and opacified fumed silica (OFS) as fillers of vacuum insulation panel are experimentally investigated for variable pressing load and vacuum level. Density change of the specimen as a function of the pressing force is measured. The thermal conductivity at center of panel is measured under various vacuum levels and pressing loads. To evaluate the radiative conductivity separately, the diffusion approximation is adopted and the extinction coefficient is measured by an FT-IR apparatus. As the density increases, the solid conductivity increases, while the radiative conductivity decreases to have their sum increased. Pore size is inversely proportional to the density of the material; however, the relation is not consistent in the case of OFS at very low density because of the highly heterogeneous porous structure at that density. Comparing the materials in terms of initial insulation per, formance at center of panel, we find that GW is superior at low pressing load and the other one is better at high pressing load. Also, OFS turns out to have a longer service-life. (C) 2013 Elsevier Ltd. All rights reserved.