Thermal conductivities of opacified powderous silica aerogels with different density, grain size and concentration of IR-opacifier were investigated. Measurements were performed in a guarded hot-plate apparatus by variation of temperature, gas pressure and external load. The lowest achieved thermal conductivity in air was around 0.017 W/(m K) at room temperature and an external load of 0.1 bar. The measurements allow determination of the gaseous, solid and radiative contributions to the thermal conductivity. The solid conductivities derived are in the range 0.002–0.004 W/(m K), depending on density, mechanical load and grain size. The specific extinction was found to be dependent on the concentrations of IR-opacifier and grain size. Typical radiative conductivities were of the order of 0.001 W/(m K) at room temperature. Gaseous conductivities observed were about 0.011–0.017 W/(m K), depending on the inter-grain pore space.