Opacified silica aerogels are composite insulating materials containing silica nanoparticles and microsize opacifier grains. The radiative heat transfer in this dispersed medium was analyzed using a realistic microstructure model to calculate the opacified aerogel’s optical properties. The aerogel matrices were simulated using aggregates generated by a DLA algorithm, where the particle sizes and numbers were determined from the basic physical parameters. The theoretical predictions of the aerogel’s optical parameters agreed well with experimental data. A geometric unit containing one opacifier particle and a large number of aerogel particles was then built to study the coupled radiation effect between the aerogel and the opacifier. The optical parameters were computed using a multi-sphere T-matrix code with comparisons with Mie scattering solutions. The results show how the opacifier’s modified optical properties reduce the aerogel’s radiative conductivity.