The thermal conductivities of nanoporous and nanocompositesilicon with incorporated amorphous phases have been computed by molecular dynamics simulations. A systematic investigation of the porosity and the width of the amorphous shell contouring a spherical pore has been made. The impact of amorphous phase nanoinclusions in a crystalline matrix has also been studied with the same amorphous fraction as the porosity of nanoporoussilicon to achieve comparison. The key parameter for all configurations with or without the amorphous phase is proved to be the interface (between the crystalline and amorphous phases or crystalline and void) to volume ratio. We obtain the sub-amorphous thermal conductivity for several configurations by combining pores, amorphous shell, and crystalline phase. These configurations are promising candidates for low cost and not toxic thermoelectric devices based on abundant semiconductors.