Three different approaches for describing combined radiation and conduction heat transfer in fiber thermal insulation al high temperatures are analyzed and compared. The considered approaches include the radiation transfer equation or its approximations, approximation of radiation thermal conductivity and the radiation diffusion approximation for radiation transfer. The first causes difficulties due to the need for experimental measurement of optical properties, while calculations based on the Mie theory may give inexact results. The second rakes into account the radiation transfer inaccurately and the experimentally measured total thermal conductivity may depend on temperature drop, thickness of sample, properties of boundaries and time parameters at transient conditions. It is shown that the most preferable approach is the radiation diffusion approximation for radiation transfer. The comparison of the radiation thermal conductivity approximation and the radiation diffusion approximation is carried out for the example of modeling the working conditions of the fibrous thermal insulation of the Space Shuttle when the vehicle enters the Earth’s atmosphere.