Due to the proven performance of fibrous insulations in various industries, including manufacturing, energy, aerospace, and construction industries. These insulations were studied in several papers. Ceramic tiles used on the space shuttle are one of the most famous high temperature applications of these fibrous insulation. Due to high porosity of these insulations, the main method of heat transfer in these materials is radiation, especially when heat transfer occurred in more than 500 degrees of Celsius. Anomalous Diffraction Theory (ADT) is one of the theoretical methods to estimate extinction coefficient of fiber insulation. Roseland theory calculates the radiation heat transfer coefficient according to the radiation extinction coefficient. Results obtained from the developed code were validated with the results of other articles. In this article, the effect of fiber diameter on the radiative heat transfer coefficient is investigated. The results show that the radiant heat transfer coefficient of the fibers changes with the change of the diameter of the fibers and also the radiant heat transfer coefficient of silica insulation will be minimum if the fiber diameter is 6 micrometers, and with increasing or decreasing the fibers diameter, the radiant heat transfer coefficient will increase. In the case of silicon fiber insulation, the fiber diameter of 4 micrometers will have the lowest radiation heat transfer.