Highly porous ceramic fiber insulations are beginning to be considered as a replacement for firebrick insulations in high temperature, high pressure applications by the chemical process industry. However, the implementation of such materials has been impeded by a lack of experimental data and predictive models, especially at high gas pressure. The goal of this work was to develop a general, applied thermophysical model to predict effective thermal conductivity, keff, of porous ceramic fiber insulation materials and to determine the temperature, pressure, and gas conditions under which natural convection is a possible mode of heat transfer. A model was developed which calculates keff as the sum of conduction, convection, and radiation partial conductivities. The model was validated using available experimental data, including laboratory measurements made by this research effort. Overall, it was concluded that natural convection is indeed possible for the most porous insulations at pressures exceeding 10 atm. Furthermore, keff for some example insulations was determined as a function of temperature, pressure, and gas environment.