Experimental and theoretical studies are reported on the determination of the effective thermal conductivities of multilayer thermal insulation systems with specially designed shields for application in high temperature fuel cells. An experimental apparatus and instrumentation have been designed and built to measure heat transfer through such insulation systems at temperatures up to 1000 °C. The insulations consisted of both fibrous and microporous materials. A theoretical model based on combined conduction and radiation heat transfer through porous materials capable of absorbing, emitting and scattering has been used to predict the temperature distribution and heat transfer in insulations comprised of the materials separated by multiple screens (shields). The experimental results obtained for several multilayer thermal insulation systems are reported, and the experimental data are compared with theoretical predictions to assess the effectiveness of the screens in reducing the effective thermal conductivity of the insulations.