Measurements of the thermal conductivity of polyimide foams have been performed in the temperature range 173-323K for different gas pressures and different gas types (CO2 and Ar). The extinction of thermal radiation has been determined by hemispherical transmission and reflection measurements in the infrared. With these data, a quantitative model has been established which predicts the thermal conductivity of polyimide foams as a function of density, gas pressure and temperature. In addition the influence of low emissivity Foils integrated into low density polyimide foams on the thermal conductivity has been calculated using a three-flux model of combined radiation and solid/gas conduction.