A thorough analysis of the thermal conductivity of virgin and spent porous Si3N4-bonded SiC sidewall materials in the Hall–Héroult electrolysis cell is reported. The thermal conductivity, measured by the laser flash method, was observed to vary significantly among the virgin materials and also within a single block of the same material. A significant reduction of the thermal conductivity in both the bottom and top part of the spent blocks was observed, which could be correlated with significant changes in the microstructure in both regions due to chemical degradation. The thermal conductivity was correlated with the phase composition, apparent porosity, and microstructure related parameters like grain-boundary resistance, pore-shape, and pore-orientation factor. Analytical models were used to analyze the influence of these factors on the thermal conductivity of the Si3N4-bonded SiC porous composites. Two different explanations for the significant reduction in the thermal conductivity in the bottom and the top parts of the spent block were proposed, and both were related to the microstructure evolution caused by the chemical degradation.