A detailed numerical pyrolysis model is presented for the thermal degradation of ethylene–vinylacetate copolymers filled with alumina-trihydrate (EVA-ATH). The formulation involves effective thermal parameters and surface radiative emissivity that continuously follow the material evolution, deduced from its evolving morphology along the degradation, as characterized and modelled based on microtomographic imaging. The comparison of simulated and actual cone calorimeter experiments suggests that the ATH dehydration occurs on a broader range of temperature than generally assumed. The analysis of the thermal balance during the material degradation shows that only 40% of the incident flux is absorbed by the material due to the radiative behaviour of alumina, 13% is released at the backface, and the ATH dehydration represents 70% of the pyrolysis endothermicity. These results confirm the importance of the front- and back-face boundary conditions in reference experiments, and the need of an improved modelling of the ATH chemical degradation scheme and kinetics.