The long-term thermal performance of vacuum insulation panels (VIP) is brought by the capacity of their barrier envelope to maintain the core material under vacuum. This study is focused on the detailed modelling of gas transfer through the defects of aluminium-coated polymer films used for VIPs' envelopes. The 3D simulations were performed with monolayer and multilayer metal-coated polymer films. They have been carried out in dynamic conditions with the SYRTHESA (R) software developed by EDF R&D. The results show that the water vapour and air permeations through a monolayer film slightly depend on the polymer substrate thickness, diffusivity and solubility, but primarily, on the defects geometry and arrangement. Regarding multilayer films, the permeation can be deduced from the ideal laminate theory. We are now able to provide and operate a numerical model, which can calculate the permeance of monolayer or multilayer metallized polymer films as a function of the coating quality and the geometry of the layers. Even if calculated permeances are ten times higher than measurements, this study improves our understanding of gas transports through VIPs' barrier envelope and allows to manage more efficiently the relations between the films microstructures and their overall permeability. This paper is split into 6 parts: physical phenomena, methodology and modelling tools, simulation results, experiments and model validation and then, discussion and conclusion.