The past decades witnessed growing interest in polymeric closed-cell porous materials. They are lightweight, robust and find applications in many different sectors, like packaging, thermal insulation or cushioning. Numerical simulations are commonly used in the construction process to predict the behaviour of the products in application. So far, gas diffusion has often been neglected, as there are no tools available to include it. Therefore, this paper aims to present an algorithm to include the simulation of gas diffusion into a commercial finite element analysis (FEA) tool, at the example of MSC Marc. Using the similarity of thermal conductivity and gas diffusion, the thermal solver of the FEA program is exploited to perform the simulation of gas diffusion. Some first two-dimensional numerical simulations of porous closed-cell models under long-term load show promising results. Effective diffusion coefficients are obtained and conclusions about the time scale are made. The results agree to analytical estimations of the diffusion process. A notable influence of gas diffusion on long-term loaded porous materials can be shown.