Experiments with CO2-blown hanging polystyrene (PS) films were carried out. The porosity of foams decreased with decreasing film thickness, indicating thus effects of heat and/or mass transfer. Taking into account the Joule-Thomson effect, mathematical model based on a simplified heat balance for the system PS+CO2 at the conditions of PS saturation with CO2 on one side and at the conditions of foam vitrification at glass transition temperature on the other side was derived to estimate the foam porosity. The model was applied to data on CO2-blown hanging films as well as literature data on foaming of PS particles. The trends of porosity dependence on saturation temperature and film thickness predicted by the model correspond to experimental results. The Joule-Thomson effect included in the modelling of the CO2 foaming process significantly affects the foam porosity and improves the predictive capabilities of simple model based on enthalpy balance and thermodynamic data. (C) 2017 Elsevier B.V. All rights reserved.