I'm trying to calculate the heat conductivity of a mold-polymer interface. I'm assuming that there's a thin layer of air between the polymer and the mold (gap conductance). According to some literature (Madhusudana, Thermal contact conductance, Springer) the heat transfer coefficient is a function (among other things) of the physical properties of the air in the gap.
I was wondering if I should use a real gas model for my calculations instead of assuming a real gas.
Under which temperature and pressure conditions should I use a real gas model?