I’ve read a while back that the thermal conductivity for Ar is lower than CO2 which would make it a better insulator from a convection standpoint. Ar is only slightly more dense than CO2. If we want to go with the density of the gas reducing the amount of convection through a window I’m surprised the window manufacturers aren’t using Kr or Xe instead of Ar.
Please correct me if I am wrong what I have just said.
Thermal conductivity (i.e., long-range energy transfer from atom to atom) is not an issue, the primary transfer mechanism is motion of the atoms themselves via mass-flow currents, i.e., convection. This is why fiberglass insulation is so effective -- it impedes the gas current flow -- and why triple-pane glass is better than double.
Incidentally, that is also the real-greenhouse effect. The glass roof prevents the thermal convection flow upwards 'into the wild-blue yonder' which occurs outside the greenhouse -- it physically traps the warm air within say 9-10 ft of the ground.