The key requirement for a GHG is that it absorbs infra-red (heat) radiation in the frequency region where a 'black-body' at earth-surface temperatures radiates. This blocks surface radiation and forces the IR heat radiation to 'outer-space' to occur from higher altitudes, where the atmosphere is colder, and the heat radiation is thus much weaker.
Clouds at night do the same thing, but clouds block the sun during the day whereas greenhouse gases do not. Argon, oxygen, and nitrogen do not absorb significantly in the IR region of interest (but ozone does, and is a GHG)
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.