Re: Why does argon have a better insulation factor than air?

 I was not aware that argon was a better insulator than air (and
I'm talking about dry air here. Moist air might yield diferent results.)
        Anyway, the reason has to do with the THERMAL CONDUCTIVITY, a
transport property of the material. Thermal conductivity provides an
indication of the rate at which energy is transferred by the diffusion
process. Pure metals, like silver and aluminum, are very good conductors
of heat, with high thermal conductivities, but gases have low thermal
conductivities and, therefore, don't conduct heat well.
        As far as the Argon vs air question, Argon (as do most monotomic
gases) has a very low thermal conductivity, even as compared to air, and
is, therefore, a better insulator.
        A good reference on thermal conduction and heat transfer in
general is "Fundamentals of Heat and Mass Transfer" by Incropera & DeWitt.
A good handbook of Chemistry and Physics might have a treatment of the
topic, too.
        (See also a related question which I answered here on moist vs.
dry air as a heat transfer medium.)
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Hope this helps.

Frederick J. Carranti, P.E.