This is not correct because the dimensions and thermal gradients are very different. Thatâ€™s where all those fluid dynamics numbers I mentioned come into play. Without bothering to do the calculation, Iâ€™m going to claim that the Nusselt number for a light bulb is much greater than unity, which means that thermal convection dominates over conduction. You can rely on experience for confirmation: the â€śheat wavesâ€ť above a hot surface (e.g., asphalt in the summer) are visual evidence of convection at work. The Grashof number is proportional to the temperature difference.

Iâ€™ll speculate that the reason there is an â€śoptimum thicknessâ€ť for the window mentioned in the Wikipedia article is precisely for this reason, i.e, that if the spacing of the panes is too great, viscosity is not high enough to prevent the onset of natural convection.

The Grashof number is inversely proportional to the square of the kinematic viscosity, which is itself inversely proportional to the density. That means the Grashof number is much larger for krypton than it is for argon, for example. Since the Nusselt number is monotonically increasing with Grashof number (see figure below), convection is easier to get going krypton. Once there is convection, thermal transfer is greatly enhanced.

It is left as an exercise to the reader to compute the optimum thickness as a function of gas fill. Too thin and thermal conduction is greater than it need be; too thick and there is free convection. The figure below gives the connection between the Grashof and Nusselt numbers.