[MarkPawelek (OP)]

The internet tells me that argon is used in double-glazing, between glass panes, because argon is denser than air. This explanation makes no sense.

I noticed that inert gases have the lowest heat capacity of all gases ( = 20.8 Joule/mol/K - that same for all ). Argon is also the most abundant, so cheapest, inert gas.

So, a low heat capacity means argon will conduct less heat between the window panes from inside to outside. This makes far more sense. Is this the real explanation?

[Bored chemist]

They use argon because it has a low thermal conductivity.

[Petrochemicals]

Double glazing is about heat transmission, not capacity. Argon is a gas that is less conductive. It is a noble gas, I am unsure what effect that has.

[alancalverd]

They use argon because it has a low thermal conductivity.

That makes sense, but carbon dioxide has an even lower conductivity and is far more available, if not from air.

[MarkPawelek (OP)]

Using units: mW/m/K, at 100 kPa pressure, and 300 K temperature, thermal conductivities of some gases are :

Xenon _____ 5.5
Krypton ____ 9.5
CO2 _____ 16.8
N2O _____ 17.4
Argon ____ 17.7

I excluded gases with: thermal conductivities greater than Argon, low boiling point, toxic, chemically reactive, CFCs, ...  I see why xenon isn't used due to its high cost. But why not krypton? Krypton costs about $0.50 / L.  Also: why are CO2 or N2O not used in place of argon?

[Bored chemist]

Krypton costs about $0.50 / L.

That's more than argon.

I don't know why they don't use CO2.
It hardly matters, the seals are not perfect, after a while the gap is full of air anyway.

[alancalverd]

A thought! Argon arc welding can be used to fabricate aluminum and stainless steel, so it's quite likely that a window factory will have argon on tap.

[Bored chemist]

They use CO2 in welding too.
If you are going to set up a double glazing sealed unit factory, I rather doubt that you make the fill gas decision on the basis of what you happen to have lying about.

[Petrochemicals]

It may have to do with radiation why Argon is used. 5his would however have concequences for the solar gain methods of house heating as it would stop the incoming solar radiation.

https://aip.scitation.org/doi/10.1063/1.435565

Also argon is one of the few elements that does not react to fluorine?

[evan_au]

How about attacking it from a different angle?
- Argon has a high atomic mass (40g/mol), compared to CO₂ (at 28g/mol) or N₂  (also at 28g/mol).
- This means that heated Argon gas would have a lower atomic velocity and rate of diffusion than CO₂ or N₂, which might lead to lower thermal conductivity
- Does this also mean that Argon would have a lower convection velocity than CO₂ or N₂?

Because CO₂ or N₂ have more vibrational modes than Ar, they have higher heat capacity
- Does this mean they can transfer more heat energy from one pane to the other, if they do manage to diffuse that far?

[Bored chemist]

Because CO2 or N2 have more vibrational modes than Ar, they have higher heat capacity
- Does this mean they can transfer more heat energy from one pane to the other, if they do manage to diffuse that far?

Not unless the gas is rather hot, but they do have rotational modes which contribute to the conductivity.

Exactly why the conductivities have the values they do is rather beside the point.
The conductivity of CO2 is a bit lower than Ar, and I think it's cheaper too.
That's why I wonder why they don't use it.

[Bored chemist]

Also argon is one of the few elements that does not react to fluorine?

If you have fluorine in the gap in your windows (1) they will fog (2) you have more serious problems than double glazing.

[Bored chemist]

It may have to do with radiation why Argon is used. 5his would however have concequences for the solar gain methods of house heating as it would stop the incoming solar radiation.

Most of the gases we are talking about, N2,O2, Ar, Kr, Xe etc have no microwave,  infra red or visible light absorptions.
They will absorb short wave UV, but that won't get through the glass.
CO2  (and water, since it's likely to be present) will absorb IR but:
It's likely that the IR would be absorbed by the glass.
It's also likely that it will be absorbed by CO2 and water vapour in the surrounding air.
Any radiation absorbed within the cavity is as likely to subsequently be transmitted out of the window as in through it.

[alancalverd]

(and water, since it's likely to be present)

Absolutely not! The usual reason for replacing double glazed panes is because they fog and ice internally! Early manufactured items incorporated strips of silica gel or other desiccants with an air filling, but pressure-filing with dry gas improves the appearance of the product as long as the seal is good.

CO2 is not used for welding aluminum - you need 100% argon to make a clean joint.

This has turned out to be a very instructive question! 

[Bored chemist]

The usual reason for replacing double glazed panes is because they fog and ice internally!

Thanks for confirming that they contain water- as I said.

[Bored chemist]

CO2 is not used for welding aluminum

It's far from clear why aluminium would be used in making modern sealed double glazing units. It's a very good conductor of heat.
And, there's certainly not a lot of arc welding  involved .

Why would you bother?
The whole lot is going to be "glued" to make it airtight, why not use that glue to hold the thing together?

[alancalverd]

The usual reason for replacing double glazed panes is because they fog and ice internally!

Thanks for confirming that they contain water- as I said.

But they shouldn't, which is why they get replaced.

[alancalverd]

It's far from clear why aluminium would be used in making modern sealed double glazing units.

Worth searching for "aluminium windows UK"  to find a whole lot that are. 

[Bored chemist]

The usual reason for replacing double glazed panes is because they fog and ice internally!

Thanks for confirming that they contain water- as I said.

But they shouldn't, which is why they get replaced.

They get replaced when there's enough water in them to be a problem. But there will always be some water there.
(and if you get bored, you can look at the specification for welding grade argon...)

[Bored chemist]

It's far from clear why aluminium would be used in making modern sealed double glazing units.

Worth searching for "aluminium windows UK"  to find a whole lot that are. 

Are they arc welded or irrelevant?