[vhfpmr]

Are there any gases that are more or less likely to leak out through the hot melt glue used for sealing the DG? CO2 is notorious for leaking out of butyl cycle inner tubes, although they're under pressure.

Early manufactured items incorporated strips of silica gel

The silica gel is injected down the inside of the aluminium spacer tubes.

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.

The frame or the spacers? The spacers are thin walled tubes, and such a small proprtion of the overall window area it wouldn't be worth fussing over. Early aluminium window frames were notorious for getting cold and attracting condensation, that's why they've been made with a thermal break for decades.

[Iannguyen]

Argon gas may be injected into double-pane windows to help insulate a space and reduce heat transmission through the glass. For years, window makers have relied on argon gas, a colorless, odorless gas. Because argon gas is denser than air, it increases the thermal insulation effectiveness of double-pane windows by adding to the glass unit. Argon gas windows, when combined with a particular low-E (low emissivity) glass coating, bring the temperature of the window closer to room temperature. When different temperatures meet, this process eventually removes air currents and draughts. While the cost of injecting argon into double-pane windows is initially higher than that of air-filled double-pane windows, the long-term advantages of energy efficiency are incalculable. Higher efficiency criteria must be satisfied when new windows are installed, as more strict energy rules have been introduced in recent years.
Consequently, Thermal windows are filled with argon gas to improve their energy efficiency and overall performance. Thermal windows, also known as insulated glass units or IGUs, are double-pane or triple-pane, meaning that each portion of the window has two or three panes of glass with a sealed area between them. To delay heat flow through the window, the voids are filled with argon or other gases. Because air includes moisture, it can condense on the inside of the glass units, clouding the window. Gases like argon are preferable to air. In addition, air insulates less well than gas fills. Argon is a non-toxic, colorless, and odorless gas found naturally and makes up less than 1% of the Earth's atmosphere.
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[wolfekeeper]

Argon is used because of a combination of properties, notable it's less conductive than air per metre of thickness, and is about as viscous.

Ideally the gap should be as big as possible to give the thickest insulation, but the viscosity matters because if the gap is too big, a convection flow starts up between the panes which conducts the heat. A more viscous gas allows greater thickness without too much convection. Argon and air have similar viscosities so the same modules can work with either, but argon has about 30% lower thermal conductivity.

By comparison CO2 has half the viscosity so the gap has to be much smaller to avoid setting up convection, but CO2's thermal conductivity per metre of thickness is nearly the same as argon's so using CO2 is a net loss compared to argon.

CO2 also has the disadvantage that it radiates IR somewhat. That's bad because, unless the outer pane is e-coated it can radiate IR through the window and cool the fill gas, in turn cooling the window. But I don't think that's as big an effect as the viscosity one.