Argon fill
Argon fill is argon gas sealed into the space between the panes of an insulated glass unit instead of plain air. Argon conducts heat more slowly than air, so it lowers the heat loss through the glass at almost no extra cost, which is why it is standard on most European double and triple units.
The space between the panes of an insulated glass unit does real work. It is a barrier, and what fills it decides how well it blocks heat. Plain dry air is a decent insulator. Argon, an inert gas that makes up about 1% of the air we breathe, is a better one because it is denser and conducts heat more slowly.
Heat crossing a double-glazed unit moves by conduction and convection inside the gap. A denser, slower gas reduces both, so the glass loses less heat for the same two panes.
Air, argon and krypton
| Fill | Relative performance | Best gap width | Typical use |
|---|---|---|---|
| Air | Baseline | About 1/2 in. | Low-cost or legacy units |
| Argon | Better than air | About 1/2-5/8 in. | Standard on most double and triple units |
| Krypton | Better than argon | Narrow gap, under about 1/2 in. | Slim triple glazing, tight sightlines |
Krypton costs more and is used only when the gap is too narrow for argon to do its job. Fills are often a mix, for example about 90% argon with air making up the rest, as the panes are filled in production.
Reading it on a spec
European data sheets write the fill inside the glass formula. "4/16Ar/4" reads as 4 mm glass, a 16 mm gap filled with argon, and 4 mm glass. The "Ar" is the argon. If a quote gives only "double glazed" with no fill, ask what is in the gap.
Argon works together with the rest of the glass package: a low-E coating and a warm-edge spacer. The whole-window U-factor is what counts, and the glass package sets the center-of-glass part of it. In my estimator the European systems are priced with argon-filled glass as standard, and triple glazing is a priced option on top.
Common questions
- Does argon gas leak out of windows over time?
- A small amount escapes over decades, because no seal is perfect. A well-made unit with a good edge seal keeps most of its fill for the life of the window. A failed seal shows as fogging between the panes, and the unit is covered or replaced as a whole.
- Is argon better than air in a window?
- Yes, for insulation. Argon conducts heat less than air, so a unit filled with argon has a lower center-of-glass U-factor than the same unit filled with air. It is invisible, harmless and inexpensive, which is why it is the default.
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