Argon vs krypton: filling the cavity
Both gases are there to slow the conduction that air would otherwise carry across the cavity. Argon is the standard fill — inert, cheap, and worth roughly a third better than dry air on the U-value of a normal cavity. Krypton is the denser gas for the thin cavities: where the gap is narrow, argon loses most of its benefit and krypton holds it, at several times the cost. The rule is to match the gas to the cavity width, not to buy the dearer one by default.
What the cavity is for
An insulated glass unit is a pane, a gap and a pane. Dry air in the gap is already an insulator, but it still moves: warm air rises on the inside face, cool air falls on the outside face, and that loop carries heat across. The gas fill slows the loop. Argon is about a third less conductive than air; krypton is heavier and slower again. The fill does not change the glass — it changes what happens in the space between the glass.
Where argon works
Argon is the default fill on the standard schedule: inert, non-toxic, no reaction with the spacer or the gasket, and effective in the cavity widths most windows use — the 12 to 16 mm range on our lines. It is mixed with dry air and sealed when the unit is assembled, and it stays put as long as the edge seal holds. On the ordinary double-IGU it is the fill that pays for itself first.
Where krypton earns its price
Krypton is the answer to a narrow cavity. When the gap is thin — the slim-frame and heritage cases where the sash depth will not carry a wide unit — argon loses most of its benefit, because in a narrow gap the conduction paths are short and the gas has less room to slow. Krypton, heavier and slower, holds a useful U-value in a gap that argon cannot help. The cost is several times the argon fill, which is why it is specified for the slim build, not the standard one.
Mixtures, and what to put on the schedule
- Argon for the standard cavity (roughly 12 to 16 mm) — the cost-effective default.
- Krypton for the narrow cavity where the frame depth cannot take a wider gap.
- An argon and krypton mixture where the gap sits between the two useful ranges.
- State the fill and the target U-value together; the NFRC or EN 673 value is quoted for the configured gas, not for the glass alone.
Questions
A small loss over the life of the unit is normal as the edge seal ages; the fill is set so the unit still meets its declared value at the end of the warranty period.