What gas fill options are available and how do they affect performance?

The cavity between panes in a sealed unit is filled with an inert gas to slow heat transfer, and the fill choice affects the U-value:

Argon is the standard and most cost-effective fill, used in the overwhelming majority of double-glazed units. It’s denser than air, noticeably improves the U-value over an air-filled cavity, and offers the best balance of performance and price — which is why it’s the default specification.

Krypton performs better than argon and works well in narrower cavities, making it useful for slim sightline units, heritage or triple-glazed applications where space between panes is limited. It’s considerably more expensive, so it’s specified where performance or a slim profile genuinely demands it.

Xenon gives the best thermal performance of the three but is rarely used in domestic windows because the cost is prohibitive.

For nearly all standard trade work, argon-filled units are the right call. Krypton is a deliberate upgrade for slim-cavity or high-performance jobs. Tell the depot what U-value you’re targeting and we’ll spec the right fill rather than over-paying for gas you don’t need.