In between the two panes in a double glazed unit is normally an inert gas, which provides insulation. This stops heat escaping your home, keeping down both your energy bills and carbon footprint.

Air is actually a good insulator in itself, so filling the spacer with a low conductivity gas will enhance the energy efficiency of the unit further.

Argon tends to be the most popular as it reduces conductive and convective heat transfers. The reason for this is because air density is not as great as argon.

Krypton is another popular choice and both gases are odourless, non-flammable, colourless and non-reactive.

When installed, the air gap is filled around 90% with one of these motionless gases, but over time some will seep out of the unit. It?s generally accepted that the double glazed unit will lose 1% of its gas every year.

Argon?s inexpensive costs give it the edge over krypton, which is 200 times more expensive per unit volume.

As Krypton works best in more confined spaces it will be used more frequently in triple glazing, where the spaces are smaller.

There are a few other gases on the market which can be used to plug the air gap. Sulfur hexafluoride and carbon dioxide are particularly useful in reducing sound emissions but they?re not known for their thermal qualities.

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