Gas filling is a method where a low-conductivity inert gas, such as Argon, is used instead of air in window cavities in order to reduce heat transmission through the window. Windows which use this method are known as Insulating gas units. Although air is most commonly used to fill cavities, the use of an inert gas such as argon and krypton can reduce window heat transfer, increasing the insulating abilities of the window. Although krypton and argon are both colourless, odourless, and non-toxic, argon is most commonly used as it insulates well without being extremely expensive. For an argon gas filled window the cost is only $30-$40 more. Krypton is more effective at reducing heat loss but is about 200x more expensive than argon. Gasses such as sulphur hexafluoride or carbon dioxide can be used to reduce sound transmission but are not as effective as the inert gasses in preventing heat transfer.
These insulating glass units are designed to regulate the temperature of homes better, keeping them warm in the cold months and cool in the hot months. Argon, Krypton, or other inert gasses are pumped inside the cavities in a window, between the panes of a sealed glazing unit, resulting in a mixture of the fill gas and air. These gases are colorless, so they do not prevent visible light from entering the home, and odourless. Argon is denser than the atmosphere, and this provides more thermal efficiency than having simply air between the panes. Krypton is again denser than argon and thus provides somewhat more thermal efficiency, although it is more expensive. It is these densities that slows the movement of heat through the window itself. Generally, a standard gas-filled window consists of two to three litres of gas sandwiched between panes of glass or other glazing material. The more gas is used, the better the window will insulate. The gas within the window will gradually diffuse overtime at an estimated rate of 0.5-1.0% per year. This leakage however does not noticably affect the performance of the window itself, with the window still being significantly more energy efficient even twenty years down the road, even while accounting for leakage.
The insulating performance of these windows can further be enhanced by adding an e-coating. When 90% argon is used to fill these windows in a window that also has a low-e coating the insulating value can be improved by 16%.