Insulation glass without low-e coating uses air between the panes as a primary source of insulation.  As air itself is a good insulator, filling the gap between the glass panes with a low-conductivity gas such as argon reduces conductive and convective heat transfers.  This is because the density of the gas is greater than the density of the air.

Argon is the most commonly used fill gas, due to its excellent thermal performance and cost-efficiency in comparison to other gas fills.  Argon gas reduces heat loss in sealed units by slowing down convection inside the air space.  It is extremely cost-efficient, and works well with Low-e coating glazing.  Argon and Krypton are colourless, odourless, non-flammable and non-reactive inert gases.

Several techniques are used to fill the IG cavity and all techniques result in a mixture of fill gas and air.  It is generally accepted that the IG unit should achieve a 90% fill gas concentration.  Over time this concentration will gradually evaporate, at an estimated rate of 0.5 to 1% per year.  IG units filled with argon do not degrade significantly until they reach 75% concentration, which means up to 20 years of durability.

A major benefit of IGU is that it is passive, so it does not require a switching-on process to work.  Argon gas is relatively inexpensive option, and quickly produces heat cost-savings that justify its cost.