Argon is a colorless, odorless, non-flammable, non-reactive, inert gas. Argon gas fills are used to reduce heat loss in sealed units by slowing down convection inside the air space. Argon gas is extremely cost-efficient, and works well with Low-E coated glazing.
As air itself is a good insulator, filling the gap between the glass panes with a low-conductivity gas such as argon improves window performance by reducing conductive and convective heat transfers. This phenomenon results from the fact that 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.