The Window Technologies of Argon and Krypton Gas Fills:
To improve thermal performance of insulated glazed windows you reduce the conductance between the glass layers. At first, air or dry nitrogen was applied before the window was sealed. Air current between the two panes of a sealed insulated glass carry heat, this heat flows to the top of the window where it cools, then settles, drops to the bottom of the window as cold pools. When this space is filled with a less conductive, thicker, or slow-moving gas, it reduces the convection current
between the glass planes, reducing the transfer of heat.

To improve thermal performance manufacturers began to fill the space between the layers of the window with argon and krypton gas. Argon is a clear, odorless, nontoxic, non-reactive gas. Krypton is a more expensive gas than argon, but has better thermal performance.