Krypton gas is one of two gases frequently used to fill the spaces between panes in double- and triple-paned windows. Like argon (the other gas used to fill windows), krypton gas is nontoxic, inert (nonreactive), colorless, and odorless. Krypton is slow-moving and denser than both air and argon.
Krypton increases the energy efficiency of a window, provides better insulation, and reduces the convection currents (currents of warm and cool air within the window), all of which prevent heat loss. It outperforms argon in insulation, but it is used less frequently due to its higher cost. It is often found in triple-pane windows, where the gaps between panes are slimmer.