Insulating Glass w/ Argon Gas
Argon, a safe, odorless, non-toxic gas which is 40% denser than air, significantly slows down the transference of heat and cold from pane to pane. The Argon can be upgraded to Krypton, which is 96% denser than air, to provide considerably more insulation value and significantly reduce heat transfer as well as energy bills.
The air-filled windows and argon-filled windows look exactly the same. It is also important to note that argon gas is nontoxic. As such, a cracked or broken argon window is just as harmless as an air-filled window.