The air sealed between two panes of glass in some windows helps to <em>reduce the flow of heat passing through the window</em>. Even though the windows are more expensive to build, that little bit of trapped air in there is a very effective INSULATION. It makes the house easier to keep warm in Winter, and easier to keep cool on Summer, and both of those save on the cost of gas or electricity for the house.
The answer is: reduce the flow of heat passing through the window.
A double glazing window or insulating glass consists of two glass window panes separated by a spacer bar and seals to create a hermetically-sealed environment. Space between glass panes is filled under vacuum with either dehydrated air or an inert gas to improve insulation and prevent condensation within the unit. The physical properties of the dehydrated air used in the space between glass panes mean that it transmits less heat than normal air, this is because the molecules of dehydrated air are relatively inert and therefore have a lower thermal conductivity.
