Double Glazing describes windows that consist of two sheets of glass (triple glazing describes windows that consist of three sheets of glass).
The purpose of Double Glazing is to insulate a building better. A great amount of heat is lost through window space as windows are thinner than walls. To reduce heat flow through the window space, Double Glazing windows are installed. A gap of air between the two sheets of glass reduces heat flow and helps keep a room warmer than the outside temperature.
The effectiveness of Double Glazing is dependent upon the distance between the two sheets of glass: if the distance between the two sheets is too great or not great enough, the Double Glazing will not be effective. Optimal performance is considered to be achieved at a distance of about 18mm.
The air in between the sheets can be almost entirely removed, creating a vacuum. This further reduces the conduction of heat. In addition, gases are sometimes used to fill the space instead of air.