Double glazing involves using two window panes separated by a gap held by a wooden frame or any suitable plastic material. The purpose of this gap between the window panes is insulation. The gap can be filled with an inert gas such as argon, xenon or krypton. Argon is the least expensive of these three and also the least effective. The gap between the panes can also be a vacuum. The gas-filled gap or vacuum between the windows reduces the heat transfer rate from one side of the glass to the other.