When a window is made by using two glass panes, it is known as double glazing. A spacing of a few millimetres should exist between the two panes and the sealing should also be airtight. Moisture between the panes must be avoided by use of a drying agent before sealing the gaps. Suitable spacing must be determined so as to ensure that a good amount of air is kept in. This air acts as insulation to the house since it has been known that heating bills reduced with secondary glazing.
