A single pane of glass provides very poor insulation because glass is a good conductor of heat. So much of the heat in your room literally goes out the window.
Install double glazing and you have a layer of air trapped between two panes of glass. Because air is a poor conductor of heat, much less heat is lost through the window.
If the inside of one sheet has Low E coating, even less heat is lost because the coating reflects heat back into the room.
Your home's double glazing operates just like fibreglass batts and woollen clothing. It traps a layer of air between two panes.
Air is a very poor conductor of heat, so the trapped layer sets up a blanket of protection between cold air on one side and warm on the other.
Glass itself is a very good conductor. So in a home without double glazing, heat is going straight out the window.
Heating (or cooling) bills are reduced in the double-glazed home.
Your supplier, a member of IGUMA, will advise you of the extra benefits of using Low E glass in the double glazing. Low Emissivity glass is completely transparent but provides an ultimate reduction of heat loss.
Sit next to a window in a double-glazed home. Notice the absence of draughts and cold spots.
See, too, how messy condensation is dramatically reduced.
Special glass types can be used in double glazing for safety toughening, security, fire resistance of designer looks.