What is Double Glazing?
Double glazed windows use two sheets of glass with a gap between them that creates an insulating barrier. The gap can either be filled with a heavy inert gas such as Argon or it can be a vacuum; both of these act to slow the movement of heat from one side of the window to the other.
What is Secondary Glazing?
Secondary glazing involves installing supplementary glazing on the inside of an existing single-glazed window. It can be permanent, although often secondary glazing panels are added as a temporary measure.
Double-glazing is considerably more expensive than secondary glazing, the reason being that if you decide to install double-glazing, you need entirely new units fitted – you cannot simply retrofit a second sheet of glass to single glazing.
In order for double-glazing windows to be effective thermal barriers, the gap between the panes of glass needs to be airtight and filled with an inert gas (or a vacuum). Hence for each window where you want double-glazing, a company will need to come around to your house and measure up and produce the windows specifically for you.
Secondary glazing is far cheaper – you can buy DIY kits or get installers to come and fit the supplementary glazing onto the inside of the existing window frame.