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What is Double Glazing?

Garry Crystal
Garry Crystal

Double glazing is the glazing process in which a window is made with two panes of glass with a space between them. The space between the glass is usually between 0.63 and 0.75 inches (16 and 19 mm) wide. In many cases, air is trapped between the panes and forms a layer of insulation. Before the unit is sealed, a drying agent is added to ensure that no moisture is present inside the finished glass unit.

Windows of this type must be airtight. If condensation appears between the panes, it indicates that there is a leak somewhere. These windows cannot be repaired; if condensation appears, the unit must be replaced.

Air space between two panes of glass in a double glazed window section.
Air space between two panes of glass in a double glazed window section.

Around 60% of heat loss in the home occurs through standard, single pane windows. Double glazing substantially stops heat loss, and the cost of the windows will often pay for itself very quickly in the money homeowners save from heating bills. Once the windows have been installed, heating costs should decrease by around 10 to 12%.

Double glazing is also very friendly to the environment. Homes cause around 28% of all carbon dioxide emissions, and replacing single pane windows helps to reduce these emissions and combat energy loss. As well as saving on heating bills, these windows are very good at cutting down on noise pollution and internal condensation.

Double glazed window.
Double glazed window.

Another reason to have this type of window fitted is the security it provides to a home. The main entry point for an intruder is through windows and doors. Double glazed windows provide extra security, as they are very hard to break and have their own locking system.

Double glazing also usually enhances the appearance of a property. There are a number of styles to choose from in terms of frames for the glass, including unplasticised polyvinyl chloride (UPVC), aluminum and hardwood. UPVC has better insulating properties than aluminum, but aluminum may be more aesthetically pleasing in older properties. Frames come in a wide range of color options.

With such windows, homeowners also have the option of secondary glazing on their existent windows. A house may have sound windows, but the owner may want to have the benefits that double glazing can provide. This can be achieved by fitting an extra glazed window internally against the original window. This can be a cheaper option than having a whole unit fitted and will keep the former look of the windows intact, as the secondary glazing is fitted internally.

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Discussion Comments


The article stated if moisture appears between the panes, not on the panes then it would need to be replaced. Humidity would not cause that to occur.

There are many attractive double panes available, but purists might still wish to use non uPVC windows. However, many composite materials are available that look just like real wood, with less maintenance.

I have found that payback is not so quick. It would depend on just how bad your old windows were, and the environment that the house is exposed to.


Could someone please explain "PT Double Glazing"? -- Alex.


Many British traditional homes, especially Victorian cottages, have been mutilated by the appearance of uPVC windows.

It is not necessary to fit double-glazing in order to prevent heat loss; secondary glazing (much cheaper) and thick curtains combined will do an excellent job.

The insertion of uPVC windows into a period property has a degrading visual effect, since the stark whiteness of the plastic clashes with the traditional materials.

As for payback being quick, it is utter nonsense. It will take years to recover the cost of double-glazing based on heating bills. Not to mention the devaluation on your property, especially if it is an attractive period one.


If condensation appears on a double glazed window; it does not mean that the window is at fault; in most cases, it is high humidity in the home. A better suggestion would be to see how high the humidity is in your home and things like placing plants on windows sills are a contributor as well.

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    • Air space between two panes of glass in a double glazed window section.
      By: stocksolutions
      Air space between two panes of glass in a double glazed window section.
    • Double glazed window.
      By: antiksu
      Double glazed window.