What is Double Glazing?

A double-glazed window consists of two sheets of glass separated by an air gap. The air gap provides an insulating layer that reduces heat loss and sound transmission. Condensation is also reduced, because the inner layer of glass remains warmer than the glass on the outside.

Both factory-sealed units and secondary glazing are used for domestic double glazing.

Sealed units are unobtrusive; secondary glazing is a cheaper option that helps to reduce the intrusion of noise from outside. Both provide good thermal insulation.

What size air gap?

For heat insulation, a 20mm (¾4in) gap will give the optimum level of efficiency. If the gap i less than 12mm (½ in), the air can conduct a proportion of the heat across it.

It's greater than 20mm (¾4in), there is no appreciable gain in thermal insulation, and air currents can transmit heat to the outside layer of glass.

For noise insulation, an air gap of 100 to 200mm (4 to 8in) is more effective. Triple glazing - a combination of sealed units with secondary glazing - may therefore prove to be the ideal solution.

Although the amount of heat lost through windows is relatively small, the installation of double glazing can halve the wastage. As a result, you will find there is a saving on your fuel bills.

But the benefit you will be aware of more immediately is the elimination of draughts. In addition, the cold spots associated with large windows (most noticeable when you're sitting still) are likely to be reduced.

Installing double glazing with good window locks will improve security against forced entry, particularly when sealed units or toughened glass are used.

However, make sure that some accessible part of appropriate windows can be opened in order to provide an escape route in case of fire.

Double-glazed sealed units

Double-glazed sealed units consist of two panes of glass that are separated by a spacer and hermetically sealed all round. The gap may contain dehydrated air - which eliminates condensation between the two panes of glass - or inert gases, which also improve thermal and acoustic insulation.

The thickness and type of glass used are determined by the size of the unit.

Clear float glass or toughened glass is commonly employed. When obscured glazing is required to provide privacy, patterned glass is used. Heat-retentive sealed units, incorporating special low-emissivity glass, are supplied by some double-glazing companies.

Generally, factory-sealed units are produced and installed by suppliers of ready-made double-glazed replacement windows.

Square-edged units are available for frames with a deep rebate, and stepped units for window frames that were originally intended for single glazing.

Double-glazed sealed units with metal-reinforced uPVC frames are rarely suitable for older houses. A secondary system that leaves the original window intact is generally more appropriate - especially if you have attractive leaded windows, which should be preserved (sealed units with a modern interpretation of leaded lights are not an adequate substitute for the real thing).

If you are located in Scotland, ask the Homesealed sales team for the best advice for windows in your home.