What are double glazed windows?

An insulated glass unit (IGU) combines multiple glass panes into a single window system. Most IGUs are double glazed (two panes of glass) with three panes (triple glazing) or more becoming more common due to higher energy costs. The panes of glass in IGUs are separated by a spacer and a still layer of air or gas. The glass is then fitted into window frames, which is made wider to accommodate the two panes.

The benefits of double glazing

Double glazed windows are an ideal energy efficient choice with the added benefit of minimising noise. The sealed air gap between the two panes acts as an added layer of insulation. This added thermal resistance reduces the amount of heat escaping in winter and keeps your home at a more comfortable temperature. Double glazing has the reverse effect in summer, preventing unwanted heat from coming into the home. This extra insulation lessens your reliance on artificial heaters and air conditioners and can ultimately reduce your energy costs.

When you are close to a window, your comfort is also affected by the temperature of the glass. With double glazing it’s harder for the unwanted outside temperature to transfer through, leaving the inside pane close to room temperature. Double glazing also reduces condensation which can result in the unhealthy formation of mould.

Sealed double glazing is effective at reducing medium to high frequency noise such as the human voice. A difference in glass thickness between the inner and outer panes will improve sound reduction even further.

Double glazed windows are considered a safer option when compared to standard single pane windows. Two sheets of glass are a lot harder to break than one and for even greater security you can specify toughened or laminated glass.

What to look for

Amount of space between panes

The typical space between panes ranges from 6mm to 20mm. A minimum space of 12mm is recommended for optimum thermal performance. Alternatively, for good acoustic control and to reduce low frequency noise such as traffic and aircraft, the optimum air gap recommended is 150mm or over. Note that such large gaps allow convection to occur between the panes and reduce insulating performance.

What’s in between

Because the space between the panes in double glazed windows is fully sealed, it acts as an insulator, limiting the transfer of cold air coming into your home. Thermal and acoustic performance can also be increased when gas fills this space. The most popular gas used is argon which has low conductivity properties to improve insulation.

Double glazed units include a spacer which is a metal or polymer strip that separates the two panes of glass. Typically, spacers contain a desiccant (drying agent) to remove moisture trapped in the air space.

Type of glass used

A wide range of different glass types, such as low-e and laminated, can be used in double glazing units to further increase energy efficiency and noise control. Low-e glass will further reduce the amount of heat escaping while thicker laminated panes disrupt sounds waves to improve acoustic performance.

Common problems

Windows need to be considered as a whole unit. The framing material you choose to complement your glass may enhance its performance or in some cases reduce its energy efficient properties. Standard aluminium window frames readily conduct heat and cold and if not thermally enhanced, may eliminate any benefit from installing expensive double glazing.

How well the cavity is sealed and the type of spacer used is also an important factor to consider. If the double glazed unit is not sealed properly or if the spacer does not contain adequate desiccant, it can reduce performance and condensation will appear on cold surfaces.