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Energy efficient glazing

An average home loses 10% of its heat through windows and doors. Good, energy efficient glazing reduces this heat loss, keeps the home warmer and reduces heating bills. It can also make the home quieter.


Double-glazed windows have two sheets of glass with a gap between them which is usually 16mm wide. It is this gap which creates an insulating barrier to slow down the rate at which heat escapes from the home. Triple-glazed windows are now also available which have three sheets of glass, and therefore two insulating gaps, to provide even better insulation.

How to choose a double-glazed window

Choosing replacement windows for a property can be a complicated process; there are many factors to consider including the type of glass, the windowframe material and whether the windows are suitable for the style of property. Here are some tips you can follow when looking for replacement windows.

The most energy efficient glass for double glazing is low emissivity (Low-E) glass. This type of glass often has an invisible coating of metal oxide on one of the internal panes of glass next to the gap. The advantage of this type of glass is its ability to let light in but cut down on heat loss. In addition, the gap between the two panes of glass in very efficient double glazing is filled with an inert, non-toxic gas. This gas has a greater density than air, so it more effectively reduces the heat loss from the property. Argon is the gas most commonly used in double-glazed windows, as it is extremely cost-effective, colourless, is non-flammable and does not react to other gases.

When your windows are replaced it is important to make sure that the property remains well ventilated, as the replacement windows will be more airtight than the originals. Ventilation is necessary as it allows fresh air into your home, and allows moisture to escape. If your property does not have much natural (passive) ventilation, look for replacement windows which have trickle vents. These allow a small amount of controlled ventilation.

To help you chose the best windows, the British Fenestration Rating Council (BFRC) has developed an energy efficiency rating scheme using an scale from A+ to G similar to those used for things like fridges and washing machines. When calculating the energy rating the entire window – both frame and glass – is taken into account. There are two other rating systems for windows provided by Certass and BSI. The most efficient windows may also carry the Energy Saving Trust Recommended logo.

How long does double glazing last?

The typical lifetime of double glazing is around 20 years, however this can vary from 10-35 years based on quality of materials, installation and where the windows are situated. Over time, the gas within the panes will gradually leak out. When around 25% of the gas has escaped, the thermal performance of the windows will be reduced and replacing the windows or installing a secondary glazing measure should be considered (see our page on secondary glazing here).

Listed properties or in a conservation area 

If your property is a listed building or in a conservation area there are likely to be restrictions on what you can do to your windows, so it is always advisable to contact your local authority before carrying out any work. If you are not able to replace the windows, there are several non-intrusive alternatives which can improve the energy efficiency and warmth of your home. These include putting up heavy, lined curtains which are closed at dusk, shutters, sealed blinds or secondary glazing - or a combination of these measures.

Choosing an installer

You can find a good double glazing installer through one of these organisations:

Fenestration Self-Assessment Scheme (FENSA) is a competent person scheme set up by the Glass and Glazing Federation (GGF) and other industry bodies. FENSA enables companies that install replacement windows and doors to self-certify compliance under the Building Regulations without the need for an assessment from Building Control:

www.fensa.co.uk | 0207 645 3700

The Double Glazing & Conservatory Ombudsman Scheme (DGCOS) offers a free service to investigate complaints about work carried out by its members. This includes free independent inspections, arbitration and a compensation fund. Installers accredited under DGCOS must offer deposit protection and a comprehensive guarantee to customers:

www.dgcos.org.uk | 0845 053 8975


Image: Energy Saving Trust

Frequently asked questions

What is low 'e' double glazing?

Low emissivity (low E) double glazing incorporates a very thin layer of metallic coating on one surface. This coating allows the sun's heat to enter the building but significantly reduces heat loss from the building by reflecting radiant heat back into the room.


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What are trickle vents?

A trickle vent is a small adjustable opening in a double glazing unit that allows a small amount of ventilation in rooms that are double glazed. This is required because fitting double glazing reduces the amount of natural (or passive) ventilation in the home.


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We can advise you about saving energy, or help you understand what grants and support you're eligible for:

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Why does condensation appear on my windows?

Air contains varying amounts of water vapour. Warm air holds more water as vapour than cold air. If warm moist air is cooled by a cold surface like a window pane it will not be able to hold the same amount of water vapour, therefore the water turns into droplets of liquid and collects on the cold surface as condensation.


Need more help?

We can advise you about saving energy, or help you understand what grants and support you're eligible for:

Contact us Or freephone: 0800 082 2234


Next question

View all frequently asked questions

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