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Energy efficient windows

Energy efficient glazing helps reduce your carbon footprint and your energy bills, whether with double or triple glazing, secondary glazing, or just heavier curtains.


Benefits of energy efficient windows include:

  • A more comfortable home - energy efficient glazing reduces heat loss through windows and means fewer draughts and cold spots.
  • Peace and quiet - not only do they keep the heat in, energy efficient windows insulate your home against external noise.
  • Reduced condensation - energy efficient glazing reduces condensation build up on the inside of windows.


wooden frame windows

Installing energy efficient glazing

Before installing double glazing, check with your local planning office if any of the following apply to your property:

  • You live in a conservation area.
  • You have an article 4 direction on your property, removing the right of permitted development.
  • You live in a listed building.

Most people have double glazing fitted by a professional. Some secondary glazing can be fitted by competent DIYers, but removing old windows and installing new ones should be carried out by a professional.

man installing a pvc window

Energy efficient window glazing explained

  • Double-glazed windows have two sheets of glass with a gap in between, usually about 16mm, to create an insulating barrier that keeps heat in. This is sometimes filled with gas.
  • Triple-glazed windows have three sheets of glass, but aren’t always better than double-glazed windows. 

section of windows showing frame and layers of glass

Window materials

Energy efficient windows come in a range of frame materials and styles. Performance criteria vary according to how well they stop heat from passing through, how much sunlight travels through the glass and how little air can leak in or out around the window.


The most energy efficient type of glass for double glazing is low emissivity (Low-E) glass. This often has an invisible coating of metal oxide, normally on one of the internal panes. 

It lets in light and heat, but cuts the amount of heat that can escape. ​

Gaps between the glass  

Very efficient windows might have gases such as argon, xenon or krypton filled in the gap between the sheets of glass.

Pane spacers

These are set around the inside edges to keep the two panes of glass apart. For maximum efficiency, look for pane spacers containing little or no metal – often known as ‘warm edge’ spacers.

Frame materials

For all frame materials there are windows available in all energy ratings.

  • uPVC frames last a long time and may be recycled.
  • Wooden frames can have a lower environmental impact, but require maintenance. They are often used in conservation areas where the original windows had timber frames.
  • Aluminium or steel frames are slim and long-lasting, and can be recycled.
  • Composite frames have an inner timber frame covered with aluminium or plastic. This reduces the need for maintenance and keeps the frame weather-proof.

sash window in profile

Costs and savings

England, Scotland and Wales


Northern Ireland


Understanding energy ratings

Some window manufacturers show the energy efficiency of their products using an energy-rating scale from A++ to E. The whole window (the frame and the glass) is assessed on its efficiency at retaining heat.

The scheme is run by the British Fenestration Rating Council (BFRC).To choose the most energy efficient window, look for the BFRC rating.


Windows that have an energy rating will have the u-value of the window displayed on the energy label.

A u-value is a measure of how easily heat can pass through a material.

  • Materials that let out more heat have higher u-values.
  • Materials that let less heat pass through them have lower u-values. 

In some cases, windows with a higher energy performance rating might have a higher u-value than windows with a better energy efficiency rating. This might seem the wrong way round as lower u-values indicate better insulation levels. However, in these cases it will be that there are other aspects of the window that make them better overall such as coating used on the glass and the gap between the glass panes.

side section of window with energy ratings behind

Finding an installer

Double glazing

The Glass and Glazing Federation (GGF) is a membership organisation whose members sign up to a consumer code. This means that you should receive excellent customer service.

If you use one of their members to fit your windows but you are unhappy with the work, you will also be able to use their free reconciliation service. You can use the Glass and Glazing Federation's website to find a member who works in your area

Building regulations

In England and Wales, the easiest way to make sure your windows are fitted to the Government’s building regulations standards is to choose an installer who is registered with one of the official Competent Person schemes. Installers registered with these schemes will give you a certificate when the job is finished that states your new windows have been fitted in compliance with the regulations. Registered schemes for windows can be found at Competent Persons.

If you use an installer who isn’t registered with one of these schemes, you will need to apply for building control approval before installing the window. Visit Planning Portal to find out how to apply for building control approval.

In Scotland, each local authority has a building standards office. Check with them first to see whether you need a building warrant for your new windows. 

Secondary glazing

As secondary glazing is more specialised than double glazing, there isn’t currently a central body that certifies these installations. Always get a number of quotes to get the best deal.

man installing glass with suction grip on the pane

Windows in conservation areas and period properties

Conservation areas

If you live in a conservation area or in a listed building, there may be restrictions on what you can do to your windows.

These areas are of special architectural or historic interest, meaning that any work you carry out on your home must preserve or enhance the character of the area. This does not necessarily mean you cannot replace your windows, but it might mean you will need to get windows that complement the character of the building and area.

An increasing number of double glazing companies offer double glazing in period properties. Double glazing can be made to look like your building’s original windows, but for any changes you do need to contact your local council’s conservation officer for guidance, and there are a number of non-intrusive window insulation options available for historic homes such as heavy lined curtains, shutters, secondary glazing and sealed blinds. However, each historic building is considered individually so check with your local council to see what options are available to you.

Listed buildings

Listed buildings have tight controls on what you can change on the outside and sometimes the inside as well, depending on their grading. Old sash windows in historic properties can be protected not only for their appearance, but also the materials and methods used to make them. But secondary glazing can be a non-intrusive way of insulating historic windows from the inside, and may be granted permission.

There are other ways to make historic buildings more energy efficient, but you will need to consult with, and apply for permission from, your local planning authority.

Visit Historic Scotland and English Heritage for ways to make a historic home more energy efficient.

Sash windows

Sash window units are common features of period properties and can be a design feature. They consist of two vertically sliding frames, but are often badly fitting and made of single pane of glass which therefore gives them poor insulating properties.

If you want to insulate your sash windows, there are a number of alternatives to conventional double glazing. If you want to keep the design and look of the sash windows, there are units available that are in keeping with the original design; these are fitted and sealed to prevent draughts and incorporate double glazing to reduce heat loss. The frames don’t need to be plastic, but can be metal or wood with an insulated core.

Replacing sash windows can be expensive, so good-quality secondary glazing may be worth considering. 
sash windows on an old police station

Doors and conservatories

Energy efficient doors

Like any other part of the home, doors can be insulated and draught-proofed to prevent heat escaping. Building regulations state that installing a new door requires approval from the relevant buildings control body, and new external doors now generally contain integrated insulation to reduce heat loss and comply with regulations.

A properly fitted new external door should include an effective draught-proofing system. Existing doors can be improved by fitting draught-proofing strips around the seals and the letterbox.

Fitting draught-proofing to the doors and windows will save the typical household around £20 a year.

man sealing in windows


Even the best quality glazing loses heat more quickly than an uninsulated cavity wall. This means that conservatories are not thermally efficient and should not be heated.

Provided they are never heated, and the doors between the conservatory and the heated house are kept shut in cold weather, they can actually reduce heat loss by acting as an extra insulating layer outside your house. You can make the most of this by installing a sealed sliding door, and sealed blinds or heavy, lined curtains to separate the conservatory more effectively from the rest of your house.

If you heat your conservatory, any benefit you may have had will soon disappear along with the heat that escapes into the outside air. Double glazing, blinds and shutters can all reduce the amount of heat wasted, but it is not possible to bring a conservatory up to the thermal standard of even an averagely insulated room.

 conservatory windows


Replacement windows will be more airtight than your original frames, so condensation may build up in your house due to the reduced ventilation. If your house does not have much background ventilation, look for replacement windows with trickle vents incorporated into the frame to let in a controlled amount of ventilation.

If you start to see condensation building up around your windows, there may be a damp problem in your home. As a general rule, damp occurs when there is inadequate ventilation, inadequate heating, inadequate insulation or a combination of these. If you’ve started to notice condensation in between the panes of glass in your double-glazing units, then it is likely that the seal is broken, and the unit will need to be replaced.

water condensation on windows

Alternatives to double glazing

If you can’t install double glazing – for example, if you live in a conservation area, period property, or listed building – you can install secondary glazing, or use heavy curtains, or do both. Find out how to fix the draughts around your windows by visiting our draught-proofing pages.

Secondary glazing

A secondary pane of glass and frame can be fitted inside the existing window reveal. This won’t be as well sealed as a double-glazing unit, but will be much cheaper to fit, and will still save energy. Low emissivity glass will improve the performance of secondary glazing.

Secondary glazing kits are available for the proficient DIYer – these cut down on costs and are a non-intrusive way of insulating your windows.

Curtains, sealed blinds and shutters

Curtains lined with a layer of heavy material can reduce heat loss from a room through the window at night and cut draughts. Hollow blinds, fitted into place with a sealed frame, and sealed shutters will also help cut draughts and keep your heat in for longer.
older woman opening curtains by window

More information

Damp and condensation

If you experience condensation dampness in your home, watch our video explaining how you can reduce it.

Watch the video


Do you know how much you could save on energy bills if you draught-proofed your home? Find out more here.

Learn more

EST Register

Browse our database of energy efficient products, which includes a category dedicated to windows.

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