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Energy Efficient Glazing – the basics

All properties lose heat through their windows. Installing energy efficient glazing is an effective way of reducing your energy bills and keeping your home warmer and quieter.

Double glazed windows use two sheets of glass with a gap between them which creates an insulating barrier, whilst triple glazed windows have three sheets of glass. Both options can deliver a high level of energy efficiency. It is not the case that you have to use triple glazing to gain the most energy efficient window.

Energy efficient windows are available in a variety of frame materials and styles. They also vary in their energy efficiency, depending on how well they stop heat from passing through the window, how much sunlight travels through the glass and how little air can leak in or out around the window.

Some window and door manufacturers helpfully use a window energy rating scheme to show the energy efficiency of their product. This is similar to the one you may have seen on appliances such as your fridge, or washing machine. A-rated windows are the most efficient. To check a window’s energy efficiency before you buy, look at the energy label.

The benefits of double glazing

  • Smaller energy bills: replacing all single glazed windows with energy efficient glazing could save you around £135 per year on your energy bills.
  • A smaller carbon footprint: by using less fuel, you’ll generate less of the carbon dioxide (CO2) that leads to global warming.
  • A more comfortable home: energy efficient glazing reduces heat loss through windows and means fewer draughts and cold spots.
  • Peace and quiet: as well as keeping the heat in, energy efficient windows insulate your home against unwanted outside noise.
  • Reduced condensation: energy efficient glazing reduces condensation build-up on the inside of windows.

The costs and savings of double glazing will be different for each home and each window, depending on the size, material and installer. To see how much you could save on your energy bills, take a look at the table below:


  £ / yr kg CO2/yr
Double Glazing £135 720


Choosing the right replacement windows

Replacement windows come in a range of styles and designs and there are particular features you should look out for to increase energy efficiency.

To find the right windows for your home, ask yourself these questions:

How energy efficient are the windows?

When choosing replacement windows, you can check its energy efficiency by looking at the Energy Saving Trust Recommended logo and BFRC energy label. The Energy Saving Trust endorses any windows rated C or above. The higher the energy rating, the more energy efficient it is. Unfortunately, at the moment there is no obligation for window manufacturers to label their products, however by opting for a high rated window you know you will be buying the most efficient.

For a list of all windows and their frame material and energy rating, visit the BFRC website.

How many layers of glass do you need?

Double glazing has two layers of glass with a gap of around 16mm between them. There’s also the option of triple glazing, which has three layers of glass. Both A rated double and tripled-glazed windows are available.

What type of glass is best?

The most energy efficient glass for double glazing is low emissivity (Low-E) glass. This often has an unnoticeable coating of metal oxide, normally on one of the internal panes - next to the gap. It lets sunlight and heat in but cuts the amount of heat that can get out again.

What’s between the panes?

Very efficient windows might use gases like argon, xenon or krypton in the gap between the 2 sheets of glass.

What keeps the panes apart?

All double glazed windows have pane spacers set around the inside edges to keep the two panes of glass apart. For a more efficient window, look for pane spacers containing little or no metal – often known as “warm edge” spacers.

The BFRC window energy rating scheme checks all the components to ensure the final window achieves the energy efficient standard claimed. This means that you just need to look for the A-G ratings and remember A is best! Alternatively, just look for the Energy Saving Trust Recommended logo which will only be found on glazing that is C rated or above.

Which frame suits your home?

The frame you choose will depend on your home and your personal taste. For all frame materials there are windows available in each energy rating.

  • uPVC frames are the most common type. They last a long time and can be recycled.
  • Wooden frames can have a lower environmental impact, but require maintenance. They are often used in conservation areas where the original windows were timber framed.
  • Aluminium or steel frames are slim and long-lasting. They 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 weatherproof

Do you need ventilation?

Because replacement windows will be more airtight than the original single glazed frames, condensation can build up in your house due to the reduced ventilation.

If there is not a sufficient level of background ventilation in the room some replacement windows will have trickle vents incorporated into the frame that let in a small amount of controlled ventilation.

Condensation can sometimes occur on the outside of new low-e glazing. This is because low-e glass reflects heat back into the home and as a result the outside pane remains cool and condensation can build up in cold weather – this isn’t a problem.

Installation and maintenance


When you plan an installation, you need to know about building regulations and what to do if double glazing doesn’t suit your property, as well as how to maintain your windows.

When you think about replacement glazing, you need to make sure your windows are installed correctly and comply with all the relevant regulations.

Building regulations

Under building regulations in England and Wales new and replacement windows must meet certain energy efficiency requirements:

  • new windows (i.e. in extensions, or where there was not previously a window) must be at least Window Energy Rating (WER) band D rated or U-value 1.8
  • replacement windows
    • In England & Wales must be at least WER band E or U-value 2.0 or centre pane U value 1.2.
    • In Scotland must be at least WER band D or U value 1.8 or centre pane U value 1.2.
    • In Northern Ireland must be at least WER band E or U value 2.0 or centre pane U value 1.2.

However, if you live in a conservation area, have an article 4 direction on your property or have a listed building, additional regulations are likely to apply. Before you do any work, make sure you check with your local planning office.

- an article 4 direction removes the right of permitted development, meaning that you will have to apply for planning permission before replacing any windows. This is often applied in Conservation areas.

How to comply with regulations

To make sure regulations are complied with, there are certain rules about the way you can install windows:

  • for DIY installations you must apply for building control approval before installing the windows. See how to apply for building control approval
  • for professional installations, your installer should be registered with a competent persons scheme or register the installation through Local Authority Building Control.
  • Competent Persons schemes in England and Wales are the Fenestration Self-Assessment Scheme (FENSA), the British Standards Institution (BSI) or Certass Glazing Scheme

Find registered installers

FENSA guarantees that its installers and frames comply with building regulations. To find a FENSA registered installer, visit the FENSA website.

Certass is another scheme that registers and approves installers. To find a Certass registered installer visit the Certass website.

Ask your installer when you will get a certificate when installation is completed, which demonstrates the installation has been completed in compliance with building regulations.

Other options for your windows

If you can’t install double glazing – for example if you live in a conservation area or in a listed building – you have other options:

Heavy curtains

Curtains lined with a layer of heavy material can reduce heat loss from a room through the window at night and cut draughts.
They will save some energy, but should only be used as a short term measure.

Secondary glazing

Secondary glazing works by fitting a secondary pane of glass and frame, inside the existing window reveal.

This is likely to be less effective than replacement windows. The units tend to be not as well sealed, however it is considerably cheaper than double glazing.

It is also an ideal solution if you are unable to fit double glazing because you live in a conservation area or in a listed building.

Low emissivity glass is available for secondary glazing, which will improve the performance.

  £ / yr kgCO2/yr
Secondary glazing £85 460

Maintaining and replacing windows

As double glazing should last for more than 20 years or more, you’re unlikely to upgrade often so it’s worth installing windows with a good energy rating straight away. Looking for the Energy Saving Trust Recommended logo is an easy way to ensure you are buying windows with a good energy rating.

Sometimes your windows will need maintenance, for example if the seal within the unit between the two sheets of glass fails. This leads to a build-up of condensation between the panes, and you may need to replace the glass unit – but you can usually do this without replacing the frame.

As with installation, windows should only be replaced by registered installers or checked through the building control process.

Getting help and advice

Advice on saving energy

We offer free, expert, impartial advice on all kinds of energy saving measures. We can help you choose double glazing, find installers and see if any grants are available. Just call the Energy Saving Trust helpline on 0800 512 012.

Double glazing products

Energy Saving Trust Recommended accredits double glazing products which are C rated or above - better than the current building regulations standard.

The British Fenestration Ratings Council (BFRC) rates all energy efficient windows in the UK. To find out more visit the BFRC website.

Registered installers in England and Wales

  • The Fenestration Self-Assessment Scheme (FENSA) can help you find registered installers. To find out more visit the FENSA website.
  • Always use an installer who is a member of a Trade Association.
  • This has two benefits: Firstly, it will protect your consumer rights and secondly it will ensure that the company replacing your windows and doors is competent to do the job.
  • Go to the Glass and Glazing Federation who can provide details of installers at your location.



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