Double Glazing

What is double glazing?

All properties lose heat through their windows. Installing energy efficient double-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.

The space between the glass panes can be filled with either a vacuum (quite rare nowadays because they require excellent sealing, otherwise the vacuum diminishes so the efficiency decreases), or a heavy inert gas such as Argon, Krypton or Xenon. Both these methods are trying to create a more effective insulating barrier, known scientifically as increasing the R-value (which is the measure of thermal resistance).

Energy efficient double-glazed windows are available in a variety of frame materials (including uPVC and more traditional wood) and styles. These windows vary in their energy efficiency, depending on how well they stop heat from passing out through the window, how much sunlight travels through the glass and the amount of air that can leak in or out around the window.

Some double-glazing 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 its energy label.

Questions to ask yourself before investing in double glazing:

1. How energy efficient are the windows?

When choosing replacement double-glazed windows, you can check their energy efficiency by looking at the Energy Saving Trust Recommended logo and British Fenestration Rating Council (BFRC) energy label. The higher the energy rating, the more energy efficient it is.The Energy Saving Trust endorses any windows rated B or above. Unfortunately, at the moment there is no obligation for window manufacturers to label their products; however by opting for a highly-rated window you know you will be buying the most efficient.

For a list of all types of double-glazed/triple-glazed windows and their frame material and energy rating, visit the BFRC website.

2. How many layers of glass is best?

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.

3. 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.

4. What is between the panes?

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

5. 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 B rated or above.

6. 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.

7. Do you need ventilation?

Because replacement double-glazed 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 – you don’t need to worry about it.

Benefits of installing double glazing

Smaller energy bills: replacing all single-glazed windows with energy efficient double-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. Savings will also vary depending on how much you currently pay for your heating fuel; these savings are based on a gas-heated home.

Installing double glazing

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 and replacement windows in existing homes in England and Wales must be at least WER band C or U-value 1.6 In Scotland must be at least WER band C or U value 1.6 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 four’ 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. 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 after installation is completed, which demonstrates the installation has been completed in compliance with building regulations.

Other options for improving the energy efficiency of your windows

If you can’t install double-glazing (e.g. 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, as the units tend to be not as well sealed, however it is considerably cheaper than double-glazing. Low emissivity glass is available for secondary-glazing, which will improve the performance.

Benefits

    • Double glazing keeps the heat in your house, reducing your utility bills as less heating is required to heat your house.
    • Approximately £150 per annum can be saved based on a 3 bed semi-detached & a B-rated product or above, this has the potential to lower CO2 emissions by 680kg per year.
    • Double glazing acts as a sound proofing mechanism, which makes it an ideal solution if you live near a busy road.
    • There are many different designs available for you to choose from to match the style of your home. Secondary glazing (which acts in a similar fashion to double glazing) can even allow you to keep your original windows in their current form.

Limitations

    • Double-glazed windows on occasion can get moisture trapped inside them if they are not installed properly, or are not completely air tight so have to be repaired / replaced.
    • During the summer, double glazing can end up trapping too much heat in the house making it uncomfortably warm.

Cost

    • The cost of double glazing your house depends on how many windows you are replacing, the type of frame you use (e.g. wood, uPVC) and the size of each of the frames. As rough guide, a sash uPVC window will cost you approximately £700, while a wooden framed equivalent will cost about £1,100.

The Energy Saving Trust endorses any windows rated B or above.