What is double glazing and how it works

Double glazing windows consist of two panes of glass separated by a space to reduce heat transfer and improve thermal insulation in buildings. This space is often filled with air or inert gases, which are poor conductors of heat. 

The dual glass and gas layers work together to create an insulating barrier, helping to keep warm air inside during the winter and hot air outside during the summer. This insulation is beneficial for reducing energy costs by minimising the need for central heating to maintain comfortable indoor temperatures. Additionally, double glazed windows can reduce noise pollution from the outside and increase the security and value of a property.

Our comprehensive guide explains everything about double glazed windows and their installation, from frame materials, styles, and windows prices, to the installation process and consumer rights.

The manufacturing process for double glazing products is quite straightforward. During the process, a double glazed unit is created by cutting two panes of glass according to set measurements. Then, the spacer bars are manufactured to size to ensure that they separate the two panes. 

These spacer bars are made using materials with a low thermal conduction rate, which reduces heat loss. One pane of glass is fitted with the spacer bar in position before being moved through an argon-filled chamber. The second pane is pressed on top to seal the unit and trap the argon gas, creating a vacuum.

Argon gas is used for double glazing because it is more dense, slow-moving and less conductive. This significantly reduces conduction through the gas because the density is greater than that of air. 

Once the frame is filled with argon gas, the unit is sealed around the edges to make the space airtight. This ensures that the gas can neither escape nor air enter, and maintains the frame’s structural integrity. Once complete, the units are ready for assembly using the chosen frame material.

Many homeowners are installing replacement windows to help reduce their heating bills as the cost of living crisis is still impacting household budgets. Besides lowering bills, double glazing offers several other benefits. 

1. Reduced heating bills
   
   Thanks to the insulating and solar gain properties of double glazing, your home will retain its warmth for longer, meaning that you can reduce your use of a boiler to heat your property. With around 18% of heat lost through windows and single glazing losing it twice as fast as double, you can save up to £235 annually. 
2. Increased security
   
   The glass in double-glazed windows is much stronger and more challenging to break than single panes of glass, and thanks to the design of the sealed units, it is almost impossible to force them open from the outside. Most models also have locks as standard, greatly adding to your home’s security measures. For even more security, choose toughened or laminated glass for windows that are hidden from the road or any neighbours.

3. Noise reduction
   
   If you live near a busy road, under a flight path or have loud neighbours, double glazing prevents that noise from causing a disturbance. The heat-insulating properties also stop sounds from outside from intruding, or noise from inside your home escaping.

4. Increased property value
   
   The addition of double glazing to your house improves its appearance and makes it attractive to potential buyers. Replacement windows can add as much as 10% to your property’s value.

5. Reduced condensation
   
   If your home is prone to condensation, it can encourage the spread of dampness throughout, giving soft furnishing and carpets a musty odour, causing mildew spores in the air and adversely affecting any woodwork. However, double glazing prevents this from occurring. Condensation happens when the window glass is cooler than the air inside the property, but thanks to the gap between the panes, the room’s warmth never meets the colder exterior pane.

6. Minimise interior damage
   
   Soft furnishings, pictures, wood and more can all be affected by sunlight streaming through your windows, but double glazing reduces the damaging UV rays. This prevents your furniture from suffering adverse effects, such as fading, and minimises temperature fluctuations that can cause wooden items to crack.

7. Eco friendly

   By reducing energy consumption and lessening the UK’s reliance on fossil fuels, double glazing is kinder to the environment. Changing from single-glazed windows to double allows you to decrease your carbon footprint by up to 410kg annually. The most popular material for double glazing frames is uPVC, and due to advancements in recycling technology and product innovation, this is recognised as a sustainable option. Window frames can be recycled as many as 10 times with no material degradation, meaning fewer go to landfill sites.

1. Difficult to repair
   
   Because double glazing is a sealed unit with a layer of gas between the two panes of glass, repairs are not as straightforward as single glazing. If the seal is broken and warm air is allowed to get between the panes of glass, the whole glass unit will have to be replaced, regardless if only one pane is broken. Blown double glazing repair can be costly. It is possible, however, to adjust uPVC windows, ensuring maximum efficiency is retained. 
2. Solar gain in the summer
   
   Double glazing windows can suffer from solar gain, whereby heat from sunlight becomes trapped inside your house. This can be useful in the winter because using sunlight to warm your home can reduce your reliance on central heating, but it may lead to an uncomfortable house during the summer. 

   Choosing low-E (low-emissivity) glass can resolve this issue. It features a microscopically thin, transparent coating that reflects thermal radiation or heat. This coating allows light to enter while preventing heat from escaping or entering, improving a window’s insulation properties. It helps keep interiors warmer in winter and cooler in summer, enhancing energy efficiency.

3. Not suitable for all properties
   
   While many companies have gone to great lengths to design double glazing windows that match traditional styles for older homes, there are restrictions on what kind of windows you can install in listed buildings, which may require planning permission to overcome. However, you can still improve your home’s energy efficiency by installing secondary double glazing, which is a pane of glass or plastic installed on the inside of a window that does not affect the outside appearance and doesn’t require planning permission.

Double glazing costs vary significantly based on materials, window type, and style. Your choice of installer can also impact the price, as some companies charge more than others for labour and materials. 

Although this might seem a significant investment, with potential savings of up to £235 per year and more if prices continue to rise, the breakeven point is nearer than ever.

According to Energy Savings Trust, installing A-rated replacement windows in an entirely single-glazed semi-detached property could save you £175 a year. If you replace single glazing windows with A++ rated double glazed options, you could save up to £235 annually.

There are several double glazing grants and loans available depending on your eligibility. The following schemes aim to reduce the costs of installing windows. 

Energy Company Obligation

The Energy Company Obligation, or the ECO4, is a scheme where energy suppliers who have signed up provide funding for energy-efficient upgrades. 

People who are part of the core group under the Warm Home Discount Scheme are eligible to apply or who receive at least one of the following benefits:

• Universal Credit
• Housing benefit
• Income-related Employment and Support Allowance (ESA)
• Income-based Jobseeker’s Allowance (JSA)
• Income Support
• Child Benefit
• Tax Credits (Child Tax Credits and Working Tax Credits)
• Pension Guarantee Credit
• Pension credit saving credit

LA Flex

LA Flex is an extension of ECO4, offering financial support to those not meeting the ECO4 criteria. Your local authority has funding available to assist families on lower incomes with the cost of installing energy-efficient home improvements, including double glazing windows.  

The British Fenestration Rating Council (BFRC) independently verifies a window’s thermal efficiency. This organisation developed a specific rating system similar to the one used on domestic appliance energy labels found on washing machines, freezers and fridges. 

According to the BFRC, double glazing windows are rated between A++, providing the highest level of energy efficiency, and E, the lowest. UK building regulations require that all new windows be at least C-rated. Most double glazing companies have developed units with different levels of energy efficiency, some as high as A++, allowing you to choose the best option for your home and your wallet. 

When buying double glazing, do ensure that BFRC has verified its rating.

Additionally, you’ll see the U-Value mentioned when looking for these energy ratings. This is a measure of how easily heat can pass through a material. Therefore, a higher U-Value means the amount of heat passing through is low. Some windows could have a low U-Value but a high energy rating because the energy rating looks at additional factors, such as how well the materials insulate and the type of gas or glass.

There are four types of window frames that you’ll most likely choose between during your double glazing windows installation process – uPVC, aluminium, composite and timber windows; but what are they, and what are the pros and cons of each? 

The below table examines their advantages and disadvantages and should help you pick the right one for your home.

Gone are the days of plain casement double glazing units, and today many styles are readily available, meaning you can match the architectural exterior of your property, enhancing its appearance.

Casement windows

Casement windows are the most popular type. They suit most UK properties and are designed to allow as much light as possible. The windows can be opened with hinges on either side or the top, opening inwards or out – the choice is yours.

Tilt and turn windows

Tilt and turn windows are usually incorporated into casement window units. The units have a dual-hinge system, allowing them to be opened by tilting inwards from the top of the more traditional side hinge. This makes them ideal if you have young children, as the tilt action prevents anyone from climbing out and having an accident.

Sash windows

Sash, or vertical sliding, windows are ideally suited to a period property or in Victorian and Georgian architecture. Modern mechanisms, such as easy-slide balances and tilt-in designs, facilitate smoother operation and easier cleaning. These windows maintain the characteristic appearance of classic sash windows, including the vertical or horizontal sliding panels, but with updated energy efficiency, security features, and low maintenance requirements.

When considering double glazing for your home, several types of glass options are available, each designed to meet different needs. Here’s a breakdown of the various choices of double glazing glass:

Low-E glass

• Best for cold regions where a lot of energy is used to heat the house
• Windows made out of this type of glass are the most energy efficient and will help you save the most on your energy bills

Acoustic glass

• This type of glass is designed to reduce outside noise
• It consists of two or more sheets of glass that are bonded together with one or more acoustic layers

Security glass

• Security glass panes range in thickness from 7.9mm to 18mm
• Security glass can withstand attack from objects like bricks and axes for up to two minutes

Solar control glass

• This type of glass reflects and filters the sun’s rays
• Solar control glass allows light into your home without causing glares that can be a nuisance  

Self-cleaning glass

• Self-cleaning glass is coated with a transparent coating
• The coating breaks down dirt and works to spread water evenly around the windows surface 

Decorative glass

• Decorative glass is an excellent way to personalise your windows with patterns, textures or colours
• You can also combine decorative glass with any of the above types for different functionality

These are the steps your double glazing installer will take when putting in your new windows.

1. Removal of old windows – Your old windows and their frames are carefully removed to make space for the replacements. If there is anything wrong with the brickwork surrounding the window, this is fixed before your new frames are fitted. Most installers include this step in their quote but check whether they charge extra for removing or disposing of the old units.
2. New frames installed – The installers will fit the new window frames into the open spaces left by your old units and check they are level.
3. Glass fitted – The sealed double glazing glass units are then carefully placed into the window frames and secured into place with beading.
4. Finishing touches – Your installers will finish the windows by sealing them around the edges on both sides with silicone and a colour-coded trim. 
5. Final quality check – Once your windows are finished, your installers will check them to ensure the installation quality is up to scratch. They will be looking for gaps in the sealants or other issues that could lead to your double glazing not performing as it should.
6. Clean up and demonstration – The crew working on your installation will clean up any dust or debris created during the process, leaving your home as before they arrived. Your installer might also offer you a demonstration to show you how to open and lock your windows, how to use any trickle vents and how to take care of them, as well as an explanation of any issues you should watch out for that might require them to return for an inspection.
7. Follow-up – Some double glazing installers will follow up with a courtesy call a week or two after your installation to find out if you’re happy with the quality of your windows and the work done. This is your opportunity to flag any issues or ask questions. You should also receive a FENSA certificate for your installation and product warranty details.

How long it takes to install your new double glazing will depend on how many windows you have installed. For the majority of homes, installation can be completed within one day. Installation costs vary between companies but will average around £80 per frame.

The below table should give you an estimate of how long the installation process will take and the total costs.  

Home improvements usually go off without a hitch, but there are horror stories of double glazing windows installations going wrong – incomplete work, incorrect installations, wrong-sized windows and even damage to people’s properties. If something goes wrong with your project, it’s essential to understand your rights as a consumer.

The most important step you can take is researching before contacting companies to gather quotes or signing any contracts. 

Once you’ve chosen the quote that best matches your budget and vision of how your new windows will look, below are the things to remember as your installation progresses.

What should be included in my contract?

Be sure to read your contract carefully before signing it to understand what work will be undertaken and what terms the company has placed on the project. In general, your contract should include the following details:

• A description of the windows you are ordering and the work to be carried out
• The total price, including all known charges and applicable tax
• A date for the installation to take place and how long it is expected to take

What are my cancellation rights?

General consumer rights state most sales contracts should include a cooling-off period of at least 14 days. This applies to agreements made online, over the phone or in person at your home, allowing the purchaser to cancel the sale risk-free.

However, if your windows are made specifically for your property or you signed the agreement at the installer’s premises, you will lose the right to cancel. This also applies if you discuss the sale with an advisor at your home and agree to it later, as you are then deemed to have had time to cool off and consider the contract.

What if there are problems after my installation?

If you discover a problem with your double glazing windows after your installation or something wrong with the work itself, you should be covered by your FENSA-backed guarantee, which is a requirement for installers to provide. You should contact the company in the first instance to resolve any issues.

You are also covered by the Consumer Rights Act 2015, which protects consumers from flawed products or services. The guidelines state any services should:

• Be carried out using all reasonable care and skills
• Use products that are fit for purpose and materials that are of satisfactory quality
• Be as described and what you ordered.

If you feel these criteria haven’t been met, your installer is required to put things right. However, it is up to the company to decide whether to repair them or provide replacements, and they will generally choose the cheaper of the two options.

FENSA also provides a complaints process if you cannot resolve directly with your installer, so long as they are a member.

Is there an ombudsman for double glazing windows?

Yes. The Double Glazing and Conservatory Quality Assurance Ombudsman Scheme offers a free consumer advice line and free access to its services, while The Glazing Arbitration Scheme (TGAS – formally the Glazing Ombudsman Scheme) can take cases to arbitration, which aims to resolve disputes without going to court; however, this scheme is only available if your installer is a FENSA or Glass and Glazing Federation (GGF) member and you have already been through their complaints process. Be aware that TGAS’s decision is legally binding, meaning you won’t be able to proceed to court action if you’re unhappy with the outcome of arbitration.

Other consumer rights

If you paid for part or all of your windows with a credit card or the windows company arranged finance on your behalf, and the sale was between £100 and £30,000, you will be covered by the Consumer Credit Act 1974, which makes your credit card provider equally as responsible as the installer for any disputes. This means you can claim up to the value of the entire sale. However, if you took out a bank loan to pay for the work, this does not apply.

Court action

Your last resort for resolving disputes with your installer is taking them to court to obtain an order for them to compensate you. Which court you turn to depends on the amount you claim – if it’s £10,000 or less in England and Wales or up to £3,000 in Northern Ireland, you can use the small claims court. In Scotland, you can turn to the Simple Procedure for claims of up to £5,000.

Be aware that court action can be costly and may take a long time to reach a decision.

Keep records of every step of your complaints process, including emails, phone logs, and other written communication with your installer, as well as pictures of any damage or faulty installation, as these can all be used as evidence to support your case.

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