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How to buy double glazing

Double glazing: your questions answered

By Liz Ransome-Croker

Article 4 of 5

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Double glazing: your questions answered

We explain the advantages of double glazing, how much you could save on your energy bills and answer your common double glazing questions.

For many households, installing double glazing will mean smaller energy bills and a warmer home. In fact, 51% of people we surveyed* who have double glazing said that they bought it to make their home warmer, and 44% to reduce their energy bills.

But it's a big expense, so you want to make the right decision for your home. Here, we answer your burning questions. 

What is double glazing?

Double-glazed windows consist of two panes of glass separated by a layer of air or gas. The air or gas, combined with the second layer of glass, acts as insulation against noise and heat loss, keeping your home quieter and warmer than single-glazed windows. 

You can also buy triple-glazed windows to boost the insulation levels still further. 

Use the links below to jump straight to the answers to some of the most common questions about double glazing. 

If you decide to go ahead and buy double glazing, we can help you choose the best double glazing company for your home. We surveyed thousands of double glazing customers to reveal what they really think about their double glazing firm. 

The top got an impressive customer score of 84%, while the bottom just 54%. To find out more, see our double glazing company reviews. It includes ratings for Anglian, Everest and Safestyle.

Is double glazing effective? 

Double glazing has many benefits over single glazing (where there's just one pane of glass and no air layer or gas):

  • Keeps warm air in, meaning your property is better insulated. This results in fewer draughts and cheaper heating bills.
  • Keeps noise out - you'll hear less noise from outside with double glazing.
  • Reduces the amount of condensation on the inside of your windows.
  • Heightens security - double glazed glass is more difficult to break than single glazing.

The most efficient double glazing has gas between the panes (such as argon), and uses low-emissitivy glass (Low-E), which has a reflective coating to help bounce sunlight back into a home.

What are the benefits of triple glazed windows? 

Triple glazed windows essentially have a third pane of glass, so they should make your home warmer and more sound-proof than double glazed windows. Some companies also use different types of gas between the panes to further prevent heat loss and Low-E glass to reflect sunlight inside.

As you'd expect, triple glazing is more expensive than double. If you're really keen on getting it but the cost is prohibitive to do your entire house with them, you could consider having triple glazing on the rooms that get coldest or suffer worst from external noise.

You can find out what the big-name companies offer by visiting our individual pages on Anglian, Eversest and Safestyle

What is secondary glazing? 

Secondary glazing involves fitting a second layer of glass inside your existing windows. It isn’t as effective as full double glazing, but it could still save you some money on your heating bills - and it's much cheaper to fit.

It could also be a good alternative for homes that can't install double glazing, for example if you live in a listed building.

Will getting double glazing reduce my energy bills? 

67% of homeowners said that their home is warmer since getting double glazing, which is one of the biggest differences people cited that it had made. However, only 35% said that they thought it has actually reduced their energy bills.  

If all of the single-glazed windows were replaced in a detached house with A++ double glazed windows, the Energy Saving Trust says that you'd save between £110 and £115 per year. For a mid-terraced house, where it is attached on both sides and so would naturally use and lose less heat, it would be £60.

The picture below is a thermal image showing the difference in heat loss between single-glazed and double-glazed windows. The house on the left has a single-glazed window where we can see more heat escaping - indicated by the bright-yellow colours.

What are energy ratings for double glazing? 

The energy-rating system for double glazing (pictured below) follows a similar pattern to appliance energy labels, with windows being rated between A++ (the best) and E (the worst). Building regulations require all new windows to be at least C-rated. Each of the big-name companies claim different levels of energy efficiency, some as much as A++. 

When looking at the energy savings, the Energy Saving Trust estimates that for a detached house with all double glazed windows, the savings for a C-rated window would be £95 - between £20 and £15 less than A++ ones. For a mid-terraced house, it would be £50 - just £10 lower.

Some companies offer additional energy-saving measures on top of their standard double glazed windows and doors. For example, Safestyles says its EcoDiamond double glazing exceeds heat retention and weather performance tests and are recommended by the Energy Saving Trust. 

Before you buy into any claims, we recommend visiting our double glazing company reviews to see what their customers really think of them.


When looking at energy ratings, you'll also see the 'U-Value'. U-Value is a measure of how easily heat can pass through something. The lower the amount of heat the material lets escape, the higher the U-Value will be. 

Some windows might have a high energy rating, but low U-Value. This is because the energy rating looks at all aspects of the window - such as types of glass or gas - not just how well the materials insulate, so they might be better overall.

When does double glazing need replacing? 

The majority of people (69%) said that the reason for getting double glazing was to replace existing ones. If you're unsure whether your windows need replacing, these signs should help you decide:

  • They're draughty.
  • They are leaking, letting water in.
  • They allowing excessive amounts of noise in.
  • They uPVC is yellowing or timber is cracking.
  • There's visible damage, particularly to the seals.
  • They're difficult to open, close and lock, or are sticking.
  • They're often covered in condensation, particularly between the panes of glass.

In some cases, such as with broken seals, repairs can be made. It's worth speaking to a double glazing expert to see what can and can't be done. Keep in mind though that they may refuse to fix problems on double glazing not installed by them.

How much do double glazing units cost? 

Double glazing can range from £515 for one 60x90cm uPVC casement window to more than £3,000 for a large 180x150cm double-hung sash window. As you can imagine, there are a lot of factors that can affect the price of your double glazing, including:

  • the type you choose;
  • the material (uPVC tends to be the cheapest);
  • and how many you get at one time. 

We have typical costs for different types of windows and doors on our double glazing prices page. It also includes advice on how to save with your double glazing, and will arm you with information before you speak to any double glazing firms so that you don't get overcharged.

It's also worth taking a look at our page on double glazing quotes and sales tactics to ensure you side-step common sales tricks as well as a rogue double glazier.

Can you replace double glazing yourself? 

New double glazing needs to meet certain building regulations. This needs to either be approved by Building Control or an installer registered with a Competent Person's Scheme who can verify the work themselves. Because of this, it's not advisable to install your double glazing yourself.

We'd also always recommend getting someone signed up to a Competent Person's Scheme, so you don't have to deal with Building Control.

You can use Which? Trusted Traders to find a double glazing company or trader in your area. They've all been through our rigorous checks to ensure they have the right qualifications and insurances to carry out the work they're employed for. 

Can double glazing reduce noise and soundproof? 

While only 20% of the people we surveyed got double glazing because they wanted to block out noise, nearly half (46%) believe that getting it has actually decreased exterior noise.

As double glazing consists of two panes of glass instead of one, it's likely to cut out more sound that single-glazing. Triple glazing should cut it down even more. 

Different companies purport different levels of noise reduction. Anglian, for example, says its A-rated windows provide a sound protection level of 31dB. But its Safe and Sound windows claim to offer higher levels - 36dB.

Everest offers acoustic glass, which it says reduces sound by 40db. Again, you can see more about the extras and options Anglian, Eversest and Safestyle offer on their individual pages.

Does double glazing cause damp or condensation? 

Condensation on a double glazed window or door can be a sign that it needs replacing. 48% of people we surveyed found that their new double glazing doesn't steam up.

However, in some cases, new double glazing can cause damp. This is because all homes need to have some ventilation. Older buildings, in particular, are built to breathe, so adding lots of energy-saving measures stops moisture in the air from escaping. 

This can be a particular problem in winter, when it's cold outside and warm in your home, resulting in the moisture collecting on cold surfaces, such as glass.

To try and avoid any problems, when getting new double glazing, make sure you think about the ventilation in that room. One way to enable a small amount of air in and moisture out is to have trickle vents in the frames of your double glazing.

It's also worth thinking about whether you should have a configuration that means you have one or two small windows, as opposed to all large ones you don't want to have to open all the time. 

You could also consider tilt and turn windows and doors, which enable you to tilt the window or door to let a small amount of air in while keeping it locked. Some windows also allow you to lock them slightly open.

Our guide to choosing double glazing explains the different types of windows and doors. If you have problems with damp, it's worth taking a look at our damp guide, as it gives advice on identifying and solving problems.

Our research

In September 2018, we asked 2,155 Which? members about their experiences with buying double glazing, as well as the company they bought double glazed windows and/or doors from, and had them installed by, in the last 10 years.