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1 October 2020

What to look for when buying double glazing

We explain what to check when you're buying double glazing, including energy efficiency, sound-proofing, security and the extras you'll need to choose.
Old window next to new double glazed windows 442755
LR
Liz Ransome-Croker

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 explain some of the more technical elements of double glazing.  First, make sure you’ve chosen the best type of double glazing for your home.

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 scorer 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.

Double glazing claims: how effective is it?

Double glazing has many advantages 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.

But its effectiveness, and how much you notice these benefits, will depend on what you’re replacing and the quality of the product you buy.  

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

Will getting double glazing reduce my energy bills?

Many firms claim that fitting double glazing will cut your energy bills, and 67% of homeowners said that their home is warmer since getting double glazing. This is one of the biggest differences people said double glazing 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 £115 and £120 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. Windows are 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++. 

The energy rating system is run by the British Fenestration Rating Council (BFRC). Look for its name on the energy rating label of your window. This shows you that its performance has been verified by the BFRC.

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 £100 – between £15 and £20 less than A++ ones. For a mid-terraced house, it would be £55 – 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. 

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

U-Value

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, including the types of glass or gas, and not just how well the materials insulate, so they might be better overall.

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 at 36dB. Safestyle says its windows typically provide around 31dB reduction in outside noise.

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

Will double glazing make my home more secure?

While two panes of glass is trickier to break than one, the most important thing is to have key-operated locks fitted to your windows.

This is vital for home insurance, which will usually specify that ground-level windows, and accessible upper-floor windows, must have adequate security measures.

Making wooden windows secure

If you have wooden casement windows (hinged on one side and opening outwards), choose locks which secure the window and frame together. This gives extra resistance (compared with only securing the handle) if an intruder tries to force open the window.

Where the window sites flush to the frame, fit key-operated mortice rack bolts and look for the British Standard 7950 kitemark – this is a specification for enhanced security performance for domestic casement windows.

Securing uPVC windows

These windows are made with an insurance-standard multi-point locking system. When the handle is turned, it engages several bolts with a plate, making the window very secure.

Making sash windows secure

Sash stops are the best way to secure the window. Bars, clasps and fasteners are not considered locks.

Sash stops prevent the window from being forced upwards. Plus you can also use them to lock a sash window open for ventilation. They are visible from the outside so act as a good visual deterrent.

Dual screws (which bolt the sash window frames together) are an alternative option, though they’re more fiddly and not as effective.

What double glazing extras do I need?

Beyond the window or door itself, handles, colour, decoration and more are important to the overall look and feel of your finished project.

Colour options

Some companies offer window frames in a range of colours, including more contemporary options. Handles typically come in white, black, chrome and gold.

Handles

Different shaped handles are available to suit every taste from modern to minimal, rustic to traditional. 

Decorative windows

You can add patterns, textures and colour to your window to make it unique. Options include:

  • leaded glass in square, rectangular or triangular patterns (which can help keep the feel of a period property)
  • obscure glass (for when you want more privacy, perhaps in a bathroom or where the window is close to passers-by)
  • jewelled or bevelled glass can add a hint of colour, and is often seen in glazed front doors.
  • window bars mimic cottage-style windows by making a single pane look like multiple small panels of glass.

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.

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