Your double glazing questions answered

We spoke to nine experts who work for Which? Trusted Trader double glazing companies to answer some common questions about double glazing, from installations to security and ventilation considerations. 

All of the traders we interviewed have been installing double glazing for at least 10 years, while one has been in the industry for over 45 years. 

All of their companies have passed our comprehensive assessment process to ensure their work and practices are trustworthy. 

To find out more about the benefits of using a smaller local company and how we ensure Which? Trusted Traders are reliable, read our guide to independent double glazing companies. 

How effective is double glazing?

Double glazing has many advantages over single glazing:

• Keeps warm air in Your property is better insulated, which means fewer draughts and cheaper heating bills.
• Keeps noise out 
• Reduces condensation 
• Heightens security Double-glazed glass is more difficult to break than single glazing.

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.

Many of the Which? Trusted Traders we spoke to cautioned that the effectiveness of double glazing is dependent on it being installed correctly.

‘A window is only as good as the person who has installed it. Be aware that you are buying both a product and a service,' says Phil Brown, of Salop Glass & Glazing.

Similarly, James Gaft of Balhousie Glazing said: 'Even the best windows will perform poorly if they're badly installed. Paying for a reputable installer who measures correctly, installs and seals properly will have the biggest impact on performance, longevity, and comfort. Poor installations can lead to draughts, leaks and early failure.'

Will double glazing reduce energy bills?

Many companies claim that fitting double glazing will cut your energy bills, and this is often the motivation for homeowners to purchase double glazing. 

However, it can be difficult to accurately tell how much money you are saving.  

In general, be wary of excessive savings claims, says Jacob Gourd, from Ideal Glass: 'Customers often think that new windows will make a big difference to their heating bills, and that the windows will somehow pay for themselves. Unless you are replacing old single glazed windows then the difference is often small.'

If you replace leaky old single glazing with new double glazing, your home is likely to feel warmer and cosier. If that means you reach for the thermostat less often, you should see a reduction in your energy bills. But don't expect to see all of your money returned.

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 could save up to £175 per year. For a mid-terraced house, where it is attached on both sides and so would naturally use and lose less heat, the saving would be closer to £60 a year.

What do the energy ratings for double glazing mean?

The energy-rating system for double glazing 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.

Some companies offer additional energy-saving measures on top of their standard double-glazed windows and doors. For example, Safestyle says its EcoDiamond double glazing exceeds heat retention and weather performance tests. 

What is 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. A lower U-Value is better – it indicates that the material lets a lower amount of heat escape, meaning it's a better insulator. 

Some windows might have a high energy rating, but high 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.

'Lower U-values mean better insulation, but the whole window system and installation quality matter just as much as the glass spec. Don't be obsessive about u-values, but don't ignore them either.' Tommy Rose, Barnet Window Company

Think about the big picture, rather than just u-values. 'The overall performance of a window depends on the whole unit working together – the frame, the glass, the spacer bars and the installation. The difference between modern, good-quality double glazing products can often be relatively small. For most homeowners, upgrading from older windows to modern double glazing will make the biggest improvement in comfort and efficiency. Ultimately, a well-specified product installed properly will have more real-world impact than focusing purely on the lowest U-value on paper.' James Gaft of Balhousie Glazing.

Does double glazing require ventilation?

Ventilation requirements for newly installed double glazing changed in June 2022, when new building regulations known as 'Approved Document F' came into force. 

The new regulations require extra ventilation to be provided to habitable rooms – living rooms, dining rooms, bedrooms, kitchens and bathrooms – when over 30% of the property’s windows are replaced. This is because new windows are likely to be more airtight, preventing fresh air from entering.

There are two ways to get fresh air into a home:

• Purge ventilation - opening doors and windows
• Background ventilation - allowing fresh air in at a low level continuously

New building regulations require background ventilation, rather than relying on you opening and closing windows. 

There are a variety of methods:

• Window trickle ventilators Holes are drilled through the window frame, with external and internal covers over the holes. A cheap but basic method, that can look messy and has poorer energy efficiency than other options.
• Passive wall ventilation This allows fresh air in through a small hole in the wall. Does not need a power supply and can be located anywhere in a room. Less obtrusive and more controllable than window vents. 
• Positive input ventilation Works like an extractor fan you would find in a kitchen or bathroom, but in reverse. Usually set to a low speed to minimise noise, it pushes filtered fresh air into a room.
• Mechanical heat recovery ventilation A whole house system that moves warm stale air and fresh air through a heat exchanger. Ducts in each room extract stale air and bring in fresh air, while the main unit has fans and filters to direct and clean the air. An effective but expensive system.

Good ventilation helps reduce condensation and damp. If these are damp issues in your home, read our guide on how to get rid of damp. 

Does double glazing reduce noise and soundproof?

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 recommend their secondary glazing to combat noise. 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. 

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.

You can spend extra money for quality locks and reinforced frames, which some of our experts recommended for peace of mind.

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.

Double glazing extras

Beyond the window or door itself, there are other choices that will affect your double glazing quote, depending on the look you are going for.

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

Decorative windows Options include:

• Leaded glass in square, rectangular or triangular patterns (which can help keep the feel of a period property)
• Obscured glass (for when you want more privacy, perhaps in a bathroom or where the window is close to passers-by)
• Jewelled or bevelled glass adds 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.