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The complete guide to double glazed windows

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Double glazed windows are characterised by their dual-pane design and allow you to increase energy efficiency and reduce your energy bills, when upgrading from single glazing. Whether you choose inexpensive uPVC windows or the more traditional timber-framed options, double glazing offers numerous benefits, from enhanced security to increased property value.
We’ve done a deep dive into everything you need to know about double windows, with in-depth information about different types of windows and how the investment can significantly improve your comfort, property and finances.

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What is double glazing?

Double glazing is a type of window consisting of two planes of glass. Standard double glazing is 24mm in depth, with 4mm panes of glass on the inside and outside. The 16mm gap between is filled with an inert gas, such as argon, xenon or krypton. These all have different levels of insulation as they are poor heat conductors, which helps retain the warmth inside your home. 

Which double glazed windows are right for my home?

As is to be expected with such an important home upgrade, choosing the right double glazed windows is all in the detail. Here is a checklist of what to consider:


Aesthetics and design

You might want to replace your windows like for like, if you live in a traditional home and are upgrading from single glazed windows, for example. Choosing similar designs and opening mechanisms will retain the original character, where possible, of the architectural design.


Alternatively, for a design remodel, you can choose a completely different type of window, with a more flush, lipped, or projected design – it depends on what you want for your home’s aesthetic. Consider sightlines, sill options and take note of any features that need to be respected, like materials or styles in conservation areas. If in doubt, contact your local planning authority.


Material type

The material you choose to frame double glazed windows doesn’t just depend on the look you want and your budget, but also on the level of maintenance you’re willing to take on board. 


  • uPVC: These windows tend to be the cheapest, but the material is available in lots of different styles and configurations; many designs come with wood grain finishes to mimic a traditional wooden frame, without the maintenance (or cost). Lots of companies also fit more decorative details to further match your windows to a period home design, such as Georgian bars, deep rails and mouldings.
  • Timber: You’ll find wooden windows on most period homes and double glazed units can be a good way to upgrade the efficiency of a house while retaining the heritage style. Double glazed timber windows also make a good investment for newer builds. The frames come at a higher cost than uPVC and need more maintenance, but it’s a timeless addition that could even increase house value due to its longevity and eco-friendly credentials. With good maintenance, double glazed timber will last for years, but without good upkeep, window frames may warp, peel and flake. 
  • Aluminium: Cheaper than timber but at a higher premium than uPVC, aluminium windows are strong, easy to maintain and available in lots of different finishes. The material can also accommodate larger panes of glass than uPVC or timber, so if you want large swathes of double glazed windows with minimal sightlines, this could be the material for you. Aluminium won’t chip or warp either, so it makes a sound frame material to choose for double glazing.  


Double glazing is inherently more secure than single pane windows due to the additional pane of glass. Many suppliers use safety glass as standard, which is tougher than regular panes, but check what the different brands offer and ask for proof of certifications (for instance, the British Standards Institution kitemark logo on the windows themselves). Lock type is important too.


Look for ​​multi-point locking and more advanced systems and windows that are Secured by Design, which is police preferred and includes one sheet of 6.8mm laminated glass as standard. This should also be displayed as Product Assessment Specification (PAS: 24) verified, which means the windows have been tested with up-to-date methods for enhanced security. Other standards are acceptable, and you can always check with your local authority to follow their advice.


Glass type

The glass itself has an essential role in keeping your house comfortable. For instance, low emissivity (low-E) glass has an invisible metal oxide coating that will reflect heat back into the home to retain warmth. It will also let light in, but deter UV rays; this helps protect furniture and textiles from colour fading and degradation from too much direct sunlight.


Additionally, you can enhance the noise reduction benefits of double glazing with acoustic glass, while obscured or opaque glass is available to reduce visibility from prying eyes in bathrooms or street-facing bedrooms. 


Opening mechanism

Handles come in different styles and finishes so you can enhance a home’s traditional feel or add a more modern look, if that sits better with your interiors. Depending on where your double glazed windows will be situated, a side handle or bottom-mounted handle might make sense. 



There are lots of double glazing companies that offer windows made in Britain. This can be a key consideration, as fewer carbon dioxide emissions are produced, fewer materials are imported and the time spent transporting the finished product will be reduced.


If you choose coloured uPVC windows, aluminium or timber, look for options that feature low levels of volatile organic compounds (low-VOC), as this means less toxins released into the environment. Timber frames generally carry the smallest carbon footprint, but uPVC and aluminium can also be recycled at the end of their lifespan. Choose a company that uses sustainably-sourced and FSC-certified wood.



Always research and commission a reputed window installer that is FENSA- certified. With period properties, choosing a local made-to-measure service might be a better option to ensure the design suits your home and is sympathetic to its location.



Double glazed windows typically come with a warranty of around 10-plus years for the frame and glass, with many suppliers offering longer or lifetime warranties also. Be sure to check the terms and conditions.

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How long do double glazed windows last?

A quality double glazing product that is fitted by a specialist should last at least 25 to 30 years, if not longer. Visible damage, draughts and condensation are all indicators that it’s time to replace or upgrade double glazed windows. If this happens within just a few years of the installation, you should be covered under the manufacturer’s or installer’s warranty, and any units that aren’t functioning as they should can be replaced. You can expect a warranty of 10 to 15 years from most reputable double glazing window suppliers.

Maintenance of the frame is also important for longevity, with uPVC being the easiest to keep clean – it usually has no grooves where dust, debris and water can collect and lead to rot issues – but it also has the shortest lifespan compared with aluminium or timber, which can last up to 45 years and 60 years, respectively. 

Different types of double glazed windows

Your windows are effectively the face of your home, so finding the right aesthetic to suit your home’s exterior design is essential, as is choosing a window style within your budget. Unless you live in a listed building or conservation area, most types of windows can be successfully fitted with double glazing for enhanced heat retention and more benefits. 

window styles graphic
The style and material of your windows will hugely impact the house’s kerb appeal. (Independent Advisor)

Double glazed casement windows

The most popular style of double glazed windows are casement windows. They work on a side hinge mechanism (although can also be top or bottom hung), allowing you to open them outwards, which is ideal for ventilation. Casement windows became popular in the 18th century, where you would find six-pane timber designs as standard. Over time, casement windows have evolved and they are now very versatile and can be finished using astragal bars.

This means the window pane is visually split by a divider to give the impression of multiple panes, mimicking traditional configurations. Casement windows are available in flush designs for a simple finish that will look good on transitional or modern homes. 

Double glazed sash windows

Sash windows came to popularity in the Georgian and Victorian eras, typically in six-over-six and one-over-one designs, respectively. Traditional box sash windows consist of two glass panels that slide up or down using a pulley and weight system hidden within the frame. Simpler sliding sash windows use a spring mechanism to open the panes and this will usually be the style used for new double glazing. 

Horizontal sliding sash windows (in which the panes slide side to side) are harder to replicate in double glazing, with big industry installers not offering them as standard. However, local suppliers may be able to offer these with bespoke double glazing.

Single pane sash windows weren’t typically well sealed but modern double glazed sash window designs are built to prevent heat loss, so you can enjoy all the style they have to offer, as well as better insulation. 

Double glazed tilt and turn windows

Double glazed tilt and turn windows make great additions to modern homes, especially if you live in a city or if you have children, as they are usually fitted with safe handle options to limit how wide the windows will open. As the name implies, the hinge mechanism allows you to tilt the window open from the top or bottom at an angle if you turn the handle 90 degrees. If you continue turning the handle, the window can then swing inwards for a wider vent opening. Tilt and turn double glazed windows come in uPVC, timber and aluminium.

Double glazed bay windows

Bay windows feature three panes of glass at different angles to create an area that projects beyond the house. Bay and bow (a variation on a bay) windows come in different configurations: splay, square, circular and single end bay, to suit the style of your home.

Unfortunately, they are prone to condensation and draughts, but with double glazing you will be able to keep the charm, bigger views and increased natural light, without such issues. These windows can be fitted easily with uPVC, timber or aluminium options. 

Double glazed bow windows

Bow windows are a type of bay window. Much like bay windows, bow windows protrude outwards from the main walls of the building. Bow windows are gently curved and generally consist of four or more panels or sections which form a rounded appearance. 

Bow windows are an excellent choice of window type for homeowners looking to add character to their homes. A bow window creates a dramatic viewpoint, allows light to flood into the home and can be just as eye-catching as a feature wall. 

Bow windows, although aesthetically pleasing, don’t come without problems; unfortunately, they’re more complicated to install and costly than standard casement windows. Bow windows protrude from the exterior wall of the property, and this can increase heat loss. 

Should I buy windows with double glazing in the winter or summer?

Double glazing is fine to install in all seasons. Something can be said, however, for replacing single glazed windows towards the end of summer, so your house benefits from the energy efficiency that comes with double glazing in the winter. Presumed better weather would also make installation a little simpler in the spring or summer months, as opposed to replacing all your windows when it’s cold, wet and windy.

Pros and cons of double glazing

Improved energy efficiency
Reduced energy bills
Improved security
Noise reduction
Increased home value
Reduced condensation
Initial cost
Hiring a professional to install
Might now be suitable for listed period properties

Frequently asked questions about double glazed windows

It can be almost 50 per cent cheaper to have double glazing replaced and reinforced, over replacing the entire unit, including the frame. This should be a viable option, provided your existing window frames and the other parts of the unit are still in good condition, but the cost will ultimately depend on the supplier.

To visually check if you have double glazing units, look closely at the edge of the pane; double glazed windows are made up of two glass panes with a sealed compartment between. The thickness of the panels and spacing between them may vary between suppliers and window types but typically they will feature two 4mm panels, with a 12-16mm gap. Single glazed windows only have one pane and no sealed compartment, while triple glazing will have three panes and two compartments.

Window manufacturers provide window ratings that run from A++ (the most energy efficient) to E, using a scheme run by the British Fenestration Rating Council (BFRC).

Additionally, manufacturers will supply U-values (the rate at which heat is transferred through a structure). The better-insulated a structure is, the lower the U-value will be. Single glazed windows would likely receive a U-value of 5.2W/m²K (watts per square metre-kelvin), whereas double glazing will generally be 1.6W/m²K.

Depending on the nature of your listed building, you may be able to install slimline double glazing, which has a smaller profile compared with standard double glazing (14mm versus 24mm). Slimline glazing has two 4mm panes, but a smaller gap (6mm). Aesthetically, there isn’t a big difference between single and slimline double glazing, which is why it is often permitted by listed building control.

Secondary glazing is another option for period homes with single glazing; this is essentially having another panel, glass or plastic, fitted onto your existing window frame. You can do this yourself, but for a better quality fit and less visibility, professionals will be able to fit the secondary glazing for a fee. Surprisingly, while secondary glazing is not as energy-efficient as double glazing, it offers better acoustic insulation.

In some existing single glazed timber windows with the correct depth, double glazing can be installed without replacing the frame, but for most windows you are likely to achieve a better finish and a more reasonable cost by replacing the entire window. Box sash windows, for example, will have been built to accommodate a single pane of glass and the cost to repair and route the timber and replace the glass might be more than the window is worth. For original windows you don’t want to replace, get them restored and install secondary glazing instead.

Although there are no specific double glazing window grants available, some households may be eligible for different UK funding schemes that can include double glazing as an upgrade: Ofgem’s Energy Company Obligation (ECO), often referred to as ECO4, being one of them. Many of the biggest suppliers of windows may also offer finance options to help spread the cost over months or years.

One problem that homeowners often report with double glazed windows is the build-up of condensation. Condensation can form on your windows when the window pane is cooler than the air in the room. To tackle condensation, open your windows to allow airflow and ventilation. Other ways to reduce condensation include dehumidifiers or spraying your windows with an anti-fog or mist solution to combat humidity.

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Hannah Holway circle

Hannah Holway

Home Tech Writer

Hannah Holway is a writer with eight years of experience in writing and editing across several different categories. As a home tech expert at Independent Advisor, Hannah researches, tests and writes about broadband services and home security gadgets.

She started her career as a freelance film and culture journalist, and has written for editorial platforms such as Wonderland and Hero magazine, as well as interviewing directors, actors and musical artists. While at Wonderland, she was also Social Media Editor for the brand and Contributing Editor for the publication’s sister print titles.

In 2020 she joined New York Magazine’s The Strategist UK, reporting on evolving shopping trends and writing about everything from period pants and pens to books and the next ‘status’ candle. She then used her consumer trends knowledge and expertise in her role as Shopping Writer for Woman and Home Digital, where she oversaw a range of shopping content, writing product reviews and other features in the realm of health and fitness, beauty, fashion and homes.

Hannah has also had her academic work published in journals and presented at conferences, and she has a BA and MA in Film Studies.