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With energy bills continuing to be a big concern for many, specifying energy efficient windows and making sure your glazing isn’t leaking precious warmth out of your home is a must. Glazing isn’t as thermally efficient as a solid wall, but modern products do a marvelous job of keeping rooms cosy.
And there are certain principles to remember, too, such as making sure your high performance window is being installed in the right way (otherwise you might have what’s known as a performance gap between expected and in-situ efficiency).
Here, we’re taking a look at the answers to key questions to ensure you’re in the know when it comes to specifying and installing energy efficient windows, including frame materials, U-values and triple glazing.
U-values are a measure of heat loss, stated in Watts per metre squared Kelvin (W/m²K), where lower numbers indicate better performance. For instance, a well-insulated wall might achieve a U-value of around 0.10-0.16 W/m²K. By contrast, a single-glazed window is likely to have a U-value of more like 5.0 W/m²K. So, traditionally, fenestration has been seen as something of a weak point in a building’s thermal performance.
U-values are a key indicator that will help designers to work out the overall performance of your planned or existing home, renovation or extension, and thus shape decisions on heating systems etc. And if you want to meet a specific eco standard, such as Passivhaus, then you’ll be required to install ultra energy efficient windows that meet certain U-value thresholds.
Depending on specification, high-performance doors and windows typically deliver U-values between 0.6 W/m²K (the best triple-glazed products) and 1.2 to 1.6 W/m²K for standard double glazing. Windows in new homes must have a U-value of 1.2 W/m²K and doors 1.0 W/m²K to meet Part L of the current Building Regulations. But you’ll want to aim for better than that if you’re looking to create a highly efficient structure. Passivhaus design, for instance, requires windows to have a U-value below 0.8W/m²K.
Always check what quoted U-values refer to: you should focus on the whole-window or whole-door U-value when specifying your project (not just the centre-pane figure).
find glazing suppliers in Build It’s Directory
This Build It Award-winning timber alu-clad design from Norrsken is a practical and stylish casement option, manufactured to operate smoothly throughout the entire lifecycle of the product
However, U-values aren’t the be-all-and-end-all. Most self builders and renovators will want to invest in the best quality products they can afford, for instance. So, look for energy efficient windows complete with robust seals to ensure they deliver their stated efficiency over the long-term. Glazed products have a key role to play in delivering natural brightness, solar gain and ventilation, too, and this is an important part of creating a healthy home.
It’s crucial to choose doors and windows that offer great performance, as well as style. Here’s an explainer for the jargon you might read on product descriptions:
A Passivhaus is essentially a really well-built, low-energy home. Building to Passivhaus principles involves looking at the physics of housebuilding to achieve an extremely efficient and airtight fabric, taking into consideration how the home is going to perform now and in the future.
Achieving these standards will mean relying mostly on passive heat resources such as solar gain and mechanical ventilation with heat recovery (MVHR), whereby you’re retaining and maximising any warmth. This will help to reduce your energy consumption significantly (usually up to 90%) and create a home with a comfortable temperature of 21°C.
The best place to start is with research. This is important to not only understand what Passivhaus really is, but how much it can help you to save and with whom you may want to collaborate on your bespoke home project. You should find an architect who is skilled in building low-energy homes and has a strong understanding of Passivhaus principles.
An architect with this knowledge and expertise will be aware of contractors who can deliver a high-quality Passivhaus to your specifications. This collaboration between architect and trades is essential in order to create a home that’s really going to deliver on both performance and design.
There are five principles and products to take into your eco house design when aiming for Passivhaus:
Having three panes of glass, ie triple glazing, instead of two will reduce heat transfer through your windows. “The gaps between the panes can be filled with air or – more usually – a super-insulating inert gas, such as argon or krypton, to maximise performance,” says Thomas Hagen, technical manager at Internorm. “As a guide, double glazed units typically achieve a U-value of around 1.4 W/m²K, whereas a modern insulated wall would hit 0.2 W/m²K or better. Triple glazed windows and doors bridge the gap between the two.”
That said, good quality double glazing can still be the right choice for some projects or potentially for specific locations within your building. Many suppliers can offer both solutions, which may give you flexibility on specification and cost.
Clearly, triple glazing requires an extra pane of glass – so, in theory it should be about 50% more expensive than double glazing. However, in 2026 standard triple glazed windows tends to cost an average of 10-20% more than double glazing. You might then choose to increase the spec of these with features like inert gas fillings (Argon or Krypton) to improve thermal performance or special coatings for particular applications. These will have their own impact on costs.
This impressive low-energy home features triple-glazed windows and lift-and-slide doors from Westcoast Windows’ timber composite range, which were installed by authorised distributor Aluminium Clad Windows. A slatted oak pergola provides solar shading for the wide spans of glazing on the ground floor. Photo: Matthew Smith
As with any home building purchase, however, it’s important to look beyond the raw costs and ensure the overall product delivers the performance and quality you’re looking for. Whether you settle on double or triple, the suppliers included in this book are a great place to start when it comes to finding the right system for your project.
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The right energy efficient windows will not only complete your home’s design but also fulfill your requirements in terms of lifestyle, thermal performance and more. Your choice of window frame material has a big part to play here, here are the most popular options:
Plastic (PVCu) windows are a widely available and budget-friendly choice, and come in a huge range of colours. High-quality versions can come surprisingly close to replicating the sightlines, grain appearance and jointing methods of timber designs – so they can be a good fit for heritage homes. “PVCu is a cheaper raw material, so these windows are at the lower end of the price scale,” says Ryan Breslin, Cherwell Windows’ managing director. PVCu window frames require practically no maintenance beyond an occasional wipe over with soapy water. A good quality system should yield excellent service for 25-35 years.
Timber window frames are an attractive option for their inherent character and charm, and can deliver very good sightlines. The wood can be stained to emphasise its natural grain, or painted to suit your design scheme. Most modern timber windows are manufactured in engineered softwood, which is very stable and robust, ensuring energy efficient windows that’ll deliver excellent long-term performance.
The 2024 Build It Award for Best Casement Windows went to Rationel for its Aura Plus range, which is available as flush casements and a range of other opening styles, as well as French doors and sliders. The windows have a solid timber core, made with sustainably-sourced timber from FSC-certified forests
Hardwoods such as oak can provide a luxury feel, but these often come in at a higher price point. Timber is a natural insulator, so good quality systems can offer great thermal performance. What’s more, factory-finished units shouldn’t need repainting for 5-10 years, and can easily last 50+ years.
Strong, robust and long lasting, aluminium window frames are a great low-maintenance option and a strong contender if you want big windows or designs with minimal framing. The material is also very resistant to corrosion, so these systems are often specified for homes in exposed locations, such as on the coast.
Aluminium can be powder-coated in a variety of RAL colours, so you can also customise your window frames to suit your property’s style. Modern aluminium windows are manufactured with an insulated thermal break to ensure minimal heat loss and achieve impressive levels of energy performance. Aluminum frames will ensure energy efficient windows and are the most popular option for those working on Passivhaus projects.
Combining two materials for your window and door frames can give you the best of both worlds – timber and aluminium. This style also means you can enjoy a low-maintenance external finish, while benefitting from the innate charm of timber inside your home. Composite windows are also available as alu-clad PVCu designs, delivering an enhanced solution that marries two easy-upkeep materials and matches the impressive performance of alu-clad timber options.
Carolyn Copperwheat built a stylish Passivhaus home in East Sussex using a timber frame superstructure from MBC Timber Frame. The airtight house features high-performance alu-clad glazing from ecoHaus Windows & Doors. Photo: Mary Gaudin
Some triple-glazed composite systems can deliver U-values down to 0.6W/m²K. Because of the combination of different materials, however, this style of windows tends to come at a premium cost.
How sustainable are the different window frame materials?
“In terms of the energy and carbon that goes into building a house, windows might constitute 20% or more of the total embodied environmental impact,” says Nigel Griffiths, Build It’s eco expert. This means it’s important to carefully consider the materials that your fenestration is made from in order to keep the carbon footprint of your project as low as possible. “One metric that we can be reasonably sure about is a material’s embodied carbon, which is the CO2 emitted to extract and process raw materials, then turn them into building products,” adds Nigel, who provided the figures in the box below.
| Material | Embodied carbon per kg (kg CO2e/kg) | Embodied carbon per linear m (kg CO2e/kg) |
|---|---|---|
| Timber (Softwood) | 0.5 | -2.5-1.5 |
| PVCu | 2.4 | 40-60 |
| Alu-clad timber | 5.0 | 80-150 |
| Steel | 1.4 | 140-200 |
| Aluminium | 8.2 | 320-500 |
“Timber has by far the lowest impact in terms of embodied energy/carbon. The values for aluminium depend heavily on whether it’s made from primary (virgin bauxite) or secondary (recycled scrap) sources. Virgin is 11-16kg CO2e/kg, whereas recycled is less than 1.0. In the UK and Europe, the average recycled content within aluminium is 30%-50% (hence the indicative value used here of 8.2).” Aluminium is also infinitely recyclable at end-of-life.
A door or window will only perform as intended if it’s properly fitted – indeed, a shoddy job could increase heat loss by as much as 30%. So, always appoint appropriately qualified installers who will make sure the frame is set back correctly within the wall envelope, and is properly packed, insulated and sealed.
“It’s important to ask who is going to be fitting the products,” says Edward Stobart, sales manager at IDSystems. “I’d recommend hiring a company that supplies and installs the doors themselves.” This way, you have a single point of contact responsible for ensuring the agreed performance is achieved.
21 Degrees’ range of high-performance windows are perfect for low energy and Passivhaus projects, offering U-values as low as 0.71 W/m²K
Liaise with your installer to check what method will be used to seal the perimeter of the window when it’s fitted. Expanding foam and Compriband tape are common options. Airtightness tape might also be required to meet your targets, and your installer may be able to advise on a suitable product.
If your existing glazing is causing excessive heat loss or is in a very poor state, then it’s time to look at a window repair or replacement. Upgrading what’s already there can be surprisingly effective, improving comfort levels while preserving the character of your home. A number of companies specialise in draughtproofing hard-to-treat features such as original sash windows, for instance, and there’s also the option to add secondary glazing internally.
Brookeswood Joinery won the 2025 Build It Award for Best Windows for the Heritage Flush Casement Window. The range is made from engineered timber and factory-finished with a durable three-coat microporous paint system. The result is a system that achieves U-values down to 0.95 W/m²K in a window that offers the look of heritage single glazing
If windows and doors are beyond cost-effective repair, or you’re dealing with inappropriate units (it was a common scourge of the late 20th century to replace original fenestration with what we would now consider to be poor-quality double glazing), then replacement will be your best bet. There are many high-quality modern options available that deliver exceptional performance in authentic heritage styles – including products that feature double or even triple glazing in very thin profiles.
Whether you’re thinking of upgrading or replacing existing windows and doors, it’s important to consider the performance of the rest of the house, too. There’s little point undertaking this work without addressing loft insulation, draughts in chimneys and fireplaces, floor voids etc. You’ll need to understand how these changes may impact internal moisture levels, and potentially install trickle vents to manage background ventilation.
Depending on the nature of your project, Part F of the Building Regs may require the inclusion of trickle vents within your chosen window and door frames. These are simple ventilation points that have an on or off position, providing a throughflow of external air to control moisture and condensation within your home. In some buildings, this is vital for promoting good indoor air quality – but the downside is that it introduces a relatively uncontrolled path for heat loss.
As a general rule of thumb, if you’re building a very high-performance house that achieves strong airtightness and uses a mechanical ventilation system (MVHR), you won’t need trickle vents. But for most other types of project (including the overwhelming majority of renovations), you will. “Trickle vents always add to the price,” says Matt Higgs from Kloeber. “How much is very product dependent, as some items need to be manufactured differently to accommodate the vents.”
A member of Kloeber’s install team fits and seals a timber window into the insulated concrete formwork (ICF) walling system at the Build It Education House. Correct installation is vital for realising long-term performance
| Want to know more about eco building solutions?
Head to Build It Live and visit the Sustainable Building Products Pavilion (in association with the Alliance for Sustainable Building Solutions) for some expert advice and to source some of the best eco products on the market. Build It Live takes place three times a year in Malvern, Exeter and Kent. The next show will be on 30th and 31st May 2026 in Malvern, Worcestershire. Claim a pair of free tickets today and start planning your visit. |
They can and, in the past, adding a low-e (low-emissivity) coating to the spec was a big focus for increasing the efficiency of glazed products. This reduces heat loss from within the home, while still allowing the passage of visible light, thereby improving thermal performance without maximising natural brightness. These days, you can expect low-emissivity as standard, rather than an upgrade. “A low-e coating is mandatory on glass now, in order to pass the latest Building Regulations,” says Matt.
For many years, Building Regulations (along with many designers and clients) focused primarily on how to retain heat in new and existing homes. The aim, of course, was to reduce energy bills and minimise carbon emissions. But we’ve now reached a standard of performance where it’s necessary to specifically address the risk of new houses and extensions becoming too hot in summer. Hence, the 2022 version of the Building Regs introduced a Part O that aims to mitigate overheating.
Specifying thermally efficient glazing can help to enable spectacular features, such as this near-frameless, glass-to-glass corner picture window by Internorm
From a quality of life point of view, few of us would want to reduce the number or size of windows in our home designs, as it would mean losing out on natural brightness and garden views. Thankfully, there are ways to prevent overheating without impacting your enjoyment of the living spaces.
One is to opt for a solar control glass coating for south-facing windows and doors. This will reduce the amount of incoming solar heat gain, and therefore minimise the risk of rooms getting too hot. These days, you can even buy smart glass that delivers both privacy and solar control at the flick of a switch.
This self build project features external solar shading from Sunshield Unique Louvres, helping to reduce solar heat gain before it reaches the glazing. This helps to maintain comfortable internal temperatures throughout the year while still allowing natural light, ventilation and outward views
You can also look to design out the worst of the high summer sun by incorporating stylish shading measures. Architectural features such as roof overhangs or slatted brises soleil are common in new build homes, for instance. Or you might look to harness the power of nature. Mature trees will be in full leaf come summertime, and the shade they cast can now be easily modelled in 3D design software.
| Window Frame Material | Window Cost (excl. VAT) | Trickle Vent Cost (additional) | Triple Glazing Cost (additional) | Trickle Vent & Triple Glazing Combination Cost (additional) |
|---|---|---|---|---|
| Softwood Timber Window | £1,150 | £35 | £50 | £85 |
| Hardwood Timber Window | £1,250 | £35 | £50 | £85 |
| Oak Timber Window | £1,365 | £35 | £50 | £85 |
| Aluminium Window | £1,015 | £80 | £185 | £265 |
| Aluminium Window (heritage glazing bars) | £1,185 | £80 | £185 | £265 |
| Aluminium Window (super-slim frame) | £1,120 | £105 | £175 | £275 |
| Alu-clad Softwood Window | £1,280 | £40 | £95 | £40 |
| Alu-clad Hardwood Window | £1,645 | £40 | £135 | £40 |
| PVCu (white) | £800 | £TBC | £TBC | £TBC |
| PVCU (timber effect) | £920 | £TBC | £TBC | £TBC |
Vasilis Giannopoulos is specifications manager at Internorm UK, a leading window brand in Europe, offering high-quality, low-energy and Passivhaus window and door systems.
Opt for windows with well-insulated frames, plus excellent seals and gaskets. Low-e glass coatings, gas-filled cavities and warm-edge spacers will reduce heat transfer, too.
Position within the insulating envelope. Ideally, windows should be installed in line with the insulation layer rather than flush with the outer brickwork/cladding.
Use high-quality airtight tapes, expanding foam and/or specialist membranes to create a robust seal around the window frame.
The junction between the window and wall must be insulated correctly in order to prevent any unwanted heat loss. This will mean engaging professionals who understand how to correctly install glazing systems into a low-energy building fabric.
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