Energy-Efficient Windows

Everything you need to know to specify energy-efficient windows for your building project
by Katherine Sorrell
30th November 2018

According to the Energy Saving Trust, around 2 million windows are installed every year in the UK, both in new builds and as replacements in existing buildings. Yet most of these only meet the minimum legal standard for energy efficiency, leaving plenty of room for improvement.

Single glazing is out of the question now, except for certain historic buildings, but did you know that many types of double glazing no longer make the grade?

A careful choice is required, particularly if, for a new build, you wish to attain a high rating against the Code for Sustainable Homes: windows can affect your rating in several areas, including materials and processes used, sound reduction, security and natural daylight.

Specifying windows

There are plenty of advantages to investing in highly efficient fenestration, from creating a comfortable internal environment to cutting your energy bills.

Choosing windows for your home? Talk to the experts about your project and learn from their years of experience at Build It Live Bicester, taking place on 2nd and 3rd October 2021.  Watch live presentations on a variety of topics, such as:

  • Choosing & ordering windows
  • Installing windows
  • Eco window options
  • How to get best value & performance
  • And more!


“An airtight arrangement also offers improved acoustic insulation,” says Ryan Breslin, managing director at Cherwell. However, there can be a lot of jargon to get your head around when you first start hunting for suitable products.

Window U-values

U-values are the measure used to describe a window’s thermal performance.

“This figure indicates the amount of warmth allowed to escape through a window – in other words, heat loss,” says Donna Muir, direct sales manager at Velfac“When assessing a unit’s ability to retain warmth, the lower the number the better.”

If you’re building a new home, the Building Regulations stipulate the U-value for whole windows and doors (frame and glazing) should be no higher than 1.8 W/m2K. If you’re replacing the fenestration as part of a renovation, the required figure is 1.6 W/m2K.

Composite windows by Velfac

Clad in aluminium on the outside and pine on the inside, these made-to-measure composite windows were supplied by Velfac

It’s important to bear in mind that this measure of thermal performance can be divided into two categories.

The Ug-value refers to the glazed centre pane, while the Uw-value looks at the energy efficiency of the whole unit, including the frame.

The latter is the figure you should be focusing on, as it’s possible to get high-quality glazing installed within a leaky frame. Therefore, it’s vital that you ascertain which one your supplier is using and ask them for the Uw-value to be sure about the performance.

According to the website, single glazing might typically have a value of 5.0 W/m2K and conventional double glazing 3.0 W/m2K.

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Modern double glazing can achieve 1.6 W/m2K and the very best windows as little as 0.4 W/m2K (anything less than 1.0 W/m2K is very good, and Passivhaus’ requirement is 0.8 W/m2K).

Window energy ratings

The ideal window offers plenty of natural light but little or no heat loss; a very few windows even offer a small net capture of heat.

To check the Window Energy Rating (WER), go to the British Fenestration Rating Council (BFRC), which has rated a small but increasing number of windows. The ratings are given in bands from A to G, just like energy-performance labels on new fridges; the minimum to satisfy Building Regulations is E, but the Energy Saving Trust recommends at least C.

The result of complex calculations, the BFRC rating assesses the performance of the whole window, taking into account solar gain and the long-term performance of the installed product.

Studies by manufacturers Swish show that fitting an E-rated window can cut the cost of heat loss by up to £113 per year, compared to an old single-glazed window; fitting a C-rated window would cut a further £45 and fitting an A-rated window would eliminate the cost of heat loss altogether.

It’s possible, in theory, for more or less any style of window to attain a BFRC A-grade, although in practice the majority of the higher-graded windows tend to be casement-style and made from PVCu frames.

Materials, glazing, and the gas used to fill the gap between the glazing and other design features all make a difference to the energy efficiency of a window.

Below are the main factors to consider for different types of windows:

Window frame materials

There are four main options when it comes to choosing the surrounds for your home’s fenestration – wood, PVCu, metal and composite. Below we’ll cover each material in more detail.

Timber windows

Timber will combine traditional aesthetic appeal with innate insulating properties. “It’s naturally more efficient than metal or PVCu,” says Donna.

The Centre for Alternative Technology recommends timber-framed windows unequivocally. Hardwood (FSC-certified) or durable softwood frames come from a renewable source, use comparatively little energy in manufacture, should last decades (with regular maintenance) and can be repaired easily.

Double or triple glazed windows are BFRC A-rated and can achieve a whole-window U-value of as little as 0.8.

They’re only available in casement style, although they can be made to unusual designs; energy-efficient timber sash windows are harder to find.

Timber windows by Internorm

Wood is a natural insulator, so is often specified as a frame material in projects where energy-efficiency is a high priority. These units were supplied by Internorm

A few joiners, including Carey & Fox, have started using AccoyaTM timber, which is modified using acetylation to create a new product that is stable, comes from sustainable sources and guaranteed to last 50 years. They can reach a centre-pane U-value of 1.1.

PVCu windows

PVCu units have come on leaps and bounds thanks to advances in the manufacturing process.

Multi-chambered frames are now incorporated to enhance performance. This cost-effective solution requires little upkeep and is available in an array of colours and woodgrain finishes.

“PVCu is a hugely sustainable material. The industry has closed the loop on the recycling process, taking old, early-generation frames out and reprocessing and recycling them for use in a new generation of advanced, low-maintenance and energy-efficient products,” says Gareth Parton, head of sales for Swish Window and Door Systems. ” This has been recognised by the BRE in its recent A+ and A Green Guide rating.”

There are A-rated PVCu windows of all designs on the BRFC website – even including some vertical sliding sashes.

The best-performing PVCu windows have four or five internal chambers in the frames, perhaps even filled with insulating foam.

Metal windows

While metal offers sleek, minimal sightlines, it’s typically outperformed by other materials.

Metal-framed windows are durable and require little maintenance, and can be ideal either for modern projects or to suit a period property.

They used to have a reputation for being cold and draughty – metal is a good conductor, making heat loss through the frames a problem.

Modern design, however, has made it possible for metal windows to be just as energy-efficient as other types.

Lightweight and strong, aluminium is a popular choice. It’s possible to find A-rated aluminium casement windows, as well as energy-efficient aluminium windows in other styles.

Make sure your metal windows are thermally-broken to provide adequate energy efficiency.

The Centre for Alternative Technology advises that aluminium involves a great deal of energy use and pollution during manufacture, and that coatings on some frames make recycling difficult.

Modern bathroom with Schueco windows

Modern bathroom with Schueco windows

On the other hand, manufacturer Beaufort claims that aluminium offers an almost unlimited life expectancy, with minimum maintenance and complete recyclability.

Stainless steel windows are renowned for being strong and slender; Crittall, one of the best-known manufacturers, says its high-performance range can reach a U-value of 1.7.

It’s also possible to buy bronze-framed windows – Vale’s hand-made metal casements are available in the delicate Heritage system, the more thermally efficient Advanced system, and even an inward-opening system.

Composite windows

Composite units offer the best of both worlds; excellent thermal performance and aesthetics.

Featuring a timber face on the inside and an aluminium finish on the outside, this type of product comes with wood’s natural insulation properties and aluminium’s innate strength.

However, this frame material is at the top end price-wise.

It’s also possible to find timber clad with PVCu and PVCu clad with aluminium. Alternatively, windows may be made of a sandwich of different materials.

Glazing options

There are two ways to make window glass more efficient – put simply, either add extra layers or use a special coating.

Double glazing will typically attain whole-window U-values down to about 1.2 W/m2K, while triple glazing can reach as low as 0.68 W/m2K. This is good to know if you’re looking to create a super efficient home, such as a Passivhaus.

Where double glazing is better than single, triple is better than double – though Tom Anderson of Green Building Store advises that triple glazing may not be worth the extra expense unless you’re building to a very high green specification overall.

Low-E coatings

Special finishes can also be applied to the glass to reduce heat loss. Low-E coatings, for instance, comprise a microscopically thin layer of metal or metal oxide.

This is applied to the inner pane of double or triple glazed windows and works to reflect warmth back into the room, rather than allowing it to escape through the glass.

Low-E coating is now standard under building regulations, and comprises a microscopically thin layer of either metal or metal oxide on the outer side of the inner layer of glass.

The coatings may be either hard, applied during manufacture, or osft, applied later. While soft coatings reduce heat loss, they also reduce solar gain, so take advice when choosing.

To increase solar gain it’s possible to use low-iron glass which, rather than having the slight green tinge associated with standard float glass, is almost colourless and has a high light transmittance. It’s ideally used as the outer pane in double glazing, with low-E glass for the inner pane.

Gas and air window fillings

Inert gas fillings can also be used to increase the energy efficiency of multi-pane units.

Argon is the most widely-used option. “Its thermal conductivity is roughly 67% that of air and it’s inexpensive,” says Donna. Krypton gas has the edge in terms of performance, although this option is more expensive to manufacture.

Idealcombi windows in a modern dining space

Idealcombi windows in a modern dining space

Double glazing traditionally used air in the spaces between the layers of glass, but modern, high-specification windows now use an inert gas, most commonly argon. Krypton is more effective and therefore more expensive, while xenon is the best-performing of all, and correspondingly the most costly.

Warm edge technology

Many high-spec modern windows also feature warm edge technology, which is designed to reduce heat loss through the spacer bars that are used to keep the glass panes apart in double and triple-glazed windows.

The panes of glass in double or triple glazing are separated by a spacer bar, conventionally made from aluminium, which conducts heat out around the edge of the unit and can encourage condensation.

Stainless steel spacer bars offer an improvement on this thermal bridging effect, but insulating spacer bars, sometimes referred to as warm edge have no metal content and are therefore considered much more energy efficient.

“These elements are traditionally made of aluminium, a highly conductive material,” says Donna. “Warm Edge spacer bars are instead manufactured with an insulating plastic composite that reduces heat loss and improves the thermal performance of the window as a whole.”

Window design & installation

Even the best windows won’t keep in as much heat as a modern, well-insulated wall. If your house design incorporates expansive swathes of glazing, you can help to offset the potential heat loss. The best way to do this is by incorporating extra insulation and airtightness measures elsewhere, such as in the walls, floors or roof. This is known as a fabric first approach.

The installation of an energy-efficient window is as important as its manufacture – a poorly installed window can be hugely compromised in its effectiveness, perhaps increasing heat loss by as much as 30 per cent. So, the thermal performance of your windows is only ever going to be as efficient as the installation itself.

“If the perimeter of the frame is not packed, sealed and insulated correctly then you’ve compromised the overall airtightness and thermal performance the unit is able to achieve,” says Matt Higgs, director and co-owner of Kloeber.

Positioning of the window in relation to the insulation, and how far back within the wall it is set, are both important factors. If the unit is fitted in a sub-optimal place, this could create a thermal bridge that leads to condensation on the inside of the window frame.

“This would mean that the surround is cold inside and not performing correctly,” says Matt. “In most scenarios it’s better to have the frame sitting further back towards the internal side of the wall. Cavity closers should always be incorporated to close off the gap between the internal and external skin of bricks.”

“When it comes to installing your glazing, one of the most significant questions to ask is who is going to be fitting the products,” says Edward Stobart, head of projects at IDSystems. “I’d recommend hiring a company that supplies and installs the doors themselves – rather than relying on inexperienced third party fitters.”

This way, you’ll deal witha  single company who is responsible for ensuring the agreed window performance is achieved.

Ecoplus3 triple-glazed windows

Green Building Store installed the Ecoplus3 triple glazed windows fpor this project

You can also liaise with your installer to double check what method will be used to seal the perimeter of the window when it’s fitted. “Will it be a Compriband expanding tape or a minimum quality expanding foam?” says James Munro, technical manager of Internorm by Cherwell.

One way to help ensure your installation is up to scratch is to work with a professional who is registered with the Fenestration Self-Assessment Scheme (FENSA).

If your contractor is not part of this scheme, you’ll need to get the installation approved by a building control officer directly.

Retrofitting windows

In most situations, replacement windows must meet building control standards – in general their U-values should be no greater than 1.8 W/m2K or 2.0 W/m2K.

In England and Wales, you will need a certificate of compliance, which comes either from building control or by using a window company that can self-certify with FENSA.

Problems arise, however, when dealing with historically sensitive buildings, or those in a conservation area.

Read more: Windows, repair or replace?

In the event that you are allowed to install double glazing in your build project, you may be required to use a ‘conservation’ style, such as the conservation sash from Mumford & Wood.

On the other hand, if double glazing is not permitted, it will be impossible to meet building regulations and you’ll need to discuss the options with your planning authority.

This article originally appeared in Build It magazine in 2012. It has been updated with additional content by Rebecca Foster in 2018.

Top image: This folding window by Norrsken allows for expansive views of the countryside

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