Specifying Energy Efficient Windows – Your Questions Answered

Which Window Materials Are Best for Thermal Performance?

The right energy efficient windows will not only complete your home’s design but also fulfil 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:

• PVCu
• Timber
• Aluminium
• Alu-Clad Composite

PVCu

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.

These white PVCu casement windows from SafeChoice achieve a traditional aesthetic using glazing bars

“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

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. Hardwoods such as oak can provide a luxury feel, but these often come in at a higher price point.

Manufactured in the UK, this flush face casement window by Mumford & Wood is made from FSC-certified timber

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.

Aluminium

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.

Alu-Clad Composite

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.

Internorm’s HV 450 timber-aluminium window has an impressively low U-value of 0.6W/m²k, and offers the option of integral blinds

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.

Who Should Install My Energy Efficient Windows & Doors?

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.

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.

Does Triple Glazing Cost More?

Clearly, triple glazing requires an extra pane of glass – so, in theory it should be about 50% more expensive than double glazing (minus any economies of scale). That rings true in ballpark window costs Build It sourced from Kloeber in early 2023: they put standard double glazing at around £153 per m² (for the glass panels only); and triple at around £225 per m².

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

The curveball is that a number of suppliers have been incentivising self builders and homeowners to move to triple glazing via significant discounts – including offers such as triple for the price of double. Naturally, this could offer great savings for your project.

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.

How Can I Specify Energy Efficient Windows for a Heritage Property?

If the existing windows and doors in your home are causing excessive heat loss or are in a very poor state, then it’s time to look at 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.

As part of their listed home renovation, Mark Camillin and Liam Denny replaced the original home’s single-glazed windows with conservation double-glazed hardwood models, alongside installing heritage-style aluminium bifolds and French doors. Photo: Richard Downer

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 also need to understand the impact these changes could have on how moisture is managed in older houses, which leads us onto the next question…

Do Energy Efficient Windows Still Need Trickle Vents?

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 feature 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, managing director at 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

Can Glass Coatings Reduce Heat Loss?

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.

Do I Need to Worry About Energy Efficient Windows Causing Overheating?

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.

Set within the urban Highgate conservation area of north London, Emil Eve Architects transformed this five-storey Edwardian terraced house with a striking and impactful glazed kitchen-diner extension. The addition features glazed sliding doors from IDSystems. Photo: Taran Wilkhu

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.

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.