A Commercial Guide to Passivhaus Certified Windows

Adopting the Passivhaus approach to home design enables architects and specifiers to make significant energy efficiencies. Passivhaus standards set performance targets that help reduce bills and improve occupant comfort, health and wellbeing. An area where significant performance improvements can be made is a building’s windows.

Growing Passivhaus Windows Standards Adoption

Passivhaus windows installation is growing in popularity in the UK – or at least the use of materials and design principles that underpin them. There are now 1,500 certified homes in the UK, with a further 7,000 in planning, under construction or almost complete. Membership of the UK Passivhaus Trust has also doubled between 2020 and 2022 alone.

There are several reasons why interest is increasing. For those working in the built environment there’s legislative pressure in the form of the Future Homes Standard, which proposes changes to Part F and Part L of Building Regulations.

Today’s building designs will need to evolve in line with these updates although thought will also need to be given to futureproofing, not least because there is a huge volume of legacy building stock and further changes to building law can be expected as the demands of net zero begin to accelerate.

It’s this idea that is leading some authorities, such as the devolved Scottish government, to consider making all new build homes Passivhaus standard or equivalent. This would include homes that are custom and self-built, as well as housing associations, councils and the private sector.

There are some key points that architects and specifiers should be aware of when choosing windows for a building developed to Passivhaus standards, even if the property itself is not being retrofitted to achieve official certification — note that windows do not need to be certified by the Passivhaus Institute to work successfully in a build.

Passivhaus Windows Materials

As it stands, Passivhaus windows are typically triple glazed and filled with argon or, more rarely, krypton gas. However, double-glazed designs with sufficient performance may eventually be possible with the advent of vacuum-filled glass units.

The suitability of materials used for Passivhaus standard windows is a subject of debate, despite PVCu, aluminium and timber-based composites all being capable of achieving certification, in terms of thermal performance.

However, the quality of installation and compatibility within the wider building design is critical. Selecting a high-performing window and installing it poorly can equate to buying a low-energy equivalent.

That said, there are some key differences that should be pointed out. For example, some Passivhaus windows suppliers’ products, such as REHAU PVCu windows, do not need a thermal break, unlike aluminium frames.

Manufacturers should offer a range of profiles to accommodate Passivhaus requirements. For example, REHAU offers different profiles (transoms and mullions) for its GENEO window system to accommodate higher wind loads.

Achieving Optimum Energy Efficiency

Airtightness is key to overall heat loss as it minimises uncontrolled air movement in and out of a building. Once a more fringe concern for the wider building industry, it’s now seen as one of the most important objectives on any building project. It is key for energy efficiency, thermal comfort and for protecting a building’s structure from damp and mould.          

Standard windows often have colder surface temperatures, creating ideal conditions for condensation. Regular moisture build-up can allow mould and other pathogens to thrive, impacting occupant health and wellbeing. Passivhaus windows eliminate this threat by managing the building’s thermal envelope, thermal bridges, air flow and ventilation.

Material Sustainability

The energy required to produce frame raw materials varies considerably. For example, the extraction of aluminium is extremely energy-intensive, requiring 190-230 megajoules of primary energy per kg of aluminium extracted and processed.

PVCu, on the other hand, has a relatively small footprint and it consumes less primary energy than any of the other commodity polymers. According to the British Plastics Federation, aluminium creates 10kg of CO2 for every 1kg of useable material produced; PVCu sits much lower at 1.9kg.

While not of direct consequence for thermal performance, the sustainability of window materials is still important to note—not least because the Passivhaus windows concept is itself rooted in a low-carbon built environment, as well as high home energy efficiency becoming a default consumer expectation.

Done correctly, PVCu recycling is also highly efficient, using a fraction of the energy required for virgin production. This concept is illustrated by the success of REHAU Windows’ PVCR—the company’s dedicated UK recycling facility.

Beyond this, there are other more practical issues for architects and specifiers to consider, especially when using timber-composite frames that are more prone to rot and water ingress. If this occurs, a window’s thermal performance could fall well below Passivhaus standards, requiring remedial work to be brought up to specification. These issues are virtually unheard of with PVCu and aluminium frames.

Passivhaus Windows for Retrofit

Passivhaus projects are principally new build, mainly because it’s very difficult, expensive and time-consuming to retrofit a property up to the required standard. That said, it’s widely accepted that existing properties need significant improvement.

For this reason, the Passivhaus Institute has developed EnerPHit certification—quality approved energy retrofitting with Passivhaus components. Significant energy savings can still be made with this method, and it’s generally agreed the principles of Passivhaus design need to be incorporated into older, inefficient properties where feasible.

In Conclusion

While the Future Homes Standards applies legislative pressure to design and material choices, the benefits of adopting Passivhaus windows standards remain clear; reducing home energy bills and improving the health and wellbeing for a home’s occupants. These are the main drivers behind the Passivhaus concept.

It is important that the Passivhaus standard should be seen as a fundamental design factor as it cannot be achieved simply by meeting each separate technical requirement, such as airtightness or window U-values. Passivhaus should become an integrated part of a building’s conception from the outset, underpinning every design aspect.

For more information about Passivhaus window standards and specifications visit: rehau.co.uk/Passivhaus

Engineering progress

Enhancing lives

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