Framing the Passivhaus homes of the future

With housing providers under pressure to deliver affordable housing that meets the UK’s net zero targets, new developments must be both environmentally conscious and consumer friendly. Midlothian Council built Passivhaus-certified social homes in two of its newest developments, with windows playing a fundamental role.

Passivhaus is a tried and tested methodology that provides a range of solutions to deliver net zero ready homes. To achieve optimal comfort levels for occupants, the standard ensures homes use very little energy for heating and cooling, relying on high levels of insulation and air-tightness, and a mechanical ventilation with heat recovery (MVHR)  system. The windows specified are vital in achieving the levels of airtightness, insulation and overall efficiency required in a Passivhaus building, which is why close attention was paid to the material choice, style and U-value of the frames. 

Tackling fuel poverty

In 2020 Midlothian Council, the local authority for Midlothian, Scotland, committed to building new Passivhaus social housing, and set this out in its housing design guide. The project’s main aims were to help the council meet wider net zero targets, while tackling the issue of fuel poverty and improving the provision of comfortable housing.

The first two developments assigned were Buccleuch Street in Dalkeith – one building housing six flats, and Burnbrae Road in Bonnyrigg – two buildings encompassing a mixture of retail and residential properties.

Knowing that achieving such rigorous standards would be no easy feat, the council appointed Smith Scott Mullan Associates, an Edinburgh-based architectural practice, and industry leader of sustainable design practices. Its team of professionals includes six certified Passivhaus designers, two PAS2035 co-ordinators, and two accredited conservation architects.

Leading the project was Claire Cockburn, Senior Architect and certified Passivhaus designer and Jarek Gasiorek, Architect and Certified Passive House designer. Specialist Passivhaus consultant, Graham Drummond, was also appointed as part of the team to advise on specific areas such as window material choice.

The Burnbrae Road challenge

Burnbrae Road brought its own set of technical difficulties. The two-block project in Bonnyrigg, delivered by contractor Flemings Buildings, comprised a mix of residential and a retail unit which required a customised Passivhaus certification process. While all 20 homes were to be designed to achieve Passivhaus certification, the retail unit was only required to have a Passivhaus compliant envelope with double-glazed windows and steel security doors.

Due to this unique challenge, the Passivhaus certifier, WARM, and Passivhaus Institute had to agree on a customised certification to make the project a reality. The average U-value targeted at design stage was 0.83 W/(m2K), across a total of 48 windows.

The Buccleuch Street challenge

The project at Buccleuch Street followed a similar specification to Burnbrae, and was delivered by contractor Morris and Spottiswood. This was to develop a new block of six flats and the refurbishment of a nearby existing building. However, the site was constrained and involved competing demands. The need for fire safety, structural stability, and space to house the MVHR system created conflicting needs that required adaptable solutions. The average U-value targeted at design stage was 0.87 W/(m2K), across a total of 32 windows. 

A not-so-common solution

Despite the nuances across each project’s design, two things were non-negotiable when it came to the windows – the thermal efficiency and the maintenance conditions. PVCu was agreed upon as the material of choice due to its low maintenance properties and more cost-effective price point against alternatives like timber. Graham then recommended REHAU’s GENEO Passivhaus windows based on his previous experience with the product and its ability to meet the rigorous technical specification.

Specialist fabricator GRM Windows was then appointed to supply the final product across both sites: the REHAU GENEO Tilt and Turn: 86mm profile depth utilising RAU-FIPRO X – a fibre composite extruded profile, with six chamber systems and triple gasket sealing. All 80 windows were supplied with anthracite grey exterior frames to match the modern design of the homes, and all were Secure by Design certified for additional resident safety.

Midlothian Council was keen to involve REHAU and GRM in all stages of the development process across both sites, from design to fit.

Project Manager Neil Davidson says: “The REHAU and GRM teams have been an integral part of the design and build process across both multi-faceted projects. The windows met Passivhaus standards and the REHAU team’s technical expertise informed many decisions throughout the build.”

Richard Gambling, Managing Director at GRM, adds: “The main challenge on this project was balancing all aspects of the design. By working in close collaboration with our glass supplier, we were able to test the GENEO to PAS 24 at a specific size for Buccleuch Street. We also achieved around 44dBA reduction on one elevation at the same time as meeting thermal and solar gain requirements of the Passivhaus design team. This wouldn’t have been possible without the support from REHAU – all queries from the designers and contractors were solved accurately and quickly.”

Framing social homes of the future

Thanks to airtight seals and membrane features, not only were sustainability targets met, but noise attenuation parameters were also maintained. Due to a local social club next door to the Buccleuch Street site, these windows were designed to an even higher specification to provide residents with peace and quiet inside their homes.

Alongside the use of other technologies including solar photovoltaics, heat storage, and electric heating, the projects were some of the first Passivhaus triumph of its kind in Scotland and have set a very high standard for the future of social homes.

Neil adds: “Across both projects, the REHAU team provided technical insight throughout the design process that will be transferable for those working on site in future Passivhaus projects.”

Samsara McDonald, Commercial Sales Manager at REHAU, concludes: “It’s testament to the REHAU team that we were chosen as the supplier for this first of its kind project in Midlothian. Passivhaus requires some of the highest industry standards to be met and REHAU products do just that. We know that our products speak for themselves with unbeatable U-values and the capability to address a range of project needs, but our team’s expertise and knowledge is what really sets us apart.”

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