PVC-U – Passivhaus Powerhouse

Midlothian Passivhaus properties

When Midlothian Council decided Passivhaus was the standard it wanted to meet with new homes, it realised windows would play a fundamental role in achieving the levels of airtightness, insulation and overall efficiency required. PVC-U was chosen over all other materials. We report.

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 in achieving the levels of airtightness, insulation and overall efficiency required in a Passivhaus building, close attention was paid to the material choice, style and U-value of the frames.



Passivhaus (or Passive House) 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) unit.


Tackling fuel poverty with sustainability champions

In 2020 Midlothian Housing and the local authority for Midlothian, Scotland, committed to building new Passivhaus social housing, and set this out in its housing design guide. The main aims of the project 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.


Architectural team

Knowing that achieving such rigorous standards would be no easy feat, the council appointed Smith Scott Mullan Associates, an Edinburgh-based architectural sustainable design practice. Its team of dedicated professionals now 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 with it its own set of technical difficulties. The two-block project in Bonnyrigg, delivered by contractor Flemings Buildings Limited, 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. 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. It 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 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 plethora of nuances across each project’s design, two things were non-negotiable when it came to the windows – the thermal efficiency and the maintenance conditions. A variety of options were considered in early technical workshops between Graham Drummond, Smith Scott Mullan Associates and contractors. PVC-U was agreed upon as the material of choice due to its low maintenance properties and more cost-effective price against alternatives. Drummond then went on to recommend Rehau’s Geneo Passivhaus windows based on his previous experience with the product and its niche ability to meet the technical specification.



Specialist fabricator and one of a handful in the UK that can manufacture windows to this specification, GRM Windows, was then appointed, after a tendering process, to supply the final product across both sites. The Rehau Geneo tilt & turn has a 86 mm 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’s expertise in all stages of the development process across both sites, from design to fit. The 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, MD at GRM says: “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, 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.


Pictures: Some of the Midlothian Passivhaus properties playing a part in net zero and decent homes objectives. PVC-U windows were chosen for their insulation and low maintenance characteristics.


Article written by Brian Shillibeer
18th June 2024


Related Articles