Passivhaus rules

Meeting net-zero targets using Passivhaus design principles

Clara Garriga suggests that following the successful opening of an innovatively designed extension to a heritage primary school building in Edinburgh, Passivhaus can become ‘the new normal’ for school design.

The brief and the sponsor

Designing a school building to Passivhaus standards to reduce its carbon footprint is not the cheapest or the fastest way to build – but it doesn’t aim to be. What it does deliver is exceptional levels of longer-term affordability, interior comfort, health, wellbeing and durability. Meeting a school sponsor who understands this, rather than just considering up-front cost, is the first step to success. And herein lies the challenge, especially in this time of extremely tight budgets.

One such ambitious sponsor is the City of Edinburgh Council, who brought us on board to design what has become the first Passivhaus primary school project in Scotland using a cross-laminated timber structure. We had previously worked with the Council on several successful school projects, and they always play an integral part in the design process.

The design brief involved a two-storey extension to Sciennes Primary School – a 19th century Grade B-listed building in the Marchmont conservation area of Edinburgh. The new-build element offers four additional classrooms that lead to shared flexible teaching areas, which encourage interaction between pupils from different classes. It opened in September 2023 at a cost of £3.2 million.

Design constraints and materials

A very tight site meant that the new extension would be overshadowed by the existing school building, reducing the opportunity for winter solar gain. To compensate, we used a fabric-first approach, addressing air-tightness and thermal performance by minimising openings and cold bridging, heavily insulating the walls and roofs, and using triple glazing. The building has achieved an impressive air tightness rating of 0.28 ach/hr@50Pa.

The use of structural timber was the perfect choice for this project, as it generally accomplishes a better air tightness than a steel structure. It also has a positive impact on user wellbeing, ensuring the classrooms and shared areas have a warm and soothing atmosphere while also effectively reducing embodied carbon when compared to alternative structural solutions.

Compromise without sacrifice

Understandably there were some questions from the Council about how a largely timber building could meet fire regulations. The process of integrating timber into the design was more complicated than expected, but we overcame this challenge by covering some of it up with fire-rated gypsum board. We did have to compromise on the amount of Cross-Laminated Timber (CLT) we could leave exposed internally – around 25%, when we had hoped for more. As more industry guidance emerges on timber buildings and fire, this process will get easier.

Happily, design-implementation was able to deliver an excellent environmental performance without compromising on natural light. The building features large windows with integrated seats and extensive rooflights, while improved landscaped areas in the playground encourage outdoor learning. It is these components which truly drive wider benefits for the pupils and offer a long-term higher quality place to learn, play and develop.

As projects like Sciennes reach completion, we hope they will demonstrate to others what can be achieved with the right mindset and that Passivhaus can become the ‘new normal’ for education buildings in a wide variety of international environments.

Embracing Passivhaus

With the success of this project under its belt, the City of Edinburgh Council is becoming increasingly committed to Passivhaus to support the decarbonisation of its education building stock, while also improving the learning environment. Last month, we submitted a planning application for the refurbishment of the existing Victorian building at Sciennes and a large, ambitious four-storey Passivhaus extension to Trinity Academy in the north of the city.

One of the reasons the Passivhaus standard is becoming increasingly attractive for schools is that it ensures a building’s actual energy use is, on average, extremely close to the amount predicted by models. Quality control on site during the construction process also ensures that any Passivhaus building will be built to very high standards.

And if a school is to be genuinely designed and built around the needs of the end user, the benefits of a Passivhaus approach will last throughout the building’s lifetime.

This is certainly the approach encouraged by the Scottish Futures Trust, as part of the Scottish Government’s Learning Estate Investment Programme funding. It stipulates that a ‘Band A’ of 67-83kWh/m2/yr must be met for a school’s overall energy use, in order to receive full funding over the 25 years following completion.

Ensuring energy standards are met

 

It’s only possible to meet these long-term energy standards if the design team consider absolutely everything, from the building form and orientation, to materials, equipment and end uses. This also means talking to teachers and will involve researching exactly what equipment teachers often plug-in during classes and for how long, to estimate predicted energy use.

Ultimately, if you put the technical expertise and intricacy aside for a moment, it’s clear what it really takes to make Passivhaus schools a reality is commitment and collaboration. A tight-knit design team is essential. Co-operation and buy-in is needed across the board, including with subcontractors and stakeholders.

Architects and contractors can help an initially dubious school sponsor take that initial plunge into Passivhaus design by showing what can be achieved with the right commitment and encouraging them to take a longer-term view of potential cost and carbon savings.

Hopefully, that job will get easier as more successful Passivhaus schools open their doors. With the UK’s ambitions for net zero gaining increasing importance, we hope that Sciennes Primary School will be the first of many educational projects of its kind.

 

Clara Garriga is a Project Architect at Holmes Miller architects

“When the doors on a new school are thrown open this must deliver the best possible learning environment for every pupil;”

 

 

Feature & Support Images: kindly provided by Holmes Miller Architects

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Trinity Academy: click on the link to read the article featured in Scottish Construction Now