An ambitious vision for a pioneering CLT Passivhaus primary school building in a historic part of Edinburgh was realised thanks to an open-minded team effort. Clara Garriga from architects Holmes Miller explains all to Roseanne Field
Despite its grade-B listed status and general grandeur, Sciennes Primary School – located in the Marchmont conservation area of Edinburgh – was also home to some dysfunctional modular temporary classrooms located in a small corner at the back of the site. Leaking and draughty, an upgrade was long overdue. The end result was a standalone extension that is the first Passivhaus building constructed in CLT in Scotland. Passivhaus certification is expected this month, and depending on timing, this could mean it is Scotland’s first Passivhaus primary school too.
Having a long working history with City of Edinburgh Council, architecture practice Holmes Miller was approached by the council to work with them and the school on a replacement. When discussions first started, the school was unsure of what they wanted, beyond the simple need for four new classrooms. “The initial brief really was as simple as ‘we’ve got to replace this with new classrooms,’” explains Holmes Miller project director and the project’s lead architect, Clara Garriga. “It could either be another set of modular classrooms, or something a bit more aspirational.”
Holmes Miller had worked on several schools with the council previously, and having been impressed with their offering, wanted “an educational space that would carry that knowledge into the Sciennes school as well,” Garriga says.
A good space for learning
As a client, the council was very “open minded,” Garriga explains, working with the practice and school to develop a brief. “As part of the general works Holmes Miller were doing with the council, we went through a lot of discussions on what a classroom is, what makes a good educational space, what’s suitable for children of different ages,” she says. As well as the architects, the council and school, also present in these meetings were interior designers, teachers and headteachers, and staff working in nursery settings. “It was a mix of specialities, all having this open discussion on what makes a good space.”
This notion of identifying ‘what makes a good educational space’ was the key driver and led to the optioneering of possible forms. “The actual shape or form of the classrooms can be very different,” Garriga says, “so having a discussion where we get everybody keen to design something that’s tailored to different schools and clients is so important.”
Holmes Miller were certain that adding more modular classrooms wasn’t the answer, and suggested creating something larger that connected with the main building, or a standalone building. Having decided upon the latter option, the team then considered the teaching that would happen within, and looked at the possibility of creating a shared space for the whole school to use, locating the new classrooms in the main building and then relocating elements such as the library into the new building. “It really transpired that what they needed was a purpose-designed ‘Primary 1’ space,” Garriga explains. She refers to the Scottish equivalent of Year 1, which children enter direct from nursery.
Given that Primary 1 children are only four and five years old, it opened a whole new range of discussions. “Primary 1 is so different to what’s required as you progress through the years,” says Garriga. “It’s almost more like a nursery in many respects, it’s play-based learning and teaching them to be more independent and confident, rather than a more formal type of teaching.” She says that designing for this age group “really gave us the opportunity of opening up the space and creating an open plan, flexible accommodation between the spaces, having a connection between floors and classrooms.”
Despite the initial brief being for four classrooms, the idea of opening up and connecting the space meant the final design resulted in an open plan space with no doors, glazed screens between spaces and a void connecting the ground floor and first floor. “You can really see everything that’s happening everywhere in the building,” Garriga says. “It makes it feel like one unit rather than four separate classrooms.” These open areas offer versatile teaching environments and create a visual connection across the various spaces.
Although Holmes Miller has plenty of experience designing primary schools, Garriga says the Sciennes extension required a different approach, which led to benefits. “Normally when we look at a school we look at the whole school and how it connects rather than a separate unit – it was quite interesting to focus on that here,” she explains. “The design really benefited, thinking more about visual links between all of the spaces internally and the void, and making sure light moved around the space.”
While an open space made the most sense from a functional point of view for the age of the children, it in turn created a host of challenges. Acoustics in particular was a crucial consideration: “The combination of open plan nature and children yelling is not great!” Garriga says. “The acoustician was involved from day one.”
The solutions offered by the acoustician were integrated with the design as much as possible so as to avoid them looking, as Garriga says, “stuck on” – such as acoustic boards that became murals or pinboards. Specifying acoustic solutions mean the classrooms can function independently as well as allowing for more open plan learning. “We have all these hugely absorbent surfaces on the walls and hanging acoustic baffles on the ceilings so we could expose all the services to create a bigger volume while also helping with the acoustics – it was all designed together because we knew who was going to be using the space, and how,” Garriga explains.
The building’s location at the rear of the school, close to the school’s boundary and therefore its neighbours, posed its own challenges. The rear playground on which the extension was built is small, meaning there were a lot of “physical constraints,” says Garriga. The original school building is three to four storeys high and therefore completely overshadows the site, but in order to avoid any overlooking of the neighbours the new building couldn’t be too high. “We had to keep good distances from the boundaries, the neighbours, and the existing school, not just for acoustics but also for fire purposes,” she explains. “We then also had to keep a good distance from the original school building, trying not to have windows overlooking, and to keep nice outdoor spaces around the new pavilion and existing school.” She continues: “It was an important technical requirement that took a lot of consideration; keeping enough distance to the boundaries while not reducing the playground space.”
Planning was also a potential sticking point, with the site’s location in the Marchmont conservation area and the original school’s grade-B listed status. “Planning was highly involved, especially in the early stages,” explains Garriga. “It’s a very beautiful area, so the planners were looking for something that wouldn’t impact negatively on the context, noting a preference for the pavilion not to be seen from the street. They were very focused on the volume, and avoiding overshadowing and imposing on neighbours.”
They designed the building as distinctly modern, not mimicking the original school but complementing it, and the planning department agreed with this approach. The design solution took cues from its surrounding context, relating to volume and height, such as keeping the horizontal lines level with the height of the terraces along the street. “There was a lot of discussion with planning on how this could sit next to the original building without trying to copy any of the features,” says Garriga.
“That really led to some of the design decisions such as the stonework and some of the horizontal lines we picked from the stonework. We tried to keep it quite simple but in dialogue with the original building.” Overall, planners agreed the proposal was “respectful of the setting,” Garriga says.
One thing that impacted the budget and the design further down the line was the decision to aim for Passivhaus certification. Discussions for what to do with the site first began in late 2018, and it wasn’t until just before 2020 that the idea was first proposed by the council, who had implemented a policy to consider a Passivhaus approach for all of their new buildings. “Everything was really pointing at it not being feasible, because of the lack of solar gain and site constraints, but we were all keen to try and do it,” Garriga says.
It was to be the first Passivhaus building delivered by not only the council, but also Holmes Miller, and later the contractor, Maxi. “Everybody was really keen to learn and do the right thing,” she says, saying that Maxi were “outstanding,” such as recording every single penetration through the external walls and photographing it.
During the design stage, the council employed their own M&E team to develop the Stage 4 information, offering informed insights into predicted usage and recommending preferred systems. Throughout the construction phase, the council’s team continued their involvement, serving as consultants to assess and validate the work carried out by the delivery M&E consultant and onsite subcontractors. Structural engineers were also heavily involved from early on, offering solutions which responded to this target.
Environmental consultant Sussed Sustainability was brought in during this phase to support the Passivhaus design and certification process. They played a crucial role aiding the design team in making informed decisions, gathering information, and subsequently offering valuable advice to the site team during construction.
The architects set a target for airtightness of 0.3, outperforming the standard Passivhaus level of 0.6. Ideally, Garriga explains, the designers would have altered the orientation of the building but this wasn’t possible due to the constrained site. In order to make up for this, U-values and airtightness were under more pressure. In the end, they achieved a final score in the airtightness test of 0.28, which as Garriga says is testament to the work done. There are “virtually no thermal bridges,” she says, with every wall tie, cable penetration and soil vent pipe calculated and thermally broken.
Access is via a single door on the ground floor leading to the shared space between the two ground floor classrooms; it has automatic closers, meaning the impact on airtightness is minimal. “In a nursery it would be more of a challenge because they want doors open all the time,” Garriga says.
The other key aspect that required work following the decision to target Passivhaus was the form factor (the ratio between the building’s external envelope where heat can escape compared to the usable internal space). The footprint of the building had to be revised to aim for a form factor of 3.04, which was achieved largely by reducing the height and simplifying the footprint to produce a more compact shape. They also had to reduce the size of the windows to mitigate heat loss, bringing the sills up to a height of 800 mm – originally these had been designed to go right down to the floor to maximise on daylight, in particular to make up for the lack of windows on the north elevation which overlooks neighbours.
Where possible, these required alterations for energy efficiency were turned into design features – the bottom of the windows was brought up and the newly-added sills turned into bench seating. “It was about changing things in a positive way,” Garriga explains. “For example, that gave us the opportunity to create seating which offers a variety of uses in the classrooms. The design actually lends itself quite well to that, it’s the part of the window that brings in the least amount of light because it’s low and the spaces are still bright.”
The design had always been predicated on having underfloor heating with an air source heat pump, but the decision to target Passivhaus meant an MVHR system had to be included as well. “That took a lot of coordination with the design team,” Garriga admits. All the services were deliberately left exposed in order to not compromise on the volume of the internal spaces. “The whole thing works together because rather than individuals coming in at separate stages we all worked together – including structural engineer, services engineer and interior designer.”
Thankfully, although some of the architectural and interior design elements had to be revised, the Passivhaus target was implemented before the technical design had been developed, meaning details such as insulation levels were considered from the start, taking into account heat loss from elsewhere. They did however encounter some hurdles at this stage where they found that many Passivhaus-certified components were more targeted at the domestic market. For example, the architects struggled to obtain the fire rated window and door components they needed such as panic hardware and push pads, and automatic openings for smoke ventilation.
The decision to aim for Passivhaus came about during 2020, so they also had the added obstacle of trying to source and specify these components and make the design tweaks during the Covid pandemic, meaning face-to-face meetings were impossible.
The windows are all triple glazed and are a combination of timber windows clad externally with aluminium, which are Passivhaus certified, and fire rated windows which aren’t Passivhaus certified. “There was some difficulty getting the backup data for those for the certification process,” admits Garriga. There are also quadruple glazed rooflights, which “bring a lot of light to the first floor.”
Despite certain design elements requiring altering and redesigning, the layout itself remained unchanged after the decision to get Passivhaus certification. “We got the educational building that we set ourselves from the beginning to have,” says Garriga. “Passivhaus is a benefit in terms of the energy usage, but the main driver was building a good educational facility.”
CLT’s primary Passivhaus benefits
One thing Garriga and the team knew they would use from early on was cross-laminated timber. “We chose it primarily because of the natural finish, internally it would give us the warmth and the space we were really looking for, for Primary 1 education,” she explains. The speed and ease of construction was also a big selling point for the team – and particularly important here due to the constrained site. “That brought the least disruption to the neighbours and the school.”
When it came to considering the Passivhaus certification, CLT’s qualities made it particularly suited for the higher airtightness required. “It’s very easy to seal, so the airtightness was largely made possible because of the CLT,” says Garriga. “It’s also biodegradable, recyclable and has benefits in terms of CO2 calculations,” with every m³ of CLT used capturing one tonne of carbon dioxide.
There were additional considerations, such as the essential issue of fire protection. The practice made two key moves to help reduce the CLT’s combustibility – firstly cladding certain parts with non-combustible boards which in turn helps conceal wall services in the internal spaces. In the areas that weren’t required by Building Control to be fire rated, the surface still needed to be treated with a matt finish transparent spread of flame coating which they used all over.
With all the CLT prefabricated offsite, with openings for doors, windows and services pre-cut, erecting the frame took two weeks, so disruption was minimal. “There was a really good relationship between the contractor and the school, agreeing certain days when they needed to keep it quiet, says Garriga.” The constrained site meant they contractors had very limited space, but they managed to secure part of the street to store materials and place cabins. “The more difficult thing was keeping the school’s fire exits open, which meant the space was even smaller,” Garriga says.
She adds: “With the landscape architect we introduced a rain garden, which is a really nice feature, and seats and planters, spaces to sit down and grow things.”
Looking ahead to future projects, Garriga is passionate about pursuing more Passivhaus-certified schemes – or at least using Passivhaus principles – wherever possible. “It needs to be the norm,” she says. “Looking at this type of fabric, the performance and this level of construction.” She strongly encourages the use of CLT in particular, but admits it is always easier with a client who “wants to do the right thing.”
The project completed in August 2023, so while the practice has as yet had limited feedback and data on the building’s performance, Garriga says comments they have heard so far have been positive. “The teachers, children and council have said they love the space,” she says. Asked to name her favourite element of the new school building, she chooses the open plan interior: “It is really different from what you get in a traditional school building, it’s a special sort of environment.”
- Form Factor: 2.86
- Airtightness: <0.6-0.3 ACH
- Annual heat demand: 15.5 kWh/m²a
- Peak heat load: 9.0 W/m²
- Average building fabric U-value: 0.85 W/m²K
- Thermal bridging heat loss allowance: 5%
- MVHR efficiency: 84%