External Building Envelope


Precast in the frame for net zero future


A precast concrete approach to constructing the building frame for the UK’s largest ever Passivhaus school has demonstrated the material’s potential for use in highly sustainable buildings. Mark McCann at FP McCann, the country’s largest manufacturer of precast concrete products explains why.


T


he development of a new educational campus has shown how precast concrete could play an increasingly important role in the construction of public buildings designed to meet the highest sustainability standards, and help the UK to achieve its net zero carbon goal.


The recently opened £120m Dunfermline Learning Campus primarily replaces two schools which had reached the end of their lifespan and no longer met the needs of 21st century education. The project to replace Woodmill High School and St. Columba’s RC High School was initiated by Fife Council in 2019 with the aim of bringing together the two secondary schools into a single building. It would also enable a new facility for Fife College to be integrated into the site to create a new education hub as part of a wider masterplan.


Rather than adopt a compliance-level approach where the new building would meet the current Building Regulations and school design standards, Fife Council had much more ambitious goals to create a state-of-the-art facility. In addition to ensuring the two schools would retain their individual identities, Fife Council sought to create a number of high quality shared facilities to enable progressive teaching and the cross-fertilisation of ideas in a building designed to push the boundaries for collaborative working.


Energy reduction and sustainability were high priority too, with Fife Council stipulating design to achieve Passivhaus Classic Certification, whilst also setting a low embodied carbon value for the project, as per RIBA 2025 targets. In addition, this was one of the first construction projects to apply the Scottish Government and Scottish Futures Trust’s new


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‘Net Zero Public Sector Buildings Standard’, a voluntary standard designed to support public bodies to define objectives for their construction project in pursuit of a credible path to net zero operational energy.


Focus on the building form


AHR Architects were tasked with designing the new building to accommodate Woodmill High School and St. Columba’s RC High School. Its Glasgow team worked closely with Fife Council’s architect John Peden to bring the vision to fruition and main contractor BAM Construction, who have delivered a number of previous projects for the council including Madras College in St Andrews.


AHR recognised the importance of embracing key Passivhaus principles from the outset, firstly in respect of the building form. In Passivhaus projects, the ‘form factor’ must be considered to ensure the ratio of the external envelope to the floor area is minimised, thus lowering the potential for heat loss. This resulted in a design with the accommodation arranged over three floors and a relatively compact form which avoided complexities such as external soffits and multiple steps in the massing.


Simplification of the build process was also significantly important for a building of this scale. In particular, how to ensure the building’s main frame could be built most efficiently whilst also achieving super-high levels of airtightness.


An analysis of the frame options was undertaken early in the design at RIBA Stage 1-2 to assess which approach would achieve the airtightness level of 0.6 air changes per hour (ACH), expressed as ≤ 0.6 h-1 @ 50 Pa. From this, the decision was made to adopt


three different methods. Precast concrete was selected for the main building frame, CLT (cross laminated timber) was chosen for the sports block and a hybrid steel frame as used to form the core of the building.


Delivering on precast concrete’s airtightness promise


Choosing precast concrete for the main building frame maximised delivery of an airtight envelope and simplification of detailing. This is because concrete is in itself inherently airtight, which makes it easier to track the airtightness line through the building, and it does not rely on membranes and tapes throughout the line.


In addition, because the joints between the precast panels could be pressure grouted, this further assists in maintaining an airtight line. Also, in general, concrete allows the airtight line to be on the internal face of the panels, allowing for ease of tracking and resolution of any issues throughout the build.


The theoretical advantages of using precast concrete, however, still required translating into reality, and this is where the expertise of BAM Construction and FP McCann came into play. We had to ensure approximately 1,500


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