The client brief for the Eco Business Centre, part of Bicester’s Eco Village, called for a zero-carbon workplace building to serve as an incubator for local start-ups and freelancers.
The result is the first building of its kind to be awarded Passivhaus Plus in the UK. The three-storey building is at the heart of the UK’s first Eco Town and provides flexible and supported workplace space as an exemplar of zero -carbon and sustainable construction.
Commissioned by Cherwell District Council and supported by European Regional Development Funding with the aim of having a positive social, economic and environmental impact, the business centre is one of the principal strategies in creating sustainable employment for the future North West Bicester community.
The building is designed to adapt to changing expectations for workplaces, coupled with a focus on health and wellbeing with high levels of thermal comfort, ample natural light and good air quality.
Designed with architects Architype to stringent environmental conditions, the building is zero carbon in use, low in Volatile Organic Compounds (VOC’s), has achieved BREEAM Excellent and is the first workplace building in the UK to achieve the Passivhaus Plus standard. Passivhaus Plus buildings go beyond net zero carbon, with a tighter total energy demand limit of <45 kWh/m², with renewable generation exceeding this by at least 15kWh/m².
The building has a reinforced concrete frame supported on a continuously insulated raft slab, thus providing high levels of thermal mass and resilience. A timber and concrete frame alternative was developed up to RIBA Stage 2 to ensure a full assessment of the options before the concrete frame was finalised. Thermal modelling indicated that heat gain from a fully working office with kitchen café space would require significant cooling unless mass was introduced into the building. An exposed concrete frame would provide the thermal mass required to regulate the internal environment, a result not achievable with a timber frame. The concrete frame would also provide the clear internal spans and flexible space to allow for a completely variable office floor plan, with a reduced structural zone lowering the overall height of the building.
The structural challenge became how to support the increased loading while maintaining a thermally isolated structure. The solution was to use a raft slab that would distribute the loads within the structure through to an insulating layer, providing a continuous external thermal zone.
The columns, walls and slab soffits are all exposed concrete to provide a complimentary aesthetic to the exposed timber on the project. This required a higher quality of construction, with the benefit of reducing follow trades, both speeding up the process and removing wet trades which improves the air quality of the finished building.
The stairs are also an exposed in-situ construction, chosen over the more traditional precast concrete solution to suit construction sequencing and avoid the need for a large crane which would have proved difficult due to restrictive access or out-of-sequence installation.
The external wall construction is a timber Larsson truss panel system filled with warm cell insulation. The restrictive access to parts of the building would have made the installing of large panels problematic so Price & Myers developed a system that would allow the panel system to be constructed on site.
The air tightness is critical for a Passivhaus construction, and great consideration was given to the details of the interfaces which can be problematic with a hybrid construction.
To simplify the construction, detailing was developed to allow for a standardised connection that would suit each situation, including the cantilever extensions at the roof, and hanging elements over the entrance. The timber frame is supported from an external free-standing frame with isolated, thermally-broken restraints. The steel frame also incorporates a steel grill floor system to permit access for maintenance and cleaning while also allowing light into the floors below.
Deep external shading was modelled to reduce unwanted solar gains while allowing good levels of natural light to penetrate the building. This was achieved with vertical timber fins sized and positioned to suit the internal daylight lighting requirements. The timber fins are European larch that weather naturally and vary in size and spacing such that each fin is a different size to its neighbour, and the depth and position varies on each elevation.