Exemplar Sustainable Building Design for The Royal Horticultural Society’s New Bramall Learning Centre

Combining Sustainability and Innovation Eco Arc Architects have recently completed a carbon negative building with the highest recorded ‘Bespoke’ public building BREEAM rating and a SBEM carbon emission rating of minus -6.

Preamble

Sustainability lies at the core of the design of the Royal Horticultural Society’s (RHS’s) recently completed Bramall Learning Centre, dictating everything from the way the building sits in the landscape to the choice of building materials and servicing strategy, providing an inspirational educational resource for generations of current and future gardeners. From inception through to construction, many challenges were overcome and the finished building has achieved the highest recorded BREEAM ‘Bespoke’ public building rating of 86.95%, leading to the building being featured on the Carbon Trust’s website as an exemplar of sustainable building design. (See Carbon Trust 5-minute case study video ) Now open the Centre is expected to provide an environmental and horticultural educational resource to around 10,000 school children a year. Construction was completed within a 10-month time frame under a NEC partnering contract worth £2.12m.

Sustainable Design

The whole premise that a building can be carbon negative is seen by many people in the construction industry to be unachievable in practical terms. However, the Bramall Learning Centre offers the potential to show that it can be both practical and achievable. An important aspect of the building’s sustainable design lies in its response to the genius loci of the landscape, which also dovetails with its environmental performance in use. The site is situated within the beautiful RHS’ Gardens of Harlow Carr with a 4m diagonally sloping gradient across the site. The Learning Centre responds to this setting by being partially earth-bermed; this not only reduces the visual impact of the two-storey structure on the sensitive site, but also offers an opportunity to enhance the thermal strategy. Utilising the natural site topography in this way necessitates that the building looks west and so, in order to increase the potential for beneficial solar gain during the winter months, a serpentine S-shaped building plan was developed to maximise the extent of the southern elevation.

The Royal Horticultural Society’s New Bramall Eco Learning Centre
timber staircase
Eco building interior space

This S-shape is repeated in the building section roof profile, which is clad with a living bio-diverse green roof enabling the building to blend in with the surrounding garden ecology. Further benefits of the warm green roof system include the provision of a natural habitat for wildlife, decreased rainwater run off and improved thermal performance providing cooling during summer through evaporation.

The building is naturally ventilated placing particularly onerous demands on the external building envelope fabric in terms of super insulation and air tightness. The exposed reinforced GGBS concrete slab and dense clay blockwork walls provide thermal mass and a triple glazed front elevation maximises natural daylighting and beneficial solar gain, while an extended roof over hang guards against summer over heating.  In principle, when required the glazed front elevation draws in the heat, which is then retained by the thermal mass in the floor slabs and rear walls.

Sustainability is also built into the building fabric through the specification of recycled and responsibly sourced building materials. The concrete sub frame extends from foundation to first floor level and contains 50% GGBS cement replacement; the timber used for the roof structure and else where throughout the building and site construction works was FSC sourced with full chain-of-custody certification. The internal non-load bearing partitions are constructed using either unfired clay bricks or dense concrete blocks manufactured using 100% recycled aggregate. These elements are plastered on the hard which assists in the regulation of both temperature and humidity. The clay bricks are included to passively absorb moisture during the winter months, whilst hydrating the air during summer. An eco-screed was used to embed the underfloor heating, incorporating 100% replacement of natural sands with recycled amorphous glass, using 1.2 tonnes of recycled glass per cubic metre of screed applied. The binder part of the screed is derived from desulphurised gypsum (DSG), a by-product of the emissions cleaning desulphurisation process carried out at a coal-fired power generating station.

The result of this stringent specification of A rated materials meant that we achieved 7 out of the 7 credits available in the materials section of the major building elements in our BREEAM rating (BREEAM Bespoke ).

Heat loss through the external envelope was minimised through the use of super insulation to ensure fabric U values in each element throughout the building being not more then 0.13W/m²K, minimising cold bridging and stringent airtight detailing. Masonry walls with 250mm wide cavities where fully filled with recycled glass wool insulation with a thermal conductivity lambda value of 32W/mK. The wall ties used were made from basalt fibre reinforced polymers, which appeared to have no cold bridging impact in the building modelling. (Moreover, by using basalt rather than stainless or mild steel, makes sense, as basalt, unlike steel, is an abundant natural resource.)

Central to reducing base loads is the passive approach using thermal mass of the structure and the earth to smooth out the daily swings (diurnal range) in temperature. This works well in both winter and summer, reducing the dependency on mechanical systems. Ventilation is primarily passive with actuated windows controlled by the central BMS which aids night time purge cooling in summer. However, for those times when the learning centre has significant occupant loads and before internal CO2 levels creep up to high levels, the BMS will bring on simple boast fresh supply air units – but they seldom run, except for short periods in the year, so their energy demands are insignificant.

To reduce the building’s carbon consumption over its lifetime, renewable energy sources have been included. The key to making this system carbon negative was to employ a 20m high 15kW wind turbine within the Harlow Carr grounds – effectively providing the majority of electricity supply during windy periods & occupied periods, with excess power exported to the grid during low demand periods. Across the year the wind turbine is predicted to generate 18MWh, although this is yet to be proven from meter readings on site.

Biomass boilers might be perceived as an obvious choice for a new building at Harlow Carr, when considering the availability of waste timber arising from tree maintenance on site, but actually the RHS did not have sufficient space on site for chipping and storage facilities, or a secure long term site supply so this idea was dropped fairly early in the design process. The main source of space heating is a ground source heat pump connected to a series of 100m deep vertical closed loop boreholes.This then feeds underfloor heating loops as the main source of space heating – a 22kWth unit provides the heating requirements for the building with an input of only 4.8kWe.

In addition, to reduce the building’s demand on conventional energy sourced for heating water, 4m² of solar thermal panels have been fixed to the roof, which is predicted to provide 1.5MWh/yr towards the domestic hot water demands for the building. Solar PV was also considered but the orientation of the remaining roof area was not ideal. Rainwater harvesting has been utilised to reduce the building’s water demand with an over flow to a site wide water store / irrigation system.

The lighting consists of generally fluorescent T5 units plus compact fluorescents. Most of the lighting is either daylight or PIR controlled. The narrow plan and careful orientation of the built form allow natural daylight to penetrate deep into most spaces during the day, with daylight switching ensuring artificial light usage is kept to a minimum.

Controls are provided by a BMS system, which provides all the required control routines for opening the windows under the correct internal and external conditions to make the most use of the thermal mass. The BMS also incorporates extensive energy monitoring which enables the building’s facilities manager to produce trend logs to keep track of where the energy is going and to help spot unusual changes in consumption. This monitoring system covers general power, lighting and services circuits, including the renewables.

Extensive IES modelling was under taken during the design phase. The current as built design predictions have led to an EPC rating of A+ with an emission of minus – 6, which means the building is carbon negative on paper. This has been achieved by a slavish adherence to the principles of Lean, Clean and Green:   Lean – to get the base loads down to the bare minimum – Clean: to use low emission technologies to deliver loads – and Green: to use renewables to drive the performance into carbon negative.

The result of this stringent energy design and specification meant that we achieved 15 out of the 15 credits available in the Energy section of the major building elements in our BREEAM rating (BREEAM Bespoke 2006).

Learning centre library
renewable energy
sustainable building design

Architectural & Structural Design

The curved and stepped subterranean retaining wall running along the building’s rear is propped at first floor level by the thermally separated first floor slab, which in turn is stabilised laterally by the in-plane rigidity of the retaining wall. In order to form a structural connection whilst minimising any cold bridging effects between the cold retaining wall and warm first floor slab, a proprietary insulating shear connector was cast in. The retaining wall is backfilled with a locally sourced recycled free-draining granular material. A triple glazed façade runs along the south / west elevation of the building; the glazing system utilises stacked timber frames sandwiched between timber mullions, with a timber ‘loose tongue’ to create a shear connection between the two elements.

The FSC timber roof structure is made up of sawn timber purlins and specially fabricated twin glulam beams, S-shaped in elevation and sheathed with a plywood diaphragm. As the soffit of the roof is primarily exposed to view from the first floor classrooms, the connection details have been sensitively detailed and fabricated, utilising steel dowel type connectors in lieu of conventional bolts where possible. The primary glulam beams to column connections employ specially fabricated ‘pinned’ connectors. Given the complexity of the roof geometry, a 3D computer model was built to aid the fabrication process.

The design of the roof diaphragm was needed to be achieve global building stability, the curved plan and section building geometry dictated that a degree of double curvature was required, and the internal soffit was to be exposed to view. Consequently FSC birch plywood was the preferred material. In order to achieve the required curvatures, the maximum panel thickness needed to be 9mm, which meant that three layers of plywood where required. Given the potential difficulty of persuading a plywood panel into double curvature, butt joints between the ply sheets were situated over the main glulam beams, reducing the residual stress in the plywood. In order to accurately predict the roof stresses and fixing requirements, a 3D Finite Element Model was generated to accurately determine the effects of in-plane forces due to horizontal wind loads.

The fabricator, Dalton Joinery, built a full-scale model of a typical bay in their workshop prior to commencement on site, enabling the team to proceed with confidence that the curvature could be achieved on site. This also allowed us to agree a plywood sheet lapping arrangement, which was then fed back into the design of the fixings. The joints on the lower layer are expressed with a routed 10mm wide x 5mm deep groove to reduce the visual impact of the plywood cut edge in the exposed internal ceiling soffit. 

Post Completion

The team felt it was important for an award winning project to demonstrate its true performance to the wider industry, to learn how it is achieved in practice. So the project has secured funding from the Technology Strategy Board to carry out a Building Performance Evaluation covering two years of operation. A programme of dissemination will follow to reveal the findings as widely as possible. The building has already been part of an early stage monitoring programme by the Carbon Trust.

Conclusion

The RHS’ new Bramall Learning Centre is highly sustainable and geometrically complex with many bespoke details, yet it was completed on programme and within budget. This was facilitated, in part at least, through close team collaboration and partnering between consultants and contractor, for example the design and detailing work that went into the roof structure meant that the doubly curved diaphragm was relatively straightforward to fix on-site. The demands of the brief required a tight specification to be accurately executed on-site, and a high level of craftsmanship was maintained throughout, including working through one of the toughest winters in recent decades. Local feedback has been very positive, the building has received six design awards and it is hoped that the building will inspire and educate in equal measure.

Text by Eco Arc & Peter Corbett ( Corbett-Tasker formally of Gifford ) Pictures Rachael Meyer

 

Credits

Client: The Royal Horticultural Society

Architect: Eco Arc Ecological Architecture Practice

Project Manager and Quantity Surveyor: Turner and Holman

Structural Engineer: Gifford LLP

M&E Engineer: Gifford LLP

Main Contractor: William Birch and Sons

Timber Fabricator: David Dalton Joinery

Photography Rachael Meyer SIRA Studio Ltd

timber joints
green architect
Eco learning centre exterior