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How 2021's six Stirling contenders tackled sustainability

Eleanor Young

Sustainable credentials are an important part of the RIBA Stirling Prize now, so what can we learn from the six shortlisted buildings? Their architects reflect on the positives – and voice a few regrets

The RIBA Stirling Prize shortlist has been announced, with six strong contenders.

  • 15 Clerkenwell Close, London (GROUPWORK): a limestone exoskeleton housing the practice offices and apartment, in one of Clerkenwell’s most intimate streets
  • Cambridge Central Mosque (Marks Barfield Architects): a timber vaulted mosque, lit from above and drawing on historic Islamic patterns
  • Key Worker Housing, Eddington, Cambridge (Stanton Williams): brick clad housing arranged in courtyards with simple cut aways that bring joy and protection
  • Kingston University London – Town House (Grafton Architects): library, performance spaces, study and social spaces flow airlily through this humane concrete structure
  • Tintagel Castle Footbridge, Cornwall (Ney & Partners and William Matthews Associates): a double cantilever of a bridge from cliff to historic island and castle
  • Windermere Jetty Museum, Cumbria (Carmody Groarke): low lying galleries and workshops clustered on the shore of Lake Windemere - dedicated to telling the stories of boats and steam

Below, their architects give some insights into the sustainability journeys the projects involved.

  • Reducing waste and bringing back green into the street: Town House by Grafton Architects.
    Reducing waste and bringing back green into the street: Town House by Grafton Architects. Credit: Grafton Architects
  • Concrete interiors are largely self finished, bar timber acoustic screens.
    Concrete interiors are largely self finished, bar timber acoustic screens. Credit: Alice Clancy

Town House, Kingston University, London
Grafton Architects for Kingston University

Shelley McNamara of Grafton sees the sustainability of the Town House, a university hub, as primarily a question creating the cultural and physical conditions for students to prosper. The plans create a porous building, the stacked colonnade, or loggia, on the outside avoids the smelly corridor, instead using external circulation. There is plenty of natural light and the large volumes help with natural ventilation. ‘It is an inside out building,’ she says.

‘We have been very relaxed about using lots of concrete in the past,’ says McNamara. ‘Now we are careful about how it works.’ Even on this building, before the significance of embodied carbon was widely recognised, the lean design with precast ribbed slabs reduced waste. Ground Granulated Blast furnace Slag (GGBS) was used as a cement replacement, up to 50% in the slab foundations and 36% in the structural frame. The biggest concrete carbon saving was in avoiding having a basement, ‘which put pressure on us as designers’ says McNamara, describing how plant was tucked into the ground floor and put behind screens on the roof.

Exposed concrete slabs are integral to the heating and cooling of the building; a thermally active building system (TABS) uses the concrete's thermal mass by running hot and cold water through the pipes in it. This of course dictates that the concrete be left exposed, cutting out an extra layer of material, although there are timber acoustic screens. The deconstruction of the earlier prefabricated building (92% was recycled) gave the opportunity to rethink the street scape with more space, trees and some biodiversity while car parking space at the front was turned over to pedestrians and cyclists.

‘Kingston University was an extraordinary client. Clients come with these fantastic visions of performance, but once these things are costed many of them are omitted,’ says McNamara. ‘We have a long way to go, legislation would help.’ But happily with Kingston, the unique structure of keeping the client on the judging panel to critique, champion and advise on the architecture is demonstrated in the Stirling shortlisted result.

High level estimates of upfront carbon circa 375-400 kgCO2e/m2 (A1-5) modelled
Gross final energy consumption, all (regulated and unregulated): 77.23 kWh/m2 /y modelled


  • Ney & Partners and William Matthews Associates.
    Ney & Partners and William Matthews Associates. Credit: Jim Holden
  • How to land lightly on such a special and complex site?
    How to land lightly on such a special and complex site? Credit: Ney & Partners and William Matthews Associates

Tintagel Castle footbridge, Cornwall 
Ney & Partners and William Matthews Associates for English Heritage

'Before carbon, the sustainability was driven by making the smallest impact on the site,’ says William Matthews. ‘We got there and wondered how on earth we could do it.’

With steep drops, no vehicular access, Sites of Special Scientific Interest at the bridge landing points, and a Scheduled Ancient Monument all around, the whole structure was built without scaffold or formwork as two independent 30m cantilevers (with a tiny – vertiginous – gap between). Materials were delivered across the site using a re-useable cable and pylon system, and minimum foundations (6m³ of concrete on each side) helped to protect the archaeology as much as reduce embodied carbon, though happily it did that too.

Competitor schemes looked at stone and concrete. Matthews and Ney and Partners chose 5t sections of virgin British steel and a local Plymouth-based fabricator rather than recycled steel shipped from the continent. The heavyweight slate that makes the deck so special comes from Delabole, just four miles away.

The client, charity English Heritage, considers itself a custodian of its site. Sustainability and EH’s commitment to maintenance over the long term played into the decisions that make this bridge at King Arthur’s legendary base a remarkable piece of future heritage.

  • The structure was originally conceived as masonry.
    The structure was originally conceived as masonry. Credit: Marks Barfield Architects
  • The depth of rooflights was a balancing act between solar gain and natural light.
    The depth of rooflights was a balancing act between solar gain and natural light. Credit: Morley von Sternberg

Cambridge Central Mosque Marks Barfield Architects  for Cambridge Mosque Trust

The dramatic timber vaulting of this mosque – the most visual symbol of a low embodied carbon approach – started out as masonry. ‘But we couldn’t make it properly structural, the structural engineers kept putting a column in the middle and we couldn’t have that,’ explains Julia Barfield. ‘One of the things that has changed in the 10 years since we started on the design is the focus on embodied energy.’

What hasn’t changed is the Mosque Trust’s sense of humility and being a custodian, which fed into other moves, along with close work with the services engineers. Air, energy and water are areas where Marks Barfield knew it could make carbon savings. Most of the spaces, from atrium to ablutions area to prayer hall, are naturally ventilated, with air extracted through rooflights. Getting the precise depth and dimensions of these was important, both to ensure they were shaded enough not to overheat the space and to optimise air flow – both when the hall is empty and if the 1000 worshippers arrive from the courtyard wet from rain. Fans act as back up to draw out stale air when needed.

There is no fossil fuel on site. Photovoltaics on the roof (not of the prayer hall, that was a no-no) provide the energy to heat the water for worshippers’ ablutions before they enter the prayer hall. They also supply 13% of the energy for heating the building. Air source heat pumps regulate the internal temperature. Collected rainwater is used to flush WCs and to irrigate the gardens that front the mosque.

‘We are trying to become carbon literate and so have done a carbon assessment of the building,’ says Barfield. As well as what the practice learnt from that, it led to some tweaks over the first year. The fans came on too frequently, and the lights were regularly on during the day even though they were intended only to come on as natural light levels fell – both were adjusted.

Barfield’s only sustainability regret is the extensive basement parking with all the concrete and embodied energy that entails. ‘We wouldn’t have got planning without it,’ she says. ‘But it would have been nice not to do such a big car park.’

Embodied carbon: circa 844 kgCO2e/m2 modelled
Gross final energy consumption, all (regulated and unregulated): 72.39 kWh/m2 /y modelled


  • Self supporting brick skins reduce the chances of cold bridging.
    Self supporting brick skins reduce the chances of cold bridging. Credit: Jack Hobhouse
  • Courtyards have a sense of green and community.
    Courtyards have a sense of green and community. Credit: Drawing: Stanton Williams

Key Worker Housing, Eddington, Cambridge 
Stanton Williams for University of Cambridge, North West Cambridge Development

The sustainability ambitions of the University of Cambridge and its Eddington urban extension are set high. The homes were designed as Sustainable Homes Code level 5, there is a district heat and power system and it has the largest storm water recycling system in the UK. With ambitions and infrastructure like this, and working to Aecom’s masterplan, Stanton Williams focused on a fabric first approach; the concrete slab includes GGBS and columns have a self supporting brick skin that allowed 250 insulation and minimal cold bridging.

The human aspects of sustainability were very much part of the brief. Community and delight as well as sustainable transport are manifested in the cycle pavilions (‘never “sheds”,’ insists Stanton Williams’ Kaori Ohsugi). Extra spacious, with climbers and a bench inside and out, they are places as much to meet your neighbour as park your bike.

Blocks were laid out corner to corner to allow good daylighting (without overheating) and a sense of intimacy. Most of the flats are dual aspect, and those that are not had rigorous requirements for views and daylight.

The storm water system starts on the roofs, where PVs sit on crushed recycled bricks that make a micro habitat of brown roofs, with water storage beneath them. But then the water percolates to the courtyards with little rills, eventually running into an attenuation pond. Simple things like bird boxes and edible plants bring wildlife into the greener courts. ‘Seeing the birds coming out makes you so happy!’ says Ohsugi.

Despite working to a design life of 120 years, Stanton Williams did push for lime mortar to be used, to help ensure the bricks could be more easily recycled. But time and cost constraints precluded it. The Code for Sustainable Homes proved a good guide on materials (via the Green Guide) but it is clear just how much knowledge about sustainability, particularly around materials and embodied carbon, has increased in the last couple of years. That is what the practice is bringing to its projects now.

Embodied carbon: Not available
Gross final energy consumption, all (regulated and unregulated): 98.25 kWh/m2 /y modelled


Clerkenwell Close, London
Groupwork for 15CC

Clerkenwell Close began as an experiment with stone to understand whether it was structurally possible, financially viable, and whether there were any sustainability advantages, writes Amin Taha of Groupwork. The entire design team knew the answers to none of these questions, having only a few precedents which used post tensioned stone (notoriously so expensive per square metre that a facade of BMW 5 Series saloons would be cheaper).

Had we accepted the circular argument that no one uses it because it’s expensive, and no it hasn’t been tested because it’s expensive, that would have been that.

Instead, we asked a French stonemason who we were already working with on a self-supporting helical stone stair. The answer was a bemused: ‘Well of course, what do you think all those cathedrals and ancient buildings are made of?’  WebbYates’ calculations showed it was structurally possible, even if we oversized stone against progressive collapse, and cheaper than ‘standard’ – stone cladding on a steel or concrete frame with various layers of fireproofing, weathering, insulation, vapour barriers, stainless steel or galvanised clamps – even without including the prelim costs saved by being faster on site.

Stone had other benefits too. Our sustainability engineers reported a saving 92% in embodied CO2 compared to a clad steel frame – because stone has no embodied CO2. Energy was used only for cutting, transportation and lifting on site (if it was from Portland – which no longer has the skills - instead of Lyon we would have saved a further 5%). Additionally, the huge exo-skeleton acts as solar shading. Moving it out an extra 50mm on its south west face reduced solar gain enough to avoid specialist glazing treatments and mechanical cooling, with operable windows being sufficient. Lastly, for the fit-out we removed all first-fix aluminium and plaster board partitions and lining and only used stone and timber. This made the building's CO2 sequestration negative, further reducing its embodied CO2.

Further work on modelling towers suggests we could build to capture carbon. We don’t need to invent machines to suck it out of the air and store it in the abyss of the deep sea. We just need to cut out, transport and lift up stones.

And a note on concrete: The lesson we learnt – after the concrete had been completed across the basement and upper floor slabs (on temporary props) over 12 months – is that we could have used CLT instead and cut around 8 months from the programme. Lower CO2 and, of course, it would have been cheaper too.

Embodied carbon: 335 kgCO2e/m2 (A1-C4) modelled post completion 
Gross final energy consumption, all (regulated and unregulated) – irrespective of source: 51.57 kWh/m2 /y modelled


  • The flood risk helped generate the forms that bed happily into the landscape.
    The flood risk helped generate the forms that bed happily into the landscape. Credit: Carmody Groarke
  • A water source heat pump is hidden under one of the jetties, allowing the museum to avoid directly using fossil fuels.
    A water source heat pump is hidden under one of the jetties, allowing the museum to avoid directly using fossil fuels. Credit: Johan Dehlin

Windermere Jetty Museum, Cumbria
Carmody Groarke for Lakeland Arts

During its construction Windermere Jetty Museum suffered from two 1:200 year floods. Sitting on the edge of Lake Windemere, the realities of flooding – and particularly extreme weather events from climate change – defined the project. The massing gave the museum a conservation gallery at normal lake level for boat access, with enhanced flood resilience and easily replacable cladding, but site waste was used to build up the rest of the buildings so galleries and public would stay above flood level. ‘That topography helped embed the building in the landscape context,’ says project architect Rowan Seaford.

The theme of water drove many sustainability moves. A swale across the site and reed beds ensure that waste water can be cleaned and filtered on site before being discharged into the lake – through the boathouse, helping keep the water moving and clear, lapping beautifully at the edge of the boats. Perhaps the most forward-looking thing for the time, says Seaford, is that the building was all electric, thanks to a water source heat pump suspended under one of the new jetties.

Sustainability was also built into the design process. The team set up a series of benchmarks to assess the project on sustainability. A significant improvement on Buildings Regulations was enshrined in the brief and it has a 20% improvement on Part L with natural ventilation through most of the museum and high levels of insulation. The process also provided a chance to work towards an extended design life of 50 to 70 years and involved the architect in a request to Windemere Lake Cruises to add a museum stop to their programme to boost sustainable travel options.

While the forms themselves are informed by passive measures including shading from the cantilevered roofs (also perfect for shelter in the changeable weather of the lakes) the practice is aware that the overall form of the building is not the most efficient. The different volumes mean more envelope and more structure. But as the first contemporary building on the lake in many years, in this most precious of National Parks, breaking up the form and scale took priority.  And the flexibility of long spans, minimal internal structure and building services built into the landscaping has certainly paid off as the museum has adapted to Covid-19 realities.

Upfront carbon: 572 kgCO2e/m2 (A1-5) modelled  
Gross final energy consumption, all (regulated and unregulated): 143.79 kWh/m2 /y modelled

The RIBA Stirling Prize winner will be announced on 14th October. The judges, chaired by Sir Norman Foster, are Annalie Riches, co-founder of Mikhail Riches, British artist, Dame Phyllida Barlow and RIBA President, Simon Allford of AHMM. The judges will be advised by Mina Hasman of Skidmore, Owings and Merrill as sustainability expert. 

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