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Good as new: CISL completes Entopia, its sustainable office retrofit

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Eleanor Young

We followed the triumphs and challenges of deep retrofit – the process of making a building sustainable in conversion and use – at the Cambridge Institute for Sustainability Leadership’s new base, through the RIBA Plan of Work. Read on for Stages 0 to 5 and an assessment of the finished building

When the Cambridge Institute for Sustainability Leadership made a 1930s telephone exchange its new home, it knew it had to bring the building into line with its values – and provide sustainable leadership in its design and delivery. We track the process and consider the fundamental questions of sustainability that must be interrogated at each point through the RIBA Plan of Work to create a powerful sustainable building exemplar. Here we examine POW Stages 0 to 4 of this ambitious, £12.8 million retrofit.

Stage 0: Strategic definition

Stage 1: Preparation and briefing

Stage 2: Concept design 

Stage 3: Spatial coordination

Stage 4: Technical design

Stage 5: Manufacturing and construction

Stage 6: Completion and handover



Stage 0: Strategic definition

Stage 0 in the RIBA Plan of Work is all about sorting out the business case that will drive a project, and starting to think about the brief. In sustainability terms, setting the aspirations for the scheme is all-important.

Cambridge University’s growing Institute for Sustainabliliy Leadership (CISL), and its then founder director Polly Courtice, spent five years scouring the university’s property portfolio to find a new home. Even then, the site they picked was not the most obvious choice for a forward thinking institute that, over 30 years, has married practical sustainability action with business as it works alongside organisations such as Asda and Chanel, and the Prince of Wales.

There was no options appraisal to review different sites or a newbuild. CISL settled on a recently-vacated 1930s telephone exchange in Regent Street, despite its small windows and rather tired interior. ‘A property was so hard to come by in Cambridge,’ says Courtice. ‘We had battled for every building available.’ It persuaded the university to let the institute take it on, rather than put it out to commercial rent; but getting it to agree to CISL undertaking a deep retrofit was more difficult. The proposal had to be taken through the university’s planning resources committee, backed by a five year business plan and a promise to fund-raise for the retrofit.

Given the institute’s twin concerns of sustainability and leadership, it saw the retrofit an opportunity to demonstrate its values (the university itself later committed to reducing its carbon emissions in 2019 when it signed up to the Science Based Targets). The retrofit is now under construction and Courtice is aware there is still a distance to go, adding: ‘We have to prove how feasible it is.’ At stake is the achievability of a deep retrofit, something the university’s ageing estate needs as much as the UK’s existing building stock does. The institute needed offices, some with hot desking, and a place bring people together – exhibitions, entertaining and a café.

The brick facade and sash windows of the 1930s telephone exchange on Regents Street in Cambridge, ahead of its retrofit..
The brick facade and sash windows of the 1930s telephone exchange on Regents Street in Cambridge, ahead of its retrofit.. Credit: Solk photography LTD

We have to prove how feasible deep retrofit is

The project was joined by John French, who had been lead academic client on the hugely innovative and sustainable Enterprise Centre at the University of East Anglia. He wrote a brief for a world class retrofit, turning values into targets with standards alongside. The Enterprise Centre had drawn on French’s work on bio-based materials; from external thatch cladding to interior finishes, it used local materials and showcased innovative products. All those aspirations were also applied to CISL’s retrofit.

The slew of project ambitions set out by French included Passivhaus’ EnerPHit and BREEAM. The brief stipulated that the building should be an example of the circular economy – in particular re-using or recycling office kits – and of bio-based materials. These materials expand the net wider than timber to include bio-based paint, wool insulation, cork finishes and more. ‘We need to move away from petrochemicals and plastic,’ says French. ‘And bio-based materials  give you a good feeling.’ In addition they act as carbon sinks. French set a ‘stretch target’ for 70% (beyond what would normally be considered achievable) of refurbishment ­materials to be bio-based. The embodied carbon target was 300kgCO2e over a 100 year life.

But that is not all. The brief required 50 innovations to be included and showcased in the building, which could be software, materials, or occupier related. And Soft Landings and post occupancy evaluation (RIBA Plan of Works Stage 7) would run for a year from practical completion to ensure the building was performing as it should and could be fine-tuned in use.

Cost was to be typical of a retrofit like this, if not quite the lick-of-paint bargain that the university had first envisaged. The client set a challenging target cost of £2500/m².

Next the consultants team had to be appointed. French is a great believer in team continuity to build on lessons learnt and relationships, and brought in Architype and BDP – which had worked together on the Enterprise Centre – through the Cambridge University’s framework, which BDP was on. This team would work on the building through Stage 3 to tender and help appoint the contractor before moving to become client-side architect.

And finally, a name. The building was to be called Entopia.


Stage 1: Preparation and briefing

At Stage 1 of the RIBA Plan of Work the architects start to get involved and work out what the brief actually means in practice.

Now, team Architype joins the job, best known for committing to Passivhaus early on, with schools in Wolverhampton and the RIBA Award winning UEA Enterprise Centre (yes that again). The key people are director Ben Humphries and associate Wendy Bishop, a Passivhaus designer. They started with the viability of the brief, first testing its sustainability and then modelling and reviewing key areas. Could the project realistically deliver the high sustainability and certification targets it had been set?

Right from the start Architype was up front about pricing the feasibility higher than normal to ensure that the many emerging issues would be explored – ‘to be competitive further along the line,’ says Humphries reassuringly. There was a lot to investigate.

On top of the standards already set, the practice proposed working with an extra one:  the WELL Building Standard. It saw a good alignment with the other aspirations. A Venn diagram of all the standard and values was drawn up and confirmed a large degree of overlap (see above). The team also looked at LEED and RICS’ SKA rating – but they didn’t make it into the mix.

So what did the high level analysis – some on Architype’s modelling tool Eccolab – of metrics and targets throw up? First was the important role that internal insulation would play. Also replacing the single glazed windows (despite some secondary layers) for thermal performance and air tightness was essential – and would play a part in improving low light levels too. Replacing poor performing windows meant a lot of glass, and with it embodied carbon. ‘You couldn’t get away from that,’ says Bishop. ‘You can’t get bio-based glass.’

Added to all that was the fact that as an existing building it couldn’t benefit from a carbon-sequestering timber structure, which knocked the targets on both bio-based materials and embodied carbon. A target of 70% of bio-based materials had been reduced to 50% by the end of Stage 1, when options and costings were reviewed. The Enterprise Centre at UEA had 400kgCO2e, partly due to the timber structure helping bring down the embodied carbon. And norms for calculation have moved on even since that was completed, with the RIBA’s 2030 Climate ChallengeLETI splitting out sequestered carbon and SKA including the reporting of end of life carbon.

The team started to talk about using and doing less. Could those existing fixed glass partitions be reused in situ or upgraded?

So the insulation and wall build up was under the microscope, with extra Wufi modelling to assess moisture migration. Was the solution Warmcel newspaper cellulose fibre between timber studs with a membrane – as often used on Passivhaus projects? Or perhaps wood fibre insulation? Whatever it was would have to be lightweight and help with moisture movement in the existing brickwork. This question remained live through the project despite early conversations – as the team was unable to open up the building to see the old wall build up until very late.

On materials, the team started to talk about using and doing less. Could those existing fixed glass partitions be reused in situ or upgraded? This didn’t bottom out until the end of Stage 2, when it was found that keeping them didn’t save much, partly because they didn’t fit the required configuration of spaces and partly due to detailing insulation in the walls alongside. It was easy to decide to keep the building core containing lift, staircase and WCs, with its existing 60-minute enclosure and block walls. Would that spiral escape stair suffice? This answer was yes (despite the potential to design something more interesting, architectural and accessible).

For the circular economy the team undertook a review of building’s materials for suitability for reuse (on site or by others), recycling potential or manufacturer take back schemes, and precedents for creative reuse of materials.

Even for those who want to be the leader in a field, precedents play an important part. By the end of Stage 1 the team had not only firmed up the brief but set up tours to buildings that might demonstrate different aspects of the project to clients, including a private members club, a WeWork building and Europe’s first WELL building, and Cundall’s London office, designed by Studio Ben Allen. So going into Stage 2 there were plenty of ideas about how to make a great sustainable office, as well as the windows to work out ahead of submitting for planning – always a tricky issue in a conservation area.


Stage 2: Concept design

At Stage 2 of the RIBA Plan of Work architects develop the conceptual design and it is signed off by the client. This is where we pick up the story of a 1930s telephone exchange that is undergoing a deep retrofit to become the new home of Cambridge Institute for Sustainability Leadership. 

It is no surprise that the original design of the telephone exchange hadn’t prioritised daylight. ‘It was quite gloomy,’ says project architect Architype’s Ben Humphries. Light modelling from BDP, and some mini-studies looking at the impact of the windows on heat demands and daylighting, proved the old sash windows needed tackling. Good daylight is a must for BREEAM and existing light levels would have scored zero credits. Options included triple-glazing to achieve Passivhaus levels of insulation. 

The circular economy is a difficult one to crack, even when you are just looking for waste

Conservation vs sustainability
Cost and the conservation area put paid to widening the openings. Architype chose the less radical, but still contentious, solution of replacing the sash windows with recessed frames, cutting out transoms and mullions and, in the process, allowing internal wall insulation and windows to sit flush, avoiding a complex thermal detail. Pitting conservation against sustainability is a very live issue, all the more so in a historically rich city like Cambridge. Architype found its presentation of findings and proposals to a gathering of colleges watched closely for the planning outcome: they knew the wider significance of a ruling either way. In the end (to jump ahead a stage) the scheme won five votes to three on the planning committee with support from some councillors for its sustainability agenda.

But back to the fabric. Surveys started to give a greater understanding of what the team had to work with. Point testing of U-values gave some information but thermal imaging was less useful as the heating had been mothballed so there was little heat leakage to pick up. It was clear, though, that areas retrofitted in the late 90s performed better thermally. Ironically, it was harder to make this section – with its steel and precast concrete – airtight. 

Refinements of the model in the Passive House Planning Package (PHPP) identified small power as an issue. This took the design team beyond the normal purview of architecture and into everything that CISL plugs in, particularly computers. Inevitably, that developed into continuing discussions about upgrades, replacements and server location. Hosting the server at the university’s data centre could have made the Passivhaus target far easier to achieve, but it didn’t work for CISL’s team. 

Another piece of software was doing some detailed analysis. Quantities from a SketchUp model of the exchange were plugged into ECCOlab – software developed by  Architype, engineers and asset managers to optimise lifecycle energy, embedded CO2 and life cycle costs. The figures proved the importance of tackling embodied carbon in floor finishes. ‘We’d have had an idea of that but not the numbers,’ explains Bishop. ‘A generic carpet tile would blow the carbon budget.’ So the raised access floor was left exposed, the glue buffed off to provide a clean surface. 

Five window replacement options – from sash to triple-glazed, recessed tilt and turn.
Five window replacement options – from sash to triple-glazed, recessed tilt and turn.

Searching for waste
University waste streams were also explored. If the projects’ progenitor, the Enterprise Centre at the University of East Anglia, had rescued and repurposed a Foster and Partners-designed desk, what riches might be found in Cambridge? Talk of using timber that was being felled on the university estate didn’t come off. As ever in design, many avenues were explored but not all led somewhere. The circular economy is a difficult one to crack, even when you are just looking for waste.

There was a palpable tension of pushing sustainability when innovation and guidance conflicted

CISL had initiated the building process and embedded itself into the project far more firmly than a normal university department, which would attend meetings as a user rather than project director. But as a professional client the university has its processes. When the project didn’t follow its design and standards brief a ‘derogation’, or variation, had to be agreed at the project engagement meeting held at the end of each stage. Talking to both Architype and project director John French you sense a mix of defiant eco warrior and quaking school child as the rule breaking was justified. There was a palpable tension in pushing sustainability when innovation and guidance conflicted – even though the university already had at least one timber framed Passivhaus building, at King’s College.

An early stumbling block at the project engagement meeting was the use of bio-based materials – specified in the brief but specifically ruled out in the university design guide. Then there were the partitions, fixed in place and on the keep list, but not meeting the mandate for future ease of reconfiguration. They were eventually ditched. Passivhaus fundamentals also had to be run past the group; using low grade heat around the ventilation system was a deviation from the norm and needed approval. 

In the meantime there was the perennial question of the cost of sustainability. French says the cost consultant added 20% for every performance criteria – which had to be resisted. ‘Yes there are costs of extra insulation and triple glazing, but it doesn’t mean it is a free for all piling on cost and risk,’ he says. There was only one air source heat pump, for instance. And French could see potential savings too, one of which was realised in the flooring – the cheaper solution of reuse the result of the quest for low embodied carbon. 

Whether these ideas could be realised or would fall at tender stage to cost and programme still had to be seen as the institute and its client-side team started to draw up what they needed from the team that would deliver the project on the ground. 


Stage 3: Spatial coordination

Stage 3 is officially labelled spatial coordination in the RIBA Plan of Work. And certainly there was a lot of testing of initial concepts in the design development for the deep retrofit for Cambridge Institute for Sustainability Leadership

So far the project manager 3PM has taken a back seat in our story. It was always part of the team with Architype as architect and BDP on MEP, but at stage 3, Architype’s role is morphing from designer to client-side monitor, and 3PM is now charged with handling the transition to the design and build contractor ISG and its team. 

The word ‘handling’ should be used advisedly. 3PM is coordinating a huge transfer of information and knowledge about decisions taken. It is also acting as broker between design teams as one lays into the other’s designs. Patrick Watson of 3PM is an old hand at this contentious process and has a whole vocabulary that comes from the world of mediation rather than the battleground.Assumptions are challenged and tested, he would say, but he admits it is tricky. 

But first the contractor team has to be appointed. There are some hurdles. Watson is familiar with the sustainable ones – he worked with lead client John French of the institute on the University of East Anglia’s landmark Enterprise Centre. But being a refurbishment elevates it to another league of complexity. ‘That was the biggest risk,’ says Watson.

Architype’s key internal strategies
Architype’s key internal strategies

A further issue was the University of Cambridge’s process of awaiting costs and funding before authorising a strip out. This meant there was no chance of getting in early to understand the built fabric that was being dealt with. Add that to demands on building performance as well as making bio materials a tenet of the project and the risk any contractor was taking on was huge. 

This defined the procurement route. The early idea of doing a single-stage tender was ditched in the face of fears of inflated costs. Instead a two-stage tender with market engagement was chosen. It should give a better chance to reduce or at least quantify the risks to cost. 

Wendy Bishop of Architype recalls: ‘There were contractors coming to find out about the building and new approaches to the job.’ Alongside the financial process there was delivery on sustainability targets. She was interested to see how keen contractors were (or weren’t) and whether it seemed as though they would give attention to detail. The structure of the design-and-build teams on offer varied – sometimes with Passivhaus consultants, sometimes with Passivhaus expertise in the architecture or M&E practice. In the case of the selected team, led by ISG, the Passivhaus expertise sat not with the architect, Feilden + Mawson, but with the MEP engineer Max Fordham.

For potential contractors, the scheme being a refurbishment elevates it to another league of complexity

Once ISG had been engaged with a pre-construction agreement, interrogation of the building and design continued. 3PM wanted to capture the risks in the main NEC OptA contract by answering some big questions. Would Architype’s ‘sensible’ stage 3 strategies around insulation really work across any structure? A WUFI survey on moisture and heat movement brought some extra knowledge to the incoming contractor team and led to plans for extra bio-based insulation. So too did survey results on the basement. This was to undergo a big change, from bog-standard basement to shared workspace – essential for income generation. Getting to grips with below-ground drainage and water ingress allowed decisions that could be reasonably costed (most significantly a raised floor with insulation below). 

The incoming design team had a steep learning curve, quizzing those in place, getting to know the building and being sure it was ready to take on the design liability. BDP’s assumptions and calculations on M&E were ‘challenged rigorously’ by Max Fordham engineers, recounts Watson, admitting: ‘It was painful at the time.’

It didn’t affect the designs much; maybe two or three-room requirements. But it threw up the question of how engaged the user, facilities managers and university were in the decisions around thermal comfort. Cue more project engagement meetings, which luckily approved the approach. 

And at the end of Stage 3, the contractor ISG was appointed on a priced Stage 4 design. Project costs had gone up but the institute had successfully gone back to funders, begging hat in hand. Planning was submitted in early summer 2019 as Stage 3 drew to a close.

‘As we had a two-stage contract, it is all about dialogue,’ says Watson. ‘It was alright once we got talking and discussing a problem. But it didn’t start like that. It is a commercial issue and that’s all.’

Being a retrofit and super sustainable ‘all added up to extra aggro’, he says. ‘It was difficult, challenging and required more facilitation than normal. It is important to learn.’ The Stage 3 process was less about new solutions, materials and technical fixes, Watson is sure about that. ‘It was the softer side about the way we approach things,’ he says.

Did that softer approach smooth things out for the new team? See in Stage 4.


Stage 4: Technical design

The exacting interrogation of the early stage designs by the contractor’s architect and services engineer, Feilden+Mawson and Max Fordham, continued through RIBA Plan of Works stage 4 – technical design. The soft skills of negotiation shifted to a detailed juggling to optimise conflicting requirements.

The team worked with a thoroughness born of the knowledge that the buck stopped there, and in terms of detailing, with Feilden +Mawson. Senior architect Chris Read is clear about that: ‘The responsibility and liability for detailing is with Feilden+Mawson.’ And detailing is really something when you are working to an air tightness of just one air change per hour at 50 Pa (approximately 1.8m³/m².h at 50Pa).

  • The wall build up modelled.
    The wall build up modelled. Credit: ISG
  • Credit: ISG
  • Credit: ISG
  • Credit: ISG
  • Credit: ISG
  • Credit: ISG

It was during stage 4 – shortly after delayed planning approval gave the OK to proceed – that Covid hit. And everything slowed down. In the early days of 2020 there had been a contractor’s site manager, who could make the odd hole to see what was hidden behind the existing linings and lift a ceiling tile here and there to see what was beneath. There were regular site visits as the team worked through each tender package. But all this stopped with the March lockdown.  

Insulation approaches

The basic plan of insulating the solid masonry walls continued. It was a knife-edge balancing act to meet the combined challenges of airtightness, internal insulation of solid masonry wall, thermal performance and moisture risk. Cambridge-based Gwilym Still of Max Fordham, an engineer and expert in EnerPHit and Passivhaus, calculated and re-calculated as Read of Feilden+Mawson was drawing details, dealing with the many varied interfaces. Between the two is an ease and mutual respect: Read a little quieter and committed, Still with keen attention and flashes of brilliance once he has turned his attention to a problem. With the challenge of EnerPHit, Still’s experience and expertise proved invaluable. Feilden+Mawson had previously done only a far smaller, residential, Passivhaus refit – and new rules bring steep learning curves. 

Different underlying wall types drove different approaches to build ups in this complex insulation job: 

  • Solid wall: Diathonite, Gutex and dry ­lining
  • Basement: tanking, cavity, Gutex, air tightness membrane, cavity (to protect membrane), stud lining and Fermacell
  • Cavity walls: Diathonite, Gutex and dry lining
  • Timber frame to extension: Space­therm or Gutex, air tightness membrane, battens and Fermacell.
  • Credit: ISG
  • Credit: ISG
  • Credit: ISG
  • Credit: ISG
  • Credit: ISG
  • Credit: ISG

In the basement the windows and doors were one focus for detailing. Once the space was tanked and insulated, how would the surfaces work with the existing reveals? On the fire encasements for the steel columns how would air tightness be achieved, and would Blowerproof seal do the trick by meeting the Diathonite on the wall next to it?

Law of diminishing returns

Perhaps the biggest single challenge for Read was when the initial strip-out, at the end of stage 4, revealed a perimeter services trench under the raised access floor. The detail ­(pictured) was the team’s answer to this additional complexity. As Still says: ‘It looks like it makes sense, it is just some rectangles… but there was a lot of backwards and forwards and detailing to limit the thermal bridging on the existing floor.’ 

Software package Therm was used to model the thermal bridges and help work out how far it was worth extending the insulation into the building, above and below the soffit – and when the law of diminishing returns kicked in. It was painstaking, says Read. ‘Each cold bridge had to be identified and a solution for insulating it developed. The detail would then be thermally modelled to understand the impact on heating demand in order to maintain a comfortable margin within Passivhaus requirements.’

Feilden+Mawson’s detail showing layers from external wall across the service trench and on to the floor slab.
Feilden+Mawson’s detail showing layers from external wall across the service trench and on to the floor slab.

In-use energy

Keeping the total in-use energy in line with EnerPHit’s complex total energy metric (PER) required working with the client, agreeing low flow showers, and as much detail as possible on copiers, printers and IT equipment. With all those big ticket items it seems surprising that tea-making took the biscuit for complexity. There was a guessing game where the numbers were trumped by estimations of human behaviour. Would a sustainably committed workforce fill the kettle just enough and not too much? ‘Parsimonius kettle filling is more honoured in the breach than the observance,’ says Still, diplomatic but hinting that while criticism might be abundant compliance is harder to reach. A Zip tap has been installed – a familiar technology from the institute’s existing building. 

Meanwhile the costs were coming in, the bulk negotiated under lockdown. No one working in construction then will be surprised to learn that they were going up and scheme exceeded its original budget. Alongside value engineering a strong case had to be made to funders, particularly the European Regional Development Fund, which luckily did increase its grants. At the same time the project lost its passionate and experienced client champion, John French, after the sign-off of stage 3, and head of the Institute, Polly Courtice, who was taking over as lead, was getting herself up to speed. Still, there was barely anything to show for all the investigating, strategising and detailing on the abandoned offices of the old telephone exchange. But as stage 4 moved to stage 5 there was action on the ground.


Stage 5: Manufacturing and construction

It is time to go on site. We have reached Stage 5 in the RIBA Plan of Works at the deep retrofit for the Cambridge Institute for Sustainability Leadership’s (CISL) Entopia Building. Stage 5 is all about manufacturing and construction and as the new home for CISL goes on site, the onset of the pandemic is an added worry to communicating new ways of working to those building the project.

The best guide to this stage is surely Peter Kelly, group director of sustainable operations at contractor ISG. He is a bit senior to be closely managing this sort of project, but when a colleague left he took a direct role on Entopia due to its technical complexity. It could have been a backward step but he has loved it. ‘Every decision was made with sustainability in mind,’ he says wonderingly. ‘It is the first time in my working life.’

We will hear about ISG’s and Kelly’s circular economy contribution shortly. And previous pieces give an idea of the early input of ISG and its team through the 55 week pre-construction contract.

But the big question is how ISG as a management contractor, entirely reliant on a network of subcontractors which are all small to medium sized enterprises, ensured everyone was on the same page? There were a series of strategies. There were training days, two videos of wall build-ups and mock-ups for air tightness. This up-skilling of local subcontractors fitted project ambitions to build regional capacity in sustainable construction: Stortford Interiors is now fully trained up to install the lime, cork and clay insulating plaster Diathonite. Daily meetings were held at the workface, getting both leads and operatives of subcontractors up to speed on airtightness and Enerphit.

Eve Waldron Design’s visualisation of the new entrance with wicker, wood and plants giving substance to the project’s biobased ideals.
Eve Waldron Design’s visualisation of the new entrance with wicker, wood and plants giving substance to the project’s biobased ideals.

Kelly was aware that just issuing reams of guidance and drawings could fail to get the message across to those who were building the project. So ISG’s in house visualisation team made a couple of simple three minute videos of different aspects of the wall build up, Kelly knew the message would be stronger if people could ‘see not read’.

ISG’s site manager was responsible for the gateway process of inspections and test points with subcontractors, generating the usual slew of photos documenting junctions, hold points and installations based on the inspection and test plan. There were plenty of such points, on the wall build up there was a stop after the first layer of Diathonite installation. A mock-up of the wall build up early on, signed off by Architype and Feilden+Mawson and air tested meant the process could be less inspection-heavy, reduced to being after the second layer of Diathonite was installed. It seems this method worked, the building has now passed its airtightness test with a permeability of 1.33 m3/h.m2 and an air change per hour of 0.6 ACH.

With not only Enerphit targets to hit but also Breeam and Well Building standards, there was endless checking that any proposed substitute products – as Covid and Brexit disrupted supply chains – were bio-based with low VOCs and low embodied carbon. Significant decisions would involve a round of meetings with architect Feilden+Mawson, Max Fordham, client as well as a sign off from Architype and BDP. ‘It was not necessarily fast decision making,’ explains Kelly. ‘It was more making the right decision. And it was a huge breadth of iterations and conversations with subcontractors.’

Getting the sequencing right also had to be done from first principles. When do you upgrade the windows if you want to avoid diesel bus and traffic fumes from the road outside entering the building? The old sash windows were kept in place until the last minute, then new windows installed from a scaffold. An air flush helped clean up the pollutants as shown by the before and after air quality test. The basement air intake and filters now have to continue that good work.

The steel PV canopy over the terrace was repurposed from a film set.
The steel PV canopy over the terrace was repurposed from a film set.

Circular economy

Kelly and ISG’s network also had a big part of to play in sourcing re-used materials and objects for the project. Of course it wasn’t as simple as grabbing something from a skip, first there was finding them and then ensuring they were checked and re-conditioned as necessary. There were three big items: reception desk, a steel canopy and lights.

A reception desk designed by Piercy&Company with Bill Amberg leather, originally for the Copyright Building in central London, where ISG was doing a refit for Netflix. This was going to be skipped due to a layout change for the new reception area, but instead went back to its makers at Benchmark to be adapted for Entopia.

Kelly found a steel structure, no longer required as part of a film set near London. After a trip up to Cleveland Steel in North Yorkshire and back to Cambridge this has been installed on Entopia as a PV canopy.

LED lights came from an ISG client who was stripping out a CAT A fit out. The complication was not only that the warranty would no longer be valid but that the lights would be going from sitting within a ceiling system to sitting proud under the soffit. But the manufacturer, Specials Lighting Design agreed to honour the existing warranty, after an inspection at its factory in Essex. This also gave the chance to add uplighters, take off the end brackets and add wires to hang the luminaires.

  • Bamboo doors and recycled materials on the tea points. Table bases were second hand with five types of table tops.
    Bamboo doors and recycled materials on the tea points. Table bases were second hand with five types of table tops.
  • Cast off lights were reworked for the project.
    Cast off lights were reworked for the project. Credit: Soren Kristensen
  • reception desk courtesy of Netflix choosing to change its reception layout with ISG.
    reception desk courtesy of Netflix choosing to change its reception layout with ISG. Credit: Soren Kristensen

These examples show the huge waste generated by discarding still-performing objects in the industry, sometimes after little or no use. ‘The circular economy has got to be how we do things in future,’ says Kelly. ‘We can’t just downcycle, there has to be a marketplace for perfectly reusable materials.’ One of ISG’s targets is an Ebay-style system for re-used materials but he admits there is a problem in scale. And there is the complexity compared to buying an off the shelf product. ‘You need a bit of luck, a bit of skill and a bit of charm,’ admits Kelly. Persuasion and making things sound possible is part of the job.  

And how will the glamourous reception desk and the clean, low VOC air be welcomed by the team at Cambridge Institute for Sustainability Leadership as they move in? Will the work on the airtightness prove itself in lower heating bills? We will see as Entopia is handed over and the team get comfortable in the building.

Stage 6: Completion and handover

Leaning on the reception desk, signing in at Entopia in Cambridge, a jagged crack in the marble catches my eye. This is the first sign that there is something special about this apparently everyday conversion. 

Of course, readers of the RIBA Journal will already know the name of Entopia, the home of the Cambridge Institute for Sustainability Leadership. We have been following its progress through each stage in the RIBA Plan of Works. Now we reach completion and handover at stage 6. As befits a project for a client like this it has high ambitions – for bio-based materials, building local sustainable construction capacity, working with the circular economy and being a great place to work. In this attempt it was aiming at three major certifications: Passive House’s EnerPhit, WELL Gold and BREEAM Outstanding.

With architects from Architype, Feilden and Mawson and others we have been through the painful guesswork of designing for airtightness before the building was opened for inspection, getting the new windows signed off despite the conservation area, working with tradespeople to ensure the knowledge was in place for the complex wall build-up, and sourcing second hand. Now it is all ready to open. The teams are coming into their new building for culture workshops and Cambridge Institute for Sustainability Leadership is ready for a more visible future in the centre of the city, on the main route from the train station into the heartlands of the university.

The cracked marble reception desk is one of the ‘dings’ the institute is embracing in its quest for the circular economy.
The cracked marble reception desk is one of the ‘dings’ the institute is embracing in its quest for the circular economy. Credit: Soren Kristensen

Except for this crack. Wendy Bishop of Architype and CISL chief operating officer Anna Nitch-Smith have known about it for a while. The reception desk is not quite new, but definitely not ‘pre-loved’ – it was a Netflix reject from the fit out of an office it took on. But when the original maker Benchmark started to resize it for CISL it struggled with the fixing – glue. And so a little crack formed, now quite a substantial one with a chunk missing too. But there is no sending it back. Bishop has just found the right gold for it to be mended in a visible way, Japanese kintsugi style. 

The mending is a statement of intent – that to use resources well you have to reuse them and accept what Nitch-Smith calls affectionately ‘marks and dings’. That’s important, as this is a deliberately ordinary building. I confess to being a little underwhelmed when I arrived; the drama is all off stage, in the process. The experience is of a good, practical, friendly office. But this was also part of the intention. One of the banners on the building reads: ‘This is not an ordinary project. But it needs to be.’

A day looking at projects in London early on helped those involved settle this. ‘Some were rough and shaggy,’ says Bishop, ‘But CISL is always talking to corporates. It didn’t want the hessian sack look, it needed to look like an office building.’ 

There are plenty of fascinating office buildings, so this seems like a missed opportunity. And there were plans for more texture, which would bring an aesthetic of re-use. Columns were meant to be left as exposed concrete. But once the boxing was off the rusting rebar made it clear some would end up more patched than original, and plasterboard seemed the kindest option. This was one of the few opportunities to add texture; the bid for airtightness excluded scrape and reveal on walls as insulation and plaster were an essential part of the wall build up.

  • A cut-through of Entopia.
    A cut-through of Entopia. Credit: Feilden + Mawson
  • Hemp fabric covers the booths.
    Hemp fabric covers the booths. Credit: Soren Kristensen
  • A reworked reception desk at the entrance to Entopia.
    A reworked reception desk at the entrance to Entopia. Credit: Soren Kristensen

What is visually as well as technically experimental is the exposed raised-access floor, the dull metal tiles stretching through office areas. The project team inspected a Brighton project where contractor ISG had used reclaimed raised access floors and one in a Cambridge building where they were newly installed as the final finish. The decision was based on the carbon cost of carpet and the floor’s hard acoustic surfaces are offset by cellulose spray on the ceilings. The regularity of the metal tiles almost strips them of materiality, but it makes you look twice. 

Often the ops team, especially when they have joined part way through a construction project, see the missed opportunities most keenly. For COO Nitch-Smith her only ‘could do better’ is the wish that CISL hadn’t underestimated the staff effort involved, doing day jobs alongside a hugely involved construction project was draining – especially since the institute roll more than doubled over that time from 60 to 140. ‘Sure, we got some things wrong, but that is part of our learning and that of others,’ she says. The building is not finished for her; the final wiring grommets are going in and will be followed by desks, and she has plans to work with local horticultural students to design planting on the rooftop. And she is figuring out how to create an asset register, probably a digital twin, that will both tell the story of some of the objects and provide maintenance and certification information for the future to bolster the beast of institutional memory.

  • A repurposed steel frame supports PVs and provides shelter up on the new roof terrace.
    A repurposed steel frame supports PVs and provides shelter up on the new roof terrace. Credit: Soren Kristensen
  • Project space under the eaves for multi-disciplinary institute teams to gather to launch new sustainability initiatives.
    Project space under the eaves for multi-disciplinary institute teams to gather to launch new sustainability initiatives. Credit: Soren Kristensen
  • SonoSpray coats the ceiling.
    SonoSpray coats the ceiling. Credit: Soren Kristensen
  • Triple-glazed windows were key to the airtightness strategy and to keeping the interior a calm and quiet space.
    Triple-glazed windows were key to the airtightness strategy and to keeping the interior a calm and quiet space. Credit: Soren Kristensen

The old telephone exchange that this building started as has gone from being the rather drab neighbour to the neo-Georgian University Arms Hotel to a handsome plainness, sharpened up by aluminium window surrounds, that holds its own on the busy street. Nitch-Smith hopes it will give the Cambridge Institute of Sustainability Leadership a higher profile with students and the university. There are already signs that it is – though perhaps not as expected. The day after its opening three slogans were stencilled on the building by Extinction Rebellion: ‘No oil, no arms, no empires’. It was a reference to the invitees, His Royal Highness Prince – now King – Charles, Rolls-Royce and Boeing and oil company BP. This shows the careful line that CISL has to tread, working alongside corporates to improve sustainability. Now this building is delivered, more of the team’s energy can go into making that happen and making change towards sustainability. Entopia should be a constant reminder of those ambitions. 

See a video walk through of Entopia here and a 60pp technical case study here from the Cambridge Institute for Sustainability Leadership.

Read more here as the building progresses

Total contract cost £10.54m
Cost per m² £3,586
Gross internal floor area 2,939 m2
Form of contract NEC3


Client University of Cambridge
User Cambridge Institute for Sustainability Leadership
Contractor  ISG 
Project manager 3PM
Cost consultant Gardiner and Theobald
Architect (Stages 1-3) and Passivhaus designer (Stages 1-3) Architype
Architect (Stage 4 onwards) Feilden + Mawson
Interior Design Eve Waldron Design
M&E Engineering (Stages 1-3), Acoustics (Stages 1-3), Structural engineering (Stages 1-3) and BREEAM and WELL consultant (all stages) BDP
M&E engineering (Stage 4 onwards), Passivhaus designer (Stage 4 onwards) and acoustics (Stage 4 onwards) Max Fordham
Structural engineering (Stage 4 onwards) CAR [Cambridge Architectural Design]
PH certifier MEAD
Principal designer iM2



Timber aluminium composite windows Internorm
External doors Moralt Ferro Passiv
Window reveals and other bespoke metalwork Harlestone Group
Diathonite insulating plaster and Gutex insulation Ecological Building Systems
Rocksilk soffit liner Knauf 
Rockfloor insulation Rockwool 


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