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It’s technical: CISL's deep retrofit hits Stage 4 of the Plan of Work

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Eleanor Young

What are the triumphs and challenges of deep retrofit – the process of making a building sustainable in conversion and use? To find out, we follow a flagship project, for the Cambridge Institute for Sustainability Leadership’s new base, through the RIBA Plan of Work. Read on for Stages 0 to 4

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 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.

Read more here as the building progresses


See the team talking about the building at Footprint in Brighton on 8th June.


Thursday 16th June, 2 -3.15pm

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