Building study: Entopia, Cambridge, by Architype


Working detail

It is hard not to conclude that the focus on the empirical has absorbed energy at the expense of other project values that architecture, in terms of disciplinary wisdom, can uniquely contribute to the practice of retrofit. While incredibly sophisticated in its building physics, it has not sufficiently established an equivalently ambitious design strategy to mediate between the host building and the new intervention. Retrofit must take qualitative issues seriously. Character and delight, while harder to evidence, give buildings their resilience and value in the long term. We know this from a city full of ancient buildings in continuous use through continuous dynamic change.

CISL set the challenging environmental and wellbeing targets of EnerPHit Classic, WELL Gold and BREEAM Outstanding, as well as low embodied carbon, maximised bio-based materials and exemplar circularity. The site is within a conservation area, with planners keen to retain the existing appearance.

The desire to retain as much as possible of the original host is understandable, yet a more radical intervention might have created a more diverse spatial language to support CISL’s role, convening interactions between the academy, business, government and finance.

Even worse, nervousness around the planners’ perceived resistance to change prevented the transformation of the defensive nature of the building – originally designed to house and protect critical infrastructure – into something more engaging. CISL’s public purpose, with its desire to ‘put the university on the high street’, is laudable but not best served by a host structure with a high cast-iron fence and moat-like lightwell, establishing an abrupt threshold to the city.

The planning committee sensibly overrode the officer’s recommendation but it appears more fundamental damage had already been done. The seemingly conflicted dialogue with planning officers resisted a more wholesale reimagination of the building that could have resulted in a potentially more positive outcome. For example, external insulation of the heavy brick construction would have avoided the high expense of the adopted internal insulation (with its Byzantine detailing around cold bridges) while adding significant thermal mass to condition the interior.

Start on site:  December 2020
Completion: July 2022
Gross internal floor area:  2,939m²
Construction cost: £10.5 million
Construction cost per m2:  £3,586
Architects: Architype (Stages 1-3) and Feilden + Mawson (Stage 4 onwards)
Client:  University of Cambridge
Structural engineers: BDP (Stages 1-3) and Cambridge Architectural Design (Stage 4 onwards)
M&E consultants: BDP (Stages 1-3) and Max Fordham (Stage 4 onwards)
Quantity surveyor: Gardiner & Theobald
Project manager: 3PM
Principal designer: iM2
Approved building inspector: Sweco Building Control
Passivhaus designers: Architype (Stages 1-3) and Max Fordham (Stage 4 onwards)
Passivhaus certifier: MEAD
Acoustic consultants: BDP (Stages 1-3) and Max Fordham (Stage 4 onwards)
BREEAM and WELL consultant: BDP
User client: Cambridge Institute For Sustainability Leadership
Interior design: Eve Waldron Design
Main contractor: ISG
CAD software used: Revit
Annual CO2 emissions: 9 kgCO2/m² (operational energy only)

Performance data

Project data

With the university’s focus on science-based targets, it is not surprising that the priority for the Entopia project has been achieving quantifiable and certifiable objectives. And in these terms, the project has excelled. This building physics-led approach sets out to establish a replicable exemplar of low embodied energy and carbon construction technologies, using natural and bio-based materials.

The vision of the Entopia Building project, its values and challenges, were enshrined in a project charter at an early stage, ensuring collective understanding and purpose-driven collaboration across the team. We periodically revisited these principles, making sure we adhered to them.
Anna Nitch-Smith, chief operating officer, University of Cambridge Institute for Sustainability Leadership (CISL)

Fundamental to the success of this deep retrofit project  was insulating the existing masonry fabric and creating an airtight building envelope while carefully modelling and managing the movement of moisture through the construction to mitigate the risk of interstitial condensation.

We live with 80 per cent of the built fabric that will exist in 2050, setting deep retrofit as the central challenge for all working to shape the built environment. Most of Cambridge city centre is a conservation area with many listed buildings. The city has declared a climate emergency, implying a dramatic transformation to reduce carbon. Noted as a ‘positive building’ in the central conservation area, the original exchange building is a mundane Ministry of Works structure. Pevsner sneered at its ‘genteel good taste’ and moved swiftly on.

The city’s success as one of the UK’s most productive places has recently spurred development, prompting the epithet ‘Cranebridge’. For the last two decades, Cambridge University has primarily focused on its western campus as well as the urban extension of Eddington. This year AstraZeneca’s Discovery Centre opened; a billion-pound palace of research on the expanding Biomedical Campus.

We understood at the outset that it would be truly challenging to deliver a building project with exemplary sustainability credentials but minimal uplift in capital cost relative to conventional projects. We embraced this as a project team, and ultimately achieved our goal by making sustainability and cost-efficiency central to all considerations from the outset.

Quite incredibly, as a means to evidence its progress, the project tracked three distinct certification regimes: BREEAM Outstanding, EnerPHit (the Passivhaus standard for retrofit) and the comfort standard WELL Gold. Both the project team and client have put extraordinary thought and care into negotiating the contradictions between these parallel standards. Finishes and furniture have been selected to embody circular economy principles, most evident in the rescuing of light fittings discarded from office fit-outs and the use of reclaimed second-hand carpet and furniture. The project claims that very low operational energy and the retention of the existing structure have saved 84 per cent in whole-life carbon compared with a business-as-usual cosmetic retrofit.

Quite incredibly, the project tracked three distinct certification regimes – BREEAM Outstanding, EnerPHit and WELL Gold – with extraordinary thought put into negotiating the contradictions between these parallel standards

Driven very much by senior adviser John French, the project reconstructs the team he used to deliver the new-build Enterprise Centre at the University of East Anglia. It aims to demonstrate ‘what can be achieved within a typical office refurbishment budget with a clear focus on cutting carbon while creating beautiful and healthy places to work’. The construction hoarding around the building proclaimed: ‘This is not an ordinary project. But it needs to be.’

We introduced circularity principles as a core element of the RIBA Stage 1 brief and considered this in our appointment of the main contractor. Our scheme includes a range of reclaimed and reused materials, including some donated, such as the PV rooftop canopy, internal lighting and furniture, fixtures and equipment. This enabled us to significantly reduce whole-life embodied carbon (over 21,000kg of CO2e were saved) and reduce the building’s natural capital footprint.

The Entopia Building is among the first of these reinventions, reworking the city’s former GPO telephone exchange, designed by George Ford at the end of the 1930s. CISL has been established to encourage global leadership for the development of a sustainable economy, working across disciplines to convene collaboration between the university, global government, business and finance. In hosting CISL, the Entopia project has positioned itself as an exemplar of deep retrofit and a model for the institute’s own mission. As well as housing CISL, half the building will provide an incubator hub for eco start-ups.

The circular economy aspect of the project has been exciting, with re-use and re-warrantying of light fittings, refurbishment of the existing lift, testing and re-use of the generator and even retention of electrical submains throughout. We’re looking forward to learning more from the in-use data and sharing this with the wider built environment community. Though it’s an exceptional project at the moment, the approaches, technologies and materials are scalable, so we hope it becomes the norm.
Gwilym Still, Passivhaus director, Max Fordham


Engineer’s view


Architect’s view

Numerous insulating and airtightness products were required throughout the project to meet the existing constraints posed by the 1930s building and the later steel-framed extensions added in the 90s. A significant portion of the building envelope was internally wrapped in insulating Diathonite plaster, acting as a buffer to reduce temperature differences through existing walls. Over this, Gutex Thermoroom insulation provides additional insulation to achieve U-values of circa 0.3W/(m2K) (varying depending on existing wall thicknesses), established through WUFI analysis to understand the risk of condensation within the newly insulated walls. An independent wall lining forms the internal finish without fixings back to the existing structure and additional soffit/slab insulation mitigates further cold bridging.

The design strategy was based on a whole-life perspective that considered sustainability and impacts and benefits across the lifetime of the building. The adoption of a fabric-first approach prioritised reducing energy demand and this led to a substantial improvement in the energy performance of the building envelope. This is essential to the achievement of the Passivhaus standard.

Our early stage embodied carbon analysis, using Eccolab, flagged that the choice of floor finishes was critical – new carpet tiles were many times higher in embodied carbon than new cork or linoleum. Exposing the existing retained raised access floor was even lower carbon than adding new layers, so this approach was adopted in the majority of areas.
Wendy Bishop, associate and Passivhaus designer, Architype

Client’s view

This focus on the city’s edges now needs to shift to the centre. Here, rather than state-of-the-art bespoke faculties, the challenge will be to remake a diverse range of built fabric (from listed heritage to faculty buildings from the 1970s education boom) in support of the university’s ambition to reach evidence-based net zero by 2038.

As a beacon project, Entopia addresses the priorities it sets itself. But in doing so, it raises some inevitable questions about how the approach might be extended and improved.

Triple-glazed windows were installed within the plane of the insulation to prevent cold bridging at openings. Iso-Chemie Winframer profiles support the windows, avoiding the need for steel brackets which would only contribute further to cold bridging. These were taped back to the existing masonry prior to the application of the Diathonite and Gutex to ensure continuity of the airtightness line back to the window.

The overall transformation of the building has been remarkable, including cutting air leakage by 80 per cent, replacing glazing to reduce heat loss, enhancing daylight and views and using bio-based insulation to deal with the challenges of internally insulating against masonry.

And yet the project was shaped by anxiety over change: Entopia was recommended for planning refusal because the strategy required replacing the building’s 97 mock-Georgian windows (despite the officer’s report acknowledging that the timber sashes had themselves replaced Ford’s original Crittall windows). The window’s chunky glazing bars blocked 60 per cent of the light, compared with the proposed triple-glazed replacements (which, despite this massive improvement, still do not meet BREEAM target daylight levels in the, at times, sepulchral interior).

The building retains its rigidly layer-cake structure. Built with a heavy structure to support the electromagnetic switchboards of the exchange, the repeating, cramped floor plan never finds a moment of upheaval where the serendipitous interactions between visitors and staff might happen.

The project’s retrofit nature has been both challenging and rewarding, particularly as the building actually has two skeletons: one from the initial build and a later extension to the east. This meant developing several sets of details, including addressing thermal performance, airtightness, and buildability. In particular, thermal bridging was more prevalent than would be the case on a new build.

Furthermore, the apparent reluctance to edit the original host means each floor is segregated by the retained remnants of fire lobbies and pass-doors from either the original exchange or the clumsy 1990s conversion into offices.

On-site installed energy generation: 8%
Heating and hot water load: 17 kWh/m²
Total energy load: 48 kWh/m²
Carbon emissions (all): 9 kgCO2/m²/yr (operational energy)
Estimated annual mains water consumption: 1.6 litres/m²/day (roughly 4m³/occupant/year)
Airtightness at 50Pa: 1.33 m³/h.m² and 0.605 ACH
Overall thermal bridging heat transfer coefficient (Y-value): 0.19 W/m²K
Overall area-weighted U-value: 0.25 W/m²K
Embodied/whole-life carbon: 408 kgCO2e/m² over 100 years
Predicted design life: 100 years


To paraphrase Aneurin Bevan, the language of priorities is the religion of sustainability. The retrofit of a former telephone exchange to house the Cambridge Institute for Sustainability Leadership (CISL) achieves its priorities while illustrating some challenges about how we rethink our built environment.

Despite the low thermal conductivity of the Winframer profiles and the timber framing of the dry lining, additional pieces of insulation were detailed to assist in reducing cold bridging.
Chris Read, associate, Feilden + Mawson

To resolve the tensions between conservation and change, town planning needs better education and resource to engage creatively in the positive renewal of our cities – not least, understanding how to pick the right battles.

For us as Passivhaus designers, MEP consultants and acousticians, the Entopia Building is a dream project with an engaged client, a forward-looking brief, and a team committed to building for the climate emergency.

The strategy from the outset was to insulate the solid brick walls internally and invisibly, but the single-glazed neo-Georgian sash windows needed to be replaced, with a potential change of appearance. Triple glazed Passivhaus windows improved the thermal performance, and the recessed frames and lack of sash bars would maximise the glazed area, with a significant improvement to daylighting levels – this helped persuade the planning committee to prioritise sustainability over conservation.

Hopefully, this impression  of segregation will soften when the spaces become occupied; the building will play a critical role in attracting people back together from home working as well as from the multiple bases from which CISL has been operating while waiting for the building’s completion.

Leadership on sustainability means understanding and controlling risk, and retrofit carries some of the most significant risks in construction. The Entopia project is explicitly an exemplar, with a focus on leadership in sustainability and a remarkable evidence base. It must invite debate on how we structure and deliver deep retrofit. Tom Holbrook is co-founder of 5th Studio