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Entopia Building

Location: Cambridge
Completion Status: Completed March 2022 Occupancy: Occupied September 2022
Architect: Architype, BDP Consultant: Max Fordham
Contractor: ISG Client: Cambridge Institute for Sustainable Leadership (CISL)
Certification: July 2022 Certifier: MEAD
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The Entopia Building, formerly a 1930's telephone exchange, now requires just 15% of the energy requirement of the original building and saves over 62,000kg CO2e in construction materials thanks to a radical EnerPHit upgrade.

A triumphant collaboration between PHT Patrons Architype, Max Fordham, and PHT member BDP, the Entopia Building is a fabric-first approach combined with embodied carbon and circular economy principles. It provides an efficient, high-quality, and comfortable working environment for the new Cambridge Institute for Sustainable Leadership (CISL) headquarters.



For us, as Passivhaus designers, the Entopia building is a dream project, with an engaged client, a forward-looking brief, and a design and delivery team committed to building for the climate emergency. The transformation of the building has been remarkable, including cutting air leakage by 80 per cent, replacing glazing to reduce heat loss, enhance daylight and views, and using bio-based insulation to deal with the challenges of internally insulating against masonry. We’re looking forward to learning more from the in-use data and sharing this with the wider built environment community. Although it’s an exceptional project at the moment, the approaches, technologies and materials are scalable, so we hope it becomes the norm.

Gwilym Still, Director, Max Fordham


Certified (August 2022) by PHT member MEADThe CISL headquarters acts as a central hub, both in Cambridge and internationally, to provide a thriving environment for staff and visitors and align with the deep-rooted values of sustainability and leadership within the institution. The new CISL headquarters houses:

  • New offices spaces, including hot desking.
  • Entertainment spaces to hold exhibitions, events, and a café.
  • A virtual hub for international offices and partner organisations, alumni, fellows, associates, researchers and visiting academics.
  • An Accelerator and Sustainability Hub to support small businesses and start-ups.
  • High tech video conferencing facilities


Key Stats

  • Project cost: approx. £12.8m

  • Construction started: March 2021

  • Completed: March 2022

  • Certification: EnerPHit, July 2022

Entopia Building Entrance. Image credit: CISL



To achieve EnerPHit certification is a major milestone in the renewal of Entopia as an ultra-low carbon, inspiring and healthy place to work. It reflects great teamwork and collaboration from the whole client, design and contractor team and enhances the building’s exemplary sustainable approach.

Wendy Bishop, Associate at Architype


Retrofitting over 28 million buildings by 2050 to meet climate pledges is an immense challenge. EnerPHit certification provides a reliable solution that may be applied at scale. The headquarters upgrade was initiated by Dame Polly Courtice, CISL's Emeritus Director, and CISL's John French, who previously led the design and build of the award-winning certified Passivhaus Enterprise Centre. It holds five years of post-occupancy monitoring data as evidence of its Passivhaus performance, a notable driver for the brief of the new Entopia Building. Following the success of this scheme, the same team, including PHT Patron Architype and PHT member BDP, were appointed to bring the Entopia retrofit from concept to completion.

Before: Entopia Building. Image credit: Soren Kristensen   After: Entopia Building. Image credit: Architype

Entopia Building: Before & After

The measures we have incorporated in this ground-breaking project go far beyond standard sustainable buildings and the integrated and collaborative approach to design, operation and management from all stakeholders has resulted in a truly efficient, world class facility. The Entopia Building stands tall as an industry exemplar and achieving EnerPHit certification is hugely significant; demonstrating how we can successfully repurpose existing buildings to be low carbon and highly efficient in both construction and operation.

Lucy Townsend, Head of Sustainability at BDP



Conservation & energy-efficiency

Striking a balance between conservation and sustainability can often present challenges in historical locations, but with a robust brief from the outset, planning was approved. However, this was not without its challenges:

  • Key challenges included upgrading the building fabric only with internal insulation to avoid drastically altering the existing façade and disrupting the character of the building. It required thermal and hygroscopic modelling to ensure that interstitial condensation would not occur, thus causing potential damage to the building fabric over time. Taking these additional measures has made for a more high-quality design, increasing the longevity and durability of the building.
  • Another design challenge was sympathetically replacing the original sash windows. Triple glazed recessed frames are the most appropriate aesthetically and practically, enabling the internal wall insulation and windows to avoid complications in thermal detailing. These common challenges serve as prime lessons for other retrofit schemes with conservation constraints.


Window design options. Image credit: Architype


The team has delivered a world class deep retrofit, true to the original vision and intent that has achieved the impressive standards of EnerPHit, and they have done this as a connected and collaborative team that has overcome difficult challenges that you would not encounter in a new build project. This team has demonstrated that there is a positive path in retrofitting our old building stock as part of the climate challenge. This represents important technical leadership to the sector supported by CISL and the University of Cambridge.

Prof John French, CISL Fellow & Advisor to the Entopia Building project




  • Internal insulation on 330-350mm solid brick.
  • 40mm cork/lime thermal plaster
  • 40mm woodfibre insulation

Pitched roof:

  • Hemp Jute between rafters
  • Sealed internally with humidity variable airtightness and vapour control membrane.


Entopia Building Construction. Image credit: Soren Kristensen
Entopia Building Construction. Image credit: Soren Kristensen

Lessons learned

  • Conservation considerations: Challenging to design and deliver a highly energy-efficient building due to conservation considerations but the operational energy savings achieved through EnerPHit certification are hugely beneficial and worthwhile.

  • Minimal product data: Minimal data was available for some products and materials; Whole life carbon (WLC) calculations were challenging. Understanding which assumptions to use in calculations was difficult and peer-reviewed to ensure consistency.

  • Existing structure replacements: Some elements were only discovered once on site due to a lack of full surveys and opening up, which increased the programme schedule. For example, concrete columns were in worse condition than anticipated, as well as steel columns encased in concrete. Without the original plans, the project team had to re-design the original plans, adding more time to the programme. Opening up the building earlier would have been a great advantage.

  • Embodied Carbon: The embodied carbon of the project increased as ageing harmful materials were discovered at a late stage. These included removal of asbestos, delamination, leaching of products through the Sonaspray, waterproofing issue, covering exposed rebar due to fire-safe columns, and beams in poor condition.

  • Material quantities: Limited materials from niche suppliers meant difficulty with order quantities for the project due to smaller manufacturers.

  • Early engagement of project team: Contractors were on-board at RIBA stage 3. Changes and challenges in the project programme were easier to manage.

  • Precedents: Reusing the existing raised floor and exposing it as a finish saved a large amount of carbon. It initially prompted concerns but was approved after the client was convinced by a similar case study.


Key team

Project Client: University Cambridge Institute for Sustainability Leadership (CISL) 

Architects:  Architype

Delivery Architect: Feilden+Mawson

M&E Engineering: Max FordhamBDP

Main Contractor: ISG


The Entopia Building construction, Image credit: MEAD


A challenging retrofit for many reasons, which also spanned a pandemic and the associated restrictions. Negating these restrictions and delays and not deviating from the original goals proves that the best results are achievable during the hardest of times. Full credit to a forward-thinking client and experienced team. I look forward to The Entopia Building, and design and delivery teams, receiving the credit that they deserve.

Kym Mead, Passivhaus Certifier, MEAD


Further information

Entopia Building

Building Entopia: The inside view of a sustainable retrofit

Building Entopia: The story behind the sustainable retrofit of CISL's home in Cambridge

Previous PHT story: Entopia EnerPHit Exemplar - 15 September 2021

Passivhaus Retrofit

Passivhaus Retrofit Masterclass lecture series

Passivhaus & Embodied Carbon

Passivhaus & Embodied Carbon webinar- on demand

Dezeen: Architype transforms old telephone exchange into Entopia workspace - 21 November 2023

Architects Journal: Entopia, Cambridge- 27 October 2022

CISL: Entopia Building achieves EnerPHit classic certification - 09 August 2022

Ecological Buidling Systems: Making the past ready for the future - 08 August 2022