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Carlton Chapel House EnerPHit

Location: Kentish Town, London
Completion Status: Completed 2019 Occupancy: Occupied since July 2019
Architect: Anne Thorne Architects Consultant: Anne Thorne Architects
Contractor: HA Marks Client: North Camden Housing Coop
Certification: November 2019, EnerPHit Certifier: WARM
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A bold overhaul of a run-down 1980s concrete social housing block has improved the lives of its residents and contributed to positive climate action. The lessons learnt from this challenging EnerPHit could provide a blueprint for upgrading swathes of similar properties.

Carlton Chapel House. Image credit: Anne Thorne Architects


North Camden Housing Coop has undertaken a deep retrofit of a three-storey block of 15 flats in Kentish Town to the EnerPHit standard. Originally designed by modernist architect Christopher Dean, thirty years on the building was experiencing a range of problems including significant structural cracks, decaying windows, and poor ventilation resulting in damp and mould. Residents were unable to afford to heat their homes adequately. In 2011, a feasibility study outlined a clear choice for Carlton Chapel House:  

  • Either undertake a traditional renovation, knowing that further costly works will be required in the future,
  • Or commission an environmental redesign of the building, aiming to eradicate fuel poverty for its members. 

The Coop decided to avoid retrofitting twice and chose the latter option.


Carlton Chapel House. Image credit: Anne Thorne Architects Carlton Chapel House. Image credit: Anne Thorne Architects

Carlton Chapel House before EnerPHit updgrade, Image credit: Anne Thorne Architects


We decided to take a bold and definitive step to address the underlying design issues and provide a sustainable, future-proofed building for the next 60 years. The drive for the decision came from the membership’s enthusiasm to create social housing that is environmentally friendly, addresses fuel poverty and is fit for the future.

North Camden Housing Coop

Key Stats

  • Build start date: April 2018

  • Date of occupation: July 2019

  • No. of units: 15 flats

  • Construction: Concrete block

  • Cost: £1.4 million 

  • Construction cost: £1,283.46/m

  • Treated Floor Area: 637 m2

Carlton Chapel House. Image credit: Anne Thorne Architects

Occupants temporarily relocated for flats to be taken back to bare walls, with plumbing, electrics, and plaster stripped out. PHT member Anne Thorne Architects was the architect and Passivhaus consultant on the project.


The Coop gave us a bold and far-sighted brief aiming for comfort and long-term environmental benefits. Detailed consultations and design workshops with the residents helped us develop that vision. The Co-op formed a design group to support the design team and make decisions throughout the project.

Fran Bradshaw, Anne Thorne Architects


Measured Energy Performance

With energy demand massively reduced, the projected heating load for each flat is just 500W.

Various heating options were explored. A communal heating system would introduce heat loss risk through the pipework, raising the energy demand and potentially contributing to overheating. Expensive heat pumps were also rejected as being ‘overkill’ for the low heat demand.



0.7 ACH @ 50 pascals

Thermal Energy Demand

20 kWh/m².yr

Thermal Energy Load

11 W/m²

Form Factor ratio



Direct electric heating, using simple wall-hung panel heaters was adopted.  Choosing electric over gas made sense for other reasons too: the standing charge alone for gas would have equalled the actual cost of heating. A rapidly decarbonising grid means that the already low level of CO2 emissions from heating these homes will only get lower in the future. The roof houses a modest array of solar photovoltaics which provide electricity directly to each flat for domestic hot water.


For the renovation, we decided to follow the Passivhaus concept. The levels of insulation meant that the heating load was greatly reduced, resulting in no need for traditional heating systems and the gas supply completely removed from the building – a great step for removing the reliance on fossil fuels.

North Camden Housing Coop


Key challenges

Form factor: A complex shape pushed the project team to develop a raft of innovative approaches to achieve the EnerPHit standard. External walkways, inset porches, and bay windows created extensive surface area and thermal bridges where heat is lost. The team simplified the building envelope by enclosing the external walkways and link the bay windows vertically to help with thermal continuity and improve airtightness detailing. Once the form was rationalised, the building was wrapped externally with a continuous airtight layer and insulation around three sides and over the roof. 


Carlton Chapel House. Image credit: Anne Thorne Architects Carlton Chapel House. Image credit: Corbett and Tasker


Insulation: The external wall insulation system included a flexible liquid-applied airtightness layer, 120mm of mineral wool insulation and a final coat of waterproof white render. Applying external insulation to a rendered masonry building should be straightforward, but the complex cavity, parapet and basement garage construction created some challenges. The cavity walls were re-filled with insulation. The innovative approaches to maintaining continuity of insulation and airtightness, where this needed to switch between internal and external strategies, were the key technical lessons from the project.

Carlton Chapel House. Image credit: Anne Thorne Architects


Airtightness: The cavity walls adjoining the neighbouring property made the discontinuity between internal and external airtightness layers difficult. The airtightness barrier needed to cross over from the outside to the inside at the party wall.

In response to this challenge, the project included a pioneering new technique for cavity construction. The airtightness barrier for the project was on the outside of the building, except at the party wall where the cavity at corners was filled with airtightness foam creating an airtight barrier column that linked the two walls. Hollow floor beam ends, built into the internal cavity walls, were also filled with airtight foam.


Carlton Chapel House. Image credit: Anne Thorne Architects

The party wall to the adjoining property also received internal airtightness treatment, as well as woodfibre internal wall insulation.The final airtest result came in at 0.66 ACH@50 pascals, an excellent result for EnerPHit, which allows up to 1 ACH rather than 0.6 ACH for Passivhaus certification.

Structural issues: The cracks in the concrete block cavity walls and hollow beam floor required extensive masonry stitching, and additional support for the cantilevers was necessary to rectify the structural issues.


Key team

Client: North Camden Housing Coop     

Architect: Anne Thorne Architects

Quantity Surveyor: Peter Gittins

M&E: Alan Clarke

Contractor: HA Marks

Structural Engineers: Corbett and Tasker

Carlton Chapel House. Image credit: Corbett & Tasker



The procurement process included a two-stage tender process, with a detailed survey of construction, structure and services, full-stage design, and a traditional contract where the client retained risk. The project was complete within the budget and programme, with only a short extension of time needed for extra structural works. The successful management of the construction tendering process and onsite procedures are replicable lessons for future projects.


The onsite work was of an exceptional standard, with the contractors appointing an effective and knowledgeable airtightness champion. There was also excellent site management, with the contractor efficiently organising work and pricing variations. The subcontractors on the project were also engaged through having been consulted. It was also important that, as project architects, we were on site weekly.

Fran Bradshaw, Anne Thorne Architects


Carlton Chapel House. Image credit: Corbett & Tasker Carlton Chapel House. Image credit: Corbett & Tasker


Residents' feedback

Residents moved back into their flats in 2019. Post-occupancy evaluation reveals that as well as improved energy savings and warmth, residents are reporting better air quality. Noise problems are reduced by removing shared flues, minimising penetrations between the flats, and thanks to triple glazing.

Carlton Chapel House. Image credit: Anne Thorne Architects

The energy consumption of the flats reduced significantly, with heating and hot water bills reduced by up to 90% for tenants. For tenants, this has meant that they’re able to live in a warm, affordable home – with fuel poverty no longer a concern. What’s more, as a co-op, we know that we’ve made the right choice for the planet as well.

North Camden Housing Coop


Carlton Chapel House. Image credit: Anne Thorne Architects Carlton Chapel House. Image credit: Anne Thorne Architects


Asthma improved. Big improvement with chronic chest infection – now seems to have cleared. Warm temperatures and no dampness in the flat.

Tenant feedback

Monitoring took place during an unprecedented UK Heatwave – and highlighted the importance of summer comfort considerations with increased frequency of extreme weather. The Trust hopes to explore this further in the 2021 UK Passivhaus Conference, alongside the 'Keeping Cool' tutorials.


We aim to use the experience we have gained on this project to re-evaluate our plans and look forward to more innovative solutions to decarbonise the whole of our stock – helping local people in North Camden, as well as the planet. 

North Camden Housing Coop


Further Information

Carlton Chapel House

Passivhaus Social Housing: Maximising Benefits, minimising costs

Previous PHT story: EnerPHit upgrade warms homes on Arctic Street - 18 November 2019

Previous PHT story: EnerPHit at scale - 15 November 2019

Triodos Bank: Kickstarting the retrofit revolution - 04 August 2021

HA Marks: Passivhaus EnerPHit - Exceeding the standard

Corbett & Tasker: Carlton Chapel House, Arctic Street