Passivhaus Affiliate

EnerPHit upgrade warms homes on Arctic Street

The UK Green Building Council recently calculated that 80% of all buildings that will be in use in the UK in 2050 have already been built. Even if we assume that all new buildings over the next 30 years, perhaps up to 230,000 dwellings a year, could be from now on built to Passivhaus standard or equivalent and their heating demand kept to 15kWh or less, the huge challenge of heating those remaining homes with low- or no-carbon energy is still to be met. The only way to do it is through retrofit at scale, and an example of how it might be done has just been demonstrated by PHT member Anne Thorne Architects, with the retrofit of a block of 15 flats to EnerPHit standard for North Camden Housing Coop.

 Carlton Chapel House, before and after


The building, designed by architect Christopher Dean Associates, was completed in 1983, and after three decades was sorely in need of attention. Flats suffered from damp and mould internally, and because tenants couldn’t afford to heat their homes adequately, cold homes stayed even colder. Decaying windows didn’t help. And structurally, the concrete block cavity walls and hollow beam floors had cracks throughout. To remedy its problems, a deep retrofit was planned, taking flats back to bare walls, with plumbing, electrics and plaster all stripped out, so as to revive the building, take it to the EnerPHit standard and bring it into the 21st century.


Key facts

Build start date:

Date of occupation:   

Number of flats:


April 2018

July 2019


£1.4 million   

The form of the building offered plenty of challenges from the start. External corridors, inset porches and bay windows all created more external surface area from which to lose heat. The external corridors have now been enclosed, and bays linked vertically to form continuous projections.

Once the form was rationalised, the building could then be wrapped externally with a continuous airtight layer and insulation around three sides and over the roof; the party wall to the neighbouring property received airtightness treatment and woodfibre insulation on the inside. The cavity walls made the discontinuity between internal and external airtightness layers particularly challenging. It was resolved by injecting an airtight foam into the cavity at the junction, creating an airtight barrier column that linked the two walls. The final airtest result came in at 0.66 ACH – an excellent result for EnerPHit, which allows up to 1 ACH rather than the 0.6 for full Passivhaus certification.

Paul Jennings and Fran Bradshaw after a successful airtightness test

Paul Jennings, airtightness expert, and Fran Bradshaw of Anne Thorne Architects following a successful airtightness test

The external wall insulation system from Sto included a flexible liquid-applied airtightness layer, 120mm of mineral wool insulation on top and a final coat of waterproof white render restoring the building’s looks. A scaffold full cover allowed thorough works to the roof, which received new decking, membrane and wood fibre insulation both between and above the rafters.

Carlton Chapel House roof and external wall insulation

With energy demand massively reduced through the EnerPHit works, attention turned to how to supply the small amount of energy that’s still needed. The projected heating load for each flat is just 500W. A communal heating system in this case would introduce heat loss risk through the pipework, raising the energy demand and potentially contributing to overheating. Heat pumps would have been expensive, and overkill for the low heat demand. So direct electric using simple wall-hung panel heaters was chosen as the space heating solution.  

Choosing electric over gas made sense for other reasons too: gas for each flat didn't add up financially, as the standing charge alone would have equalled the actual cost of heating. And a rapidly decarbonising grid means that the already low level of CO2 emissions caused in heating these homes will only get lower in the future. This project, too, makes its own small contribution to renewable energy generation: photovoltaics on the roof (somewhat limited in size by the retained original form of the roof, required by the planners) provide electricity directly to each flat for their domestic hot water.

Key team



Quantity Surveyor: 



Structural Engineers:    

North Camden Housing Coop     

Anne Thorne Architects

Peter Gittins

Alan Clarke

HA Marks

Corbett and Tasker

The project was completed within budget and on time; tenants have now moved back in, and post occupancy evaluation is taking place.


Further information

EnerPHit at scale 

Passivhaus Social Housing: Maximising Benefits, minimising costs

Anne Thorne Architects

Zero carbon, resilient retrofitting for low income tenants with Alan Clarke | AECB webinar

Passivhaus EnerPHit - Exceeding the standard | HA Marks 

Carlton Chapel House, Arctic Street | Corbett & Tasker


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18th November 2019

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