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Pursuit of zero carbon lifestyle drives Oxford EnerPHit Plus

What happens when a climate scientist’s pursuit of zero ‘lifestyle’ carbon emissions are put into action? This radical retrofit demonstrates what’s possible with an existing home, joining a growing number of Passivhaus powerhouses.

Oxford EnerPHit Plus, Image credit: Dan Paton

Completed in the autumn of 2020, this retrofit of a typical 1930’s semi-detached masonry home in Oxford has become the second certified EnerPHit Plus in the UK. All energy demands are met by the installed solar PV, with excess generation exported to the grid. EnerPHit is the Passivhaus retrofit standard. It uses the same methodology as the Passivhaus standard with a slight relaxation in the targets to allow for the complexities of existing buildings. In the case of the Oxford retrofit, complexities included a single-storey rear extension and the shared party-wall.


Key Stats

Units: 1

Occupants: 1

Build start date: February 2020

Completion: September 2020

Certification date: December 2020

TFA: 79m2

Construction: Masonry brick & block.

Solid-wall & insulated cavity-wall

Cost/m2: £2,700

Oxford EnerPHit Plus, image credit: Dan Paton


As a physicist and climate scientist, the homeowner’s goal was to reach zero-carbon for ‘lifestyle’ emissions, driving the project’s priority to radically reduce the 40% of household emissions related to home energy use. Prior unconventional choices, such as limiting occupancy to two rooms and keeping heating off throughout most of the year, were leading to mould & rot damaging the building’s fabric. 


My ambition is to be zero carbon, and I had already radically reduced my carbon footprint by managing travel, diet and heating energy demand, but I realised this was causing damage to the building fabric and wasn’t very comfortable.


At the project’s inception the architect, PHT member Eco Design Consultants, proposed several retrofit options. The client chose EnerPHit Plus standard to radically reduce the building’s energy demand and keep the building fabric safe, without compromising on a comfortable living environment, and make use of the excess energy generated by on-site renewables to offset the carbon footprint of the building’s upgrade.




Original house: solid-wall construction, insulated with 200mm EWI.

U-value <0.1 W/m2.K

Existing extension of insulated cavity-wall construction. EWI added where possible, but in party-wall situations the inner blockwork leaf was removed & replaced with an insulated construction.

U-value: 0.09 W/m2.K


Floor construction replaced with 'warm raft’ construction (also providing flood resilience): waterproof / tanked base, 225mm structural PIR sheet insulation, 18mm OSB board with air-tight membrane, 150mm timber joists insulated with mineral wool, finished with pine floorboards.

U-value: 0.072 W/m2.K


Original house: pitched-roof re-tiled with wind-tight membrane to prevent air movement in loft void. 600mm mineral wool added to loft.

U-value: 0.056 W/m2.K

Existing extension: flat roof removed & replaced with insulated pitched-roof, angled to optimise solar PV installation.

U-value: 0.086 W/m2.K

Oxford EnerPHit Plus, image credit: Dan Paton
Oxford EnerPHit Plus, Image credit: Green Building Store


Airtightness was a key concern owing to the age of the building and presence of different construction build-ups; this is a common challenge with properties that have been extended or altered. Solutions included: removal, application of airtightness detailing, and then replacement of the stairs; excavating and redesigning the ground floor construction; airtightness detailing in the roof voids.

Party-wall junctions were also a key focus, and creative solutions included careful detailing of external to internal wall insulation at junctions with the rear extension and adjoining property. As a physicist, the engaged client himself conducted thermal bridge calculations for every junction in the building, ensuring a high-performing continuous thermal envelope. The neighbouring property also benefited from installation of EWI to the front and rear, to mitigate thermal bridging at the properties’ boundary.


Energy performance

Airtightness (≤0.1ACH@50pascals)

0.73 ACH@50pascals (Measured)

Thermal Energy Demand (≤25kWh/m².yr)

16 kWh/m².yr (Predicted)

Primary E Demand (≤120kWh/m².yr)

100 kWh/m².yr (Predicted)


The design adopts a centralised MVHR system. An air-source heat pump supplies the heating demand and can provide air-conditioning if required in the future. Roof-integrated solar panels are installed on south-east and south-west facing roof; on the single-storey rear extension the flat roof was removed and replaced with a pitched-roof designed to optimise the solar PV installation.

Although not currently part of the EnerPHit accreditation, the homeowner conducted embodied carbon analysis for the materials and services used in the retrofit, which predicts that (alongside other lifestyle changes such as not owning a car) the embodied carbon of the build will be paid-back by 2025, achieving the owner’s aim of being zero-carbon.

Oxford EnerPHit Plus, Image credit: Dan Paton


By retrofitting the house to the EnerPHit Plus standard I am now exporting more than 3,000 kWh a year to the grid and avoiding energy consumption for heating. The embodied carbon in the project amounts to almost 20 tonnes, the biggest chunk of which is insulation materials: considering the annual energy savings and the net export of electricity to the grid, I’m hopeful that I’ll have met my ambition to be zero-carbon by 2025. The project was a huge team effort.


The homeowner’s fastidious ambition played a crucial role in the project’s success, future-proofing the building and producing a durable climate-resilient home. The design team was led by PHT Member Eco Design Consultants and received EnerPHit Plus accreditation in December 2020 from PHT Patron WARM.


Key team

Client: Private homeowner

Architect & PHPP consultant (to technical design stage): Eco Design Consultants

Architect (tendering / project management): Transition by Design

Contractor: AMC Build

MVHR consultant: Green Building Store

Passivhaus Certifier: WARM

Oxford EnerPHit Plus, image credit: Eco Design Consultants


Are you exploring solutions to upgrade your own home? Follow these simple steps to assess if EnerPHit is right for you & your property.

The Trust is leading a cross-industry retrofit working group. We hope to publish guidance jointly with partners including LETI, RIBA, the AECB and others later this year which identifies common building typologies and proposed upgrade works. Following this publication, the Trust shall also be launching some exciting new Net Zero Retrofit tutorials this September. Watch this space!


Further information

Radical Retrofit: Oxford EnerPHit Plus

Passivhaus Powerhouses (Plus & Premium)

Passivhaus Retrofit

Is Passivhaus retrofit right for you?

23rd July 2021

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