First UK retrofit certified to Passivhaus Plus

This Bedfordshire renovation surpassed EnerPHit criteria to become the UK's first certified Passivhaus Plus retrofit!

Completed in 2023, Greenfield Passivhaus Plus is a holistic retrofit of a 1960s detached home in Bedford. The design, by PHT member A D practice, entailed reconfiguring the internal space, adding a mezzanine and a new roof. The clients desired a new lease of life for their home whilst reducing their environmental impact and creating a dwelling capable of adapting with them through life.

Interiors have been completely modernised and flooded with daylight.

Key stats

Construction: Hybrid
TFA: 192 m²
Build start date: 2022
Completed: 2023
Certified: Passivhaus Plus

Greenfield Passivhaus Plus © AD Practice, Matthew Smith Architectural Photography

Construction

The existing ground floor was rreplaced with a new highly insulated slab with underfloor heating and cooling. The existing brick cavity walls were externally insulated using wood fibreboard that was finished with a painted render and vertical Brimstone Ash cladding. The new first-floor gable walls are highly insulated timber I-beams with cellulose insulation and clad in vertical Brimstone Ash cladding to match the upgraded brickwork. The existing roof was removed and turned 90 degrees. The new roof was constructed similarly to the new walls, forming a gable at the front and rear, maximising the views across the garden and countryside beyond.

U-values
Floor: 0.127W/m²K	Exterior - interior: Concrete slab, 160mm PIR insulation, Concrete screed with under floor heating, Floor finish.
Wall: 0.126W/m²K	Retrofitted Wall: 200mm woodfibre EWI, Existing insulated brick and block cavity wall, Airtightness membrane, Service Void, Plasterboard and skim. New First Floor Wall: 200mm woodfibre insulation, OSB, 200mm woodfibre insulation in between timber studs, SmartPly, Service void, Plasterboard and skim.
Roof: 0.117W/m²K	60mm rigid woodfribre insulation, 300mm blown cellulose insulation in between timber I joists, Smartply Service void, Plasterboard and skim.

Further information can be found at the International Passivhaus Database listing

Before and after interiors. Top row after - kitchen, bedroom, living room. Bottom row before.

Building performance

Designed energy performance, calculated according to PHPP

Airtightness n₅₀ (≤ 0.6ACH @ 50 Pa)

0.6 @ 50 Pa

Space Heating Demand** (≤ 15 kWh/m².a)

18 kWh/m².a

Heating Load** (≤ 10 W/m²)

10 W/m²

Primary Energy Demand** (≤ 120 kWh/m².a)

72kWh/m².a

PER demand 30

Primary Energy Renewable (PER) Demand** (≤ 60 kWh/m².a*)

71 kWh/m².a

Primary Energy Renewable Generation

79 kWh/m².a

*+/-15 kWh/m².a allowance if offset by energy generation. See Passivhaus criteria.

**Space Heating Demand, Heating Load, Primary Energy Demand, Primary Energy Renewable Demand - All currently undergoing monitoring.

Greenfield Before - Back Exterior © AD Practice

Greenfield After - Back Exterior © AD Practice, Matthew Smith Architectural Photography

Before and after (back exterior)

Services

The client wanted a 'gas-free' house. The home does not use fossil fuels. A ground source heat pump and an entire edge-to-edge roof pitch dedicated to 12.5kW solar PV help meet much of the home's energy requirements.

Greenfield Solar PV © GB-Sol Limited

Lessons learned

Working with roof-integrated solar PV was a huge success.
Building & reorientating a new roof eliminated thermal bridges and allowed a full roof slope of solar PV to be achieved.
Making old cavity walls airtight where they had become internal through a previous extension and acted as a chimney result in meeting 0.6ach rather than the 1.0 for EnerPHit.

Greenfield Before - Front of House © AD Practice

Greenfield After - Front of House © AD Practice, Matthew Smith Architectural Photography

Before and after (front exterior)

Key team

Client: Private
Architect & Passivhaus Designer: PHT member A D Practice Ltd
Contractor: Apollo Construction Projects Ltd
Structural engineer: Rawlings Structural Design Ltd
Passivhaus Certifier: PHT member Mead Consulting

Achieving the Passivhaus Plus standard for a retrofit is a huge accomplishment! Congratulations to the whole team & AD Practice, who have fully embraced the Passivhaus retrofit. It is not the architect's first - Harpenden EnerPHit Plus was a finalist in the 2023 UK Passivhaus Awards. We eagerly await their next Passivhaus project!

Architects view

We are always very keen to support clients who see the environmental benefits of upgrading existing properties to EnerPHit standard or, as in this case, beyond, rather than the often cheaper but less sustainable option of a demolition and rebuild. Our clients are delighted with the end result, and to have the exclusive tag of living in the first Passivhaus Plus retrofit in the UK!”

Heather McNeill, Director, Architect & Certified Passivhaus Designer, A D Practice Ltd

Similar projects

The projects below are all EnerPHit Plus certified. Take a look at our Plus & Premium page and Retrofit page for further examples, and to get started on your Passivhus self-build journey, visit the Passivhaus goes Personal page. For more guidance, check out the How to Build a Passivhaus: Good Practice Guide or our FREE interactive Introduction to Passivhaus online course.