Passivhaus Affiliate

Carrstone House certified Passivhaus Plus

Carrstone House is a large detached three bedroom home designed by PHT member Eco Design Consultants, that was certified Passivhaus Plus in September 2017. The clients desired for a highly environmentally sustainable life and aspired to achieve certification from the onset. 

Carrstone House ©Eco Design Consultants

The living spaces are arranged in an open plan format facing south to use maximum solar gain. The master bedroom and lounge are the most used rooms and so are located to the south maximising the views. A central utility spine separates private and public area within the house, providing a clear design feature.

Carrstone House ©Eco Design Consultants Carrstone House aerial view ©Eco Design Consultants


Eco Design Consultants have been a superb partner for us in the realisation of our dream to build a Passivhaus. The end result is a distinctive building with a comfortable living environment that doesn’t cost the earth to run.


The house optimises high levels of natural light, reducing energy usage with the aim to improve wellbeing. Photo voltaic (PV) panels incorporated into the roof generate energy to power and heat the home, as well as charge an electric car. Efficient water fittings, water butts collecting surface water from the roof (with overflow to rain garden and pond), and sustainable drainage solutions reduce the buildings water usage.



The development is built with a timber frame construction.


300mm MBC Timber frame, twin stud wall with plywood gussets filled with Warmcell insulation. Finished inside with OSB, service void and plasterboard and outside with Medivent, ventilated cavity and render on a render board. U-value: 0.143 W/(m2K)


MBC flooring system of 300mm EPS with concrete raft above with integral underfloor heating. U-value: 0.12 W/(m2K)


400mm MBC twin frame roof with 225mm joists inside 86mm outside separated with plywood gussets and filled with Warmcell insulation. Inside service void and plasterboard ceiling, outside membrane, then PV roofing system U-value: 0.11 W/(m2K)

Windows (Uninstalled) 

Internorm HF200-Safety (6.4/16/3/16/6.4 Laminated Light Coating) Ug-value: 0.5 W/(m2K) 


One of the challenges of the project was the PV, we had to apply for a power station license (yes anything over a minimal amount needs one!) which the network limit to 25kWP and thus eliminated meeting Passivhaus Premium for this project. Rather than a roof full of PV, we introduced glass panels around the edges.

Originally going down the eco route was suggested by the planner who had refused a previous scheme. The client were not averse to this, and became convinced of Passivhaus the more they found out about it and the comfort levels. Convincing them to go for Passivhaus Plus was not difficult as the client is a Quantity Surveyor, who had researched the additional cost of the full roof PV system, the saving on alternative roof finish and the projected income. 

Alan Budden, Director, Eco Design Consultants


Key Stats:

TFA: 221m2

Form Factor Ratio: 3.1

Location: Maulden

Start: May 2016

Certified: September 2017

Carrstone House ©Eco Design Consultants
Carrstone House Interior ©Eco Design Consultants Carrstone House ©Eco Design Consultants


The house is designed to be low lying at the back of the existing house, with roofs facing east and west pitched at 15 degrees filled with Photo Voltaic (PV) panels, and a glass border, providing 24kWP. Underfloor heating & the hot water tank is supplied by an air source heat pump with electricity from the on-site PV panels.  A Paul Novus 300 MVHR provides ventilation.

Thermal Energy demand (≤15kWh/m2.yr)

13 kWh/m2.yr 

Thermal Energy load (≤10W/m2) 

9 W/m2

PER (Demand ≤45kWh/m2.yr) 

(Generation ≥60kWh/m2.yr)

Demand: 30 kWh/m2.yr

Generation: 95 kWh/m2.yr

Airtightness n50 ACH@50pascals (≤0.6ach@50pascals 


The scheme was a recent participant of the 2017 International Passivhaus Open Days in November and has won the Bedfordshire Association of Architects Design and Craftmanship award for sustainability. 

Carrstone House ©Eco Design Consultants

Key Team

Client: Private

Architect: Eco Design Consultants

Building services consultants: Williams Energy Design,

Contractor: Nick Hull Builders / MBC Timber Frame

Certifier: CoCreate Consulting

The clients on this project took on the role of principle designers throughout the construction phase of the project. 

All images ©Eco Design Consultants


Further Information

Eco Design Consultants

House Planning Help - Taking on the principle designers role.

The new Passivhaus classes - Passipedia

International Passivhaus Buildings Database (Project ID: 5111)

14th December 2017

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