Passivhaus Retrofit

In June 2010 we took on a passive house (Passivhaus) retrofit in Chagford.

What is Passivhaus?

The term passive house (Passivhaus in German) refers to a rigorous, voluntary, standard for energy efficiency in a building, reducing its ecological footprint. It results in ultra-low energy buildings that require little energy for space heating or cooling.

Why Passivhaus retrofit?

In order to tackle the threat of climate change, there is an urgent need to refurbish the UK’s existing building stock and make existing buildings fit for the future.

The project

Westcott House lies on the outskirts of a picturesque village in the foothills of Dartmoor National Park, with views towards Castle Drogo and the rolling hills of Devon.

We were commissioned by the client to carry out all building works involved in retrofitting the house to Passivhaus standards laid out in the detailed plans from Gale and Snowden Architects. For more information on the specification, technical data and a handful of internal shots please visit this link:

The main part of the house dates back to the 1800’s and is made up of 500mm solid granite exterior walls. There has been a 1990’s addition of a single storey extension, which is of traditional block cavity wall construction. The client required us to turn the whole house passive, alter the existing extension and build a two-storey extension to the rear elevation. The extension was built of solid concrete blocks with external wall insulation (EWS) fixed to the face.


Here is an overview of the Environmental aspects of the design

Sustainable and Environmental Design Strategy:

  • Thermal mass
  • Super Insulated Building Envelope
  • Minimal thermal bridging
  • Mechanical ventilation with high efficiency heat recovery
  • High levels of airtightness
  • Daylight design
  • Low water use strategies
  • Low carbon technologies and on-site renewables
  • Rainwater collection
  • Efficient appliances

Material Selection and Healthy Buildings:

  • Natural / recycled materials where practical
  • Organic paints, waxes and stains throughout
  • Specification of timber from sustainably managed woodlands (e.g. FSC certified)
  • Reduction of the use of composite timber panel products
  • Avoid the use of PVC by careful product selection
  • Reduced use of heavy metals
  • Electrical wiring in bedrooms is radial to lessen the impact of electromagnetic fields (EMFs)
  • Use of sustainably sourced materials to ISO14001
  • Use of materials with low embodied energy, where apprpriate
  • Locally sourced materials, wherever possible
  • Prevention of dust-mite infestation by specification of easily cleanable surfaces

What we did

The first part of the project was quite typical of a normal renovation: rip out, move layouts, repair defects etc. It was when we started the first fix stage that we started to step away from the normal routines.

We started with the existing foundations and drainage. This was all done to the air tight and thermal regulations of a Passivhaus.

Externally, on the non-granite walls, 250mm of EPS (polystyrene) was used to insulate the building. The new triple glazed insulated windows fitted to a 75x75mm sub frame were bolted to the walls.

Internally, on the granite walls, 100×50 stud walls were constructed with a 20mm cavity. The walls were backed with a breathable membrane (to stop contact), insulated with 100mm mineral wool and then the Intelo membrane was stapled on and counter battened. The counter batten acts as a service void. Finally, 50mm of closed cell insulation was put between the counter battens before plaster board.

One crucial element of a retrofit is to ensure the connection between various components of the structure are air tight. Here this is achieved using various tapes from the Intello range, OSB, Ply, DPM, existing & new render.

The ground floor of the existing building (suspended timber floor) was removed and refitted with hardcore, concrete, 250mm EPS and screed. The DPM was used as water, air and radon barrier and linked to the walls. The attic had a layer of OSB laid over the top of the joists, which was taped and linked through to the walls below. 500mm of EPS was then fitted over.


Upon completion we are very pleased with the end results, and feel that a lot has been learned. Working on a project like this has made us aware that current Building Regulations and building practice are lagging behind what is technically possible. We have gained an enormous amount of knowledge on how to improve the work we do to surpass Building Regulations and are able to put that knowledge into action on all our projects, and hopefully helping the future of our planet and the environment around us.

Westcott house is currently being certified. Indications are at the moment that we have not only achieved Passivhaus EnerPHit standard but also Passivhaus new build standard making it the sixth in the world. We will update when certification is complete.

More of Our Previous Projects