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.
This robust approach to building design allows the designer to minimise the ‘Heating Demand’ of the building and in some residential buildings only specify a heated towel rail as means of conventional heating. This heat can then be recovered and circulated by a Mechanical Ventilation and Heat Recovery (MVHR) unit.
This has led to the following functional definition of a Passivhaus…
“A Passivhaus is a building, for which thermal comfort can be achieved solely by post-heating or post-cooling of the fresh air mass, which is required to achieve sufficient indoor air quality conditions – without the need for additional recirculation of air.”
Meaning the heating requirement in a Passivhaus is reduced to the point where a traditional heating system is no longer considered essential. Cooling is also minimised by the same principles and through the use of shading and in some cases via the pre-cooling of the supply air. Night purging and the use of natural cross-ventilation through open windows is encouraged during the summer months.
As well as being an energy performance standard, Passivhaus also provides excellent indoor air quality. This is achieved by reducing the air infiltration rates and supplying fresh air which is filtered and post heated by the MVHR unit.
The Passivhaus Institute has developed a series of certification processes to ensure the quality of any official Passivhaus.
The Passivhaus Planning Package (PHPP), used to inform the design process and to assess or verify compliance with the Passivhaus Standard.
Certification for designers who have the expertise to deliver Passivhaus buildings.
A certification process for Passivhaus buildings, which applies both to the proposed design and the completed building.
A number of UK organisations have been approved to assess and issue the Quality Assured Passivhaus Certificate.
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 EnerPHit Standard has been developed by the Passivhaus Institute for retrofits where the existing architecture and conservation issues mean that meeting the Passivhaus standard is not feasible. EnerPHit recognises the difficulty of achieving a full Passivhaus standard in existing buildings and has slightly relaxed requirements for airtightness and space heating demand.
The most cost-effective approach to reducing the carbon dioxide emissions from the home is to first pay attention to the energy efficiency of the building fabric, before looking at such options as micro-renewables or other forms of ‘carbon offsetting’. This approach, sometimes called eco-minimalism, focuses on ways of saving and retaining energy in buildings, rather than looking for bolt-on energy-generating approaches which, while appearing ‘green’, can often be expensive and ineffectual. This approach leads us to be passionate proponents of Passivhaus design, which concentrates on super insulation, airtightness and use of passive solar gain to cut a building’s space heating needs by 90% compared to the average UK building.
By using the simple ‘tea cosy’ effect of super-efficient insulation and excellent airtightness coupled with a ‘comfort ventilation’ system, Passivhaus design can create healthy and comfortable homes that require minimal heating. Heat generated from the sun through windows, occupants’ body heat and cooking and showering activities are often all that are needed to warm a Passivhaus home. Passivhaus dwellings typically achieve an energy saving of 90% compared to the average existing house and 75% saving in space heating compared with a new house built to our current Building Regulations.
Insulation is central to low energy construction but airtightness and windtightness are also important to reduce unnecessary heat loss. Research shows that up to half of all heat loss in buildings can be due to air leakage and uncontrolled ventilation. Insulation levels have increased substantially over the last few decades but heated air is still escaping and can be pinpointed as a major source of energy loss. In addition, heat that escapes from buildings carries a significant amount of moisture which can lead to damage to the building fabric.
Very substantial energy savings are possible with refurbishments of existing buildings. The main focus is on the following aspects:
Improved thermal insulation
Minimisation of thermal bridges
Use of excellent quality windows
Ventilation with efficient heat recovery
Efficient heat generation
Use of renewable energy sources
If you are thinking about undertaking a new build or are contemplating a radical refurbishment of an existing property, we would strongly recommend the Passivhaus approach for your 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: www.ecodesign.co.uk/projects/residential/westcott-house
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:
Super Insulated Building Envelope
Minimal thermal bridging
Mechanical ventilation with high efficiency heat recovery
High levels of airtightness
Low water use strategies
Low carbon technologies and on-site renewables
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.