One Member’s Local Eco-Build Project
“Our aim was to build an extension using the Passivhaus1 principles of high efficiency insulation and
low energy maintenance. Passivhaus building construction originated in Germany and focused on
reducing the energy requirements for heating and cooling as well as creating excellent indoor air
quality and comfort levels. The principles incorporate low carbon-footprint building materials
which is the reason we opted for a timber-framed extension.
The foundations for the extension required large polystyrene slabs2 below ground level to act as a
ground insulation barrier. The external walls were also clad and we changed our doors and
windows for Passivhaus certified triple-glazing3. Rather than building a conservatory extension we
opted for enlarging the living spaces and incorporating multiple roof lights. We also included an
aquaponics4 room separated by a sheet-glass wall from the dining and living area. It will become an
internal feature with a view of the fruit and vegetable grow beds and fish bed.
One important feature of Passivhaus construction is the requirement to create air-tight interior
spaces to minimise heat loss. For example, UPVC window frames degrade and warp after a few
years and become drafty so we opted for sturdy wooden frames protected externally with
aluminium cladding. The frames were sealed internally to the walls with tape before being
plastered to minimise heat loss and also reduce road noise. All joints and building junctions
everywhere such as the loft hatch, light fittings and even light switches were made air tight.
The emphasis on internal air quality and room air changes is important in the Passivhaus concept.
This may be relevant now as there is increasing scientific evidence that diesel exhaust is harmful
and may contribute to the increasing incidence of lung disease. Using mechanical ventilation and
heat recovery (MVHR), fresh air from outside can be filtered through charcoal and HEPA filters and
then circulated around the house refreshing the air in each space. This system also mitigates
bathroom condensation and cooking smells. The MVHR system will recover over 93% of all the heat
produced in the house.
We replaced the gas boiler with an air-source heat pump5 (ASHP) which stores hot water in a
super-insulated hot-water tank and heats the radiators. Located outside, it works like an airconditioning unit in reverse and at an efficiency of over 300%. This means that for every one kW of
energy required by the electric fan, the ASHP generates over three kW of heat for the house. A
future project will be to add solar-thermal-photovoltaic panels (PVTs) to back up the ASHP. These
are not currently covered by government grants and are not easily available in the UK so we will
wait for the market to catch up. In anticipation of retro fitting PVTs we have installed all the
necessary pipework.
The government updated its domestic energy grants through the Renewable Heat Incentive
scheme in April 20146. This will make a modest contribution towards our start-up costs. Passivhaus
insulation however, should significantly reduce our energy requirements by up to 80%. We
estimate that we will recover the capital cost of the ASHP in 3-4 years.
The construction company employed for this project designed and built the extension as well as
upgrading the rest of the house to Passivhaus standard. We worked closely with the company
director during the works modifying design ideas and materials without incurring extra cost to us.
In fact overall, the project will come in at less than 5% above the cost of a conventionally brickbuilt double-glazed extension.
It is worth noting that the construction of energy-efficient houses requires building standards that
exceed the current construction regulations. The Passivhaus Trust UK5 has certified over 300 newbuild houses in the UK. Eventually we will invite them to inspect and test our refurbishment for
certification.”
If you are interested in discussing any aspect of this project or having a tour of the house, contact
MRA who can pass on your details.
References
1. www.passivhaus.org.uk and www.passivhaustrust.org.uk
2. 500mm high density polystyrene in foundations (25% heat loss), 200mm external wall cladding
(50% heat loss including windows)
3. www.munsterjoinery.co.uk (supply and fit for new build only from 2014)
4. www.baqua.org.uk (British Aquaponics Association)
5. Mitsubishi Ecodan 8.5kW
6. www.ofgem.gov.uk/domestic-rhi (version 1.0 April 2014)