Eco Home: Zinc House Sustainability Review

Part II of the four part series covering the recently published Zinc House looks at the projects sustainability credentials, covering design choices, cost impact and lessons learned.

The Zinc House has achieved a near to zero net carbon footprint, delivering an energy efficient all-electric dwelling, combining electricity generation from PV panels and energy efficiency from a ground source heat pump installation,  in order to satisfy the entire heating and power requirements of the property. Whilst those key sustainable technologies employed contribute significantly to its energy efficiency, first and foremost sensible and intelligent passive design forms the foundation of its performance. A consideration of the path of the sun around the property, generous glazing carefully placed so as to benefit from solar gain without excessive or uncomfortable glare, a highly efficient and air tight thermal envelope, openable doors, windows and rooflights for natural cooling, all work together to support the technologies used here.

The all timber structure, a cross laminated timber panel system (see journal post I/IV), is sourced from sustainably managed forests, and avoids the energy intensive alternatives of steel and concrete, while benefitting from large amounts of stored C02 through sequestration prior to manufacture. Manufacture offsite significantly also reduces waste (although these must be offset against the environmental impact of delivery to site). 

The house is faced in long-life 100% recyclable cladding (zinc) formed on site with mobile machinery, sitting above a plinth of handmade brick sourced locally. Exposed timber finishes internally significantly reduce the amount of plasterboard and gypsum and/or cement based products in the build, and improve internal air quality.

Super-insulated walls and roofs achieve u-values that significantly outperform building control requirements. The ‘warm’ roof construction allows a seamless overcoat of insulation, improving thermal values and eliminating cold bridging present in a cold/vented deck system. 

A ground source heat pump provides highly efficient space and water heating with 300 linear metres of GSHP pipework buried in trenches in the adjacent paddock.

At the time of procurement the cost of the ground source heat pump installation was around 12k versus 6k for a conventional gas boiler/megaflow system. A 7k grant was obtainable under the Renewable heat incentive (payable over 7 years)  brought costs into direct comparison. Benefiting from the efficiency of the GSHP, whole house annual heating and electric bills fall within 10% of the annual returns on the 3.2kW solar PV array, making running costs more or less net zero. 

Underfloor heating provides radiant heat via an exposed and self finishing cast concrete screed over the entire ground floor of the property. This, working together with the airtight envelope, provides a very comfortable internal environment highly consistent in air temperature and free of draughts.

A wood burning stove tops up winter heating with coppiced timber from the site and provides a snug hearth to the house. The heat pump is weather compensated to respond to external conditions via sensors, and rooms benefit from zonal thermostatic controls, so that distinct areas of the house can be heated to different set points as required (for instance the guest bedroom differently to the master bedroom). 

The house is all electric and a 3.2Kw solar PV array sells power back to the grid with a net zero cost against all utility expenditure  for the house. Electricity is sold to the grid and bought back as required. Planning requirements had to be balanced against the desire to maximise performance of the PV system, where massing restrictions dictated some overshadowing of parts of the roof. Careful sun shading analysis was undertaken in dialogue with the PV suppliers to ensure the dual array system was configured to maximise output. 

A sedum roof was planted to encourage further biodiversity on the site. The green roof also attenuates rainfall (often consistent and heavy in this part of the country!) and provides an additional layer of insulation contributing further to the excellent performance of the envelope, as well as protecting the single ply membrane below from UV degradation over time. The roof requires occasional maintenance (general weeding once a year) but otherwise flourishes without special attention. 

The inner structural and outer insulated envelope of the building were both carefully sealed at all joints and interfaces using specialist tapes to enhance the thermal performance of the house and eliminated all draughts.

The garden design utilises reclaimed stone slabs unearthed and collected across the site. Sometimes forming steps, elsewhere finishing benches, the stone adds a visual patina that contrasts to the factory made finishes, and echoes a sense time of what came before. 

The property SAP report and calculations returned an excellent A rating of 93%.

Three further sustainable technologies that were seriously considered but not adopted here were:

  • solar thermal panel for hot water generation from solar energy.
  • whole house ventilation via a heat recovery unit.
  • greywater recycling via a buried water storage tank.

Whilst these would have further enhanced the sustainability credentials of the property to an A++ rating, a balance of advantages and disadvantages where discussed with the client (such as the tendency with whole house ventilation towards a ‘closed window’ approach to living in the house) and on analysis local property values did not appear to support the additional investment required.