Project Overview

Zero carbon house was a retrofit to an existing home, which aimed to be zero carbon in operation and create inspirational architecture.


Zero carbon house has been designed so that there is no need to use fossil fuels, with no fuel bills either. It was completed in 2009 to Code for Sustainable Homes Level 6.

The home is highly energy efficient and only uses on-site renewable energy. Low energy demand has been achieved through high quality insultation, with U-values of 0.1 W/m2.K to floors and walls, 0.08 W/m2K in the roof; air tightness of 0.57 m3/hm2 in both the new build and retrofit elements; a combination of mechanical ventilation heat recovery (MVHR) and natural ventilation; and excellent natural lighting and low energy artificial lighting.

Energy used on the property is sourced from on-site solar photovoltaic (PV) panels, and solar thermal hot water heating.


Zero carbon house used ultra-low embodied carbon measures and materials to minimise the carbon contribution from construction and throughout the building lifecycle. This included load-bearing unfired clay block walls, clay screeds, lime plaster, reclaimed timber and bricks, and cellulose insulation.

The home minimised water resource requirements by using a 2,500L rainwater harvesting tank and low water usage.


Biodiversity was enhanced with six new wildlife habitats, including six bird-boxes, two bat-boxes, hedgehogs, and lacewings in the garden.


The project aimed to inspire others through regular open days which have attracted numerous visitors. Engagement with University research projects, students and community groups continues. John Christophers (architect) advises on net zero carbon policies and design with local authorities and housing providers.

Key Sustainability Objectives And Outcomes

  • Code for Sustainable Homes rating: Certified Level 6 (CfSH score 95)
  • Heat demand*: 3kWh/sqm.yr (cf Passivhaus 15 kWh/sqm.yr)
  • Primary energy demand*: 41 kWh/sqm.yr (cg Passivhaus 120 kWh/sqm.yr) Useful solar thermal energy, hot water*: 3,653 kWh/yr
  • On-site PV electricity generation*: 3,966 kWh/yr Operational carbon total*: -122kgCO2/yr
  • Health & well-being, daylight factor: <5%
  • *Embodied carbon and whole-life carbon: University research paper figures to be published June 2021.

*Monitored data, 2011-2016 and ongoing.

Approaches Used

Zero carbon retrofit, 50% new-build, 50% retrofit.

Three-story load-bearing unfired clay block structure with lime mortar, giving high thermal mass to avoid overheating, while avoiding carbon emissions from cementitious material.

RIBA Architecture Award, RIBA Manser Medal (House of the year) shortlist, Civic Trust Commendation, inaugural Retrofit Award, BRE Award.

Lessons Learnt

  • The innovative nature of the work was facilitated through an open-book negotiated contract.
  • Earth-based clay screeds and block walls needed careful site handling, but worked well on site.
  • Designing in the required airtightness and thermal breaks to the existing building required detail forward planning.
  • At the design stage, future-proofing for a gas boiler and un-heated “sun-space” were included. As building performance has exceeded expectations, these were later considered unnecessary.