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How LEAP Architects created the UK’s most airtight home

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Continuing our series on ways to keep homes warm without killing the planet, Mark Siddall of LEAP Architects talks about his practice’s all-timber Larch Corner house – the third most airtight home in the world

At the northern edge of the Cotswolds is the sleepy village of Upper Quinton – not the kind of place you’d expect to find a house that redefines the rules. A celebration of modern timber design and engineering techniques, Larch Corner is a timber lover’s dream. Almost all of this modern, three-bedroom, single-storey home is built of sustainably sourced timber, reducing emissions during processing and manufacture, and carbon emissions in use.

It is also the UK’s most airtight home. Prior to Larch Corner’s blower door test, the World Record Academy stated that the world’s most airtight home achieved 0.05 ACH@50 Pascals. Larch Corner improves on this. Having checked the Passivhaus database at the time of testing, in 2019, Larch Corner was apparently the third most airtight house in the world, with the other two dwellings both in Austria. Its database ranking has not changed since then.

  • Larch Corner temporary shading in use during a summer heat wave.
    Larch Corner temporary shading in use during a summer heat wave. Credit: Mark Siddall
  • The sawtoothed profile of the front elevation responds the corner plot and the curve of the road.
    The sawtoothed profile of the front elevation responds the corner plot and the curve of the road. Credit: Mark Siddall
  • The sawtoothed profile of the front elevation responds the corner plot and the curve of the road.
    The sawtoothed profile of the front elevation responds the corner plot and the curve of the road. Credit: Mark Siddall
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How we did it
The structure is built from 35 internally exposed cross-laminated timber (CLT) panels, held together by 21,000 screws. With its Air Permeability of 0.041 m3/hr/m2@50Pa, Larch Corner is 195 times more airtight than required by the Building Regulations. To put this in perspective, if you gathered all its leaks together, the equivalent leakage area (ELA) is just 196mm2 – the size of a 1p coin.

The home not only has Passivhaus accreditation but also zero carbon status, assisted by a 9.3kWp photovoltaic array. In fact, according to the EPC, its emissions are actually negative -2.2 tCO2e/yr. This ‘excess’ energy is used for charging a fully electric car.

Monitoring between 2019 and 2020 identified that while the photovoltaic array displaced 1,817kgCO2e of emissions, due to grid connection, total imported electricity (1,578.4kWh) resulted in 692kgCO2e of emissions. A little over 75 per cent of that total occurred in the period between 1 November and 31 March when photovoltaics do not cover the house’s energy demand – the ‘winter gap’. Emissions rose in part due to seasonal variation between supply/generation and demand. 

Such analysis identifies an important performance gap and shows how poor calculation procedures within the SAP model can mask reality and so support false projections that net-zero carbon can be achieved while the National Grid has not achieved zero emissions status.

  • The insulated CLT structure wrapped in the windtight breather membrane.
    The insulated CLT structure wrapped in the windtight breather membrane. Credit: Mark Siddall
  • Paul Jennings of airtightness consultant Aldas, undertaking the blower door test.
    Paul Jennings of airtightness consultant Aldas, undertaking the blower door test. Credit: Mark Siddall
  • The dining table is made from offcuts of cross-laminated timber.
    The dining table is made from offcuts of cross-laminated timber. Credit: Mark Siddall
  • The open-plan living area includes the kitchen, dining and living room.
    The open-plan living area includes the kitchen, dining and living room. Credit: Mark Siddall
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Analysing the performance in operation
The house’s temperature exceeded 25°C for 5.5 per cent of the year, bearing strong correlation with predicted exceedance of 5 per cent made by the Passivhaus Planning Package. And using relative humidity as a proxy for indoor air quality, the house remained within the Good to Acceptable range for over 95 per cent of the year.

Factoring in the value of exporting electricity to the grid and renewable heat incentive payments, in 2021 the house’s owner, Mick Woolley, made almost £200 per year profit on his energy bills. Even when the price of electricity and his standing charge double, his annual bill is likely to only be about £230.

At design stage, predicted EUI (energy use intensity) was 21kWh/m2.a GIA. After a year’s monitoring it was confirmed the house used 22kWh/m2.a GIA of operational energy – a 40 per cent improvement on RIBA 2030 Challenge Operational Energy EUI target (35 kWh/m2.a GIA). Regarding whole-life carbon, the house achieved a 29 per cent improvement on the RIBA 2030 Challenge target (444 kg CO2e/m2 GIA vs. 625 kg CO2e/m2 GIA).

Lessons learned from the project, are being shared with others at the International Passivhaus Open Days and at PassivhausSecrets.co.uk

Mark Siddall is principal architect/director of research, LEAP Architects, Durham

 

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