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Larch Corner Passivhaus

A Passivhaus certified 3-bedroom single storey home been designed by Mark Siddall from LEAP Architecture and constructed by Mac Eye Projects.  The Cotswolds-based project has made great efforts to minimise carbon emissions on the project both by building the project to the Passivhaus standard but also through its use of sustainably sourced timber throughout. It is also currently the UK’s most airtight home.

21° supplied the ULTRA triple glazed timber windows and doors at the project, as well as designing, supplying and commissioning the MVHR system at the project.  Pro Clima airtightness products and Compacfoam insulation from 21° were also specified for the project.  


Larch Corner was designed to be the client’s Forever Home and was considers Lifetime Homes standards providing an accessible ground floor and incorporating adaptive opportunities for prams, walking frames, wheelchair users.

The home is built from Cross Laminated Timber (CLT) which is exposed internally. Larch cladding is used externally and a sedum roof has been planted.

As you move through the house the ceiling level changes to define various zones. The taller defines the open plan family room, the lower one, the rest of the house.


At Larch Corner timber products reduced environmental impact and affect lifecycle carbon emissions, including embodied carbon in materials, construction and maintenance as follows:

  • By limiting embodied energy/ carbon. 100 year global warming potential from manufacturing and construction is estimated at 0.51/0.62 kgCO2e.
  • By reducing heat loss related carbon emissions
  • Almost every element of the house comes from sustainably sourced PEFC and FSC timber. The CLT used on the project has Natureplus certification.

Energy use

The house has been designed, constructed and certified to achieve the Passivhaus Standard. It  eliminates cold surfaces and uncomfortable drafts to provide a high standard of thermal comfort during heat waves and the depth of a frosty winter. By attaining the Passivhaus standard the space heating demand and related heating bills are 90% less than the average UK home.

The walls are insulated with 440mm wood fibre insulation (U value: 0.092 W/m2K) and the roof is insulated with 366mm of wood fibre insulation (U value: 0.12 W/m2K).The floor has a U value of 0.07   W/m2K.

The airtightness/ air permeability  for the project came in at an exceptional 0.041 m3/hr/m2@50Pa. To place this in perspective, if you gather all leaks together, it is estimated that the Equivalent Leakage Area (ELA) of a Passivhaus is equivalent to having a letter box size hole in a house. At Larch Corner the ELA is just 196mm2 – an area that fits on a 1p coin. This, the UK’s most airtight home, is 244 times more airtight than required by Building Regulation.

A combination of good design, good detailing and meticulous care during construction contributed to the airtightness result. During construction the role of airtightness and thermal integrity champion was developed to help the contractor fulfil the ambitions of the project.

Space heating and hot water is provided by an air source heat pump. As a certified Passivhaus integrating 9.3 kWp photovoltaic array, the house achieves Zero Carbon status because, according to the EPC, emissions are -2.2 tCO2e/yr. The ‘excess’ energy is used for changing a fully electric car.

The AECB Water Efficiency Standard was adopted for the project, to help minimise the amount of energy used to provide hot water.  Measures included: minimising the volume of ‘dead legs’  and a super-insulated storage cylinder.

Triple glazed timber windows

21°s ULTRA triple glazed timber  windows were used for the project achieving a whole window  U value of 0.74 W/m2K. The project specified the ULTRA  tilt and turn windows, as well as single and double doors in RAL 7024.

The timber frame of the ULTRA windows was integrated into the wood fibre wall insulation to minimise thermal bridging, helping to raise the surface temperature above 13C and prevents condensation even if the external temperature drops to -3C.

I’m always impressed with the quality of 21°’s products, and the ULTRA windows and doors at Larch Corner are no exception. Without products of this outstanding calibre there is no way we would have achieved extraordinary standards of airtightness.”

Mark Siddall


Superior air quality is achieved by supplying fresh air and maintaining a suitable relative humidity by using mechanical ventilation with heat recovery (MVHR).  The MVHR also reduces ventilation heat losses by 92% and the whole house heat losses by 50%.

Alan Clarke was the mechanical engineer on the project and worked with 21° to deliver the MVHR system. 21° designed, supplied and commissioned the MVHR system on the project. The project used a Paul Novus 300 MVHR  unit, with a 200mm Maico grille on the exterior wall. 

When it comes to MVHR systems for Passivhaus buildings, 21° really understand the nuances and offer great support. I believe it’s their depth of understanding that makes working with them so incredibly effortless.

Mark Siddall

Project details

Architect/ Passivhaus Designer

Mark Siddall, LEAP,

Mechanical Engineer

Alan Clarke

Building Contractor

Andy Mackay, Mac Eye Projects


Larch Corner was the Winner in the Small Projects category of the UK Passivhaus Awards 2021. 

Larch Corner was one of 6 projects shortlisted for the ASBP Awards 2020. 

Life changing homes