From farm building to Passivhaus

A steel-framed agricultural farm building has been converted into a certified Passivhaus standard 5-bedroom home using a ‘box within a box’ timber frame construction.

 

The building received planning permission through Class Q planning policy, where existing agricultural barns can be converted into residential properties under certain conditions. Planning stipulations meant that the barn’s steel frame and the existing steel insulated roof had to be retained. To achieve the full Passivhaus standard, a timber frame house was built inside the existing building. The timber framed construction was insulated with blown Warmcel insulation and clad with British cedar cladding.

 

Key stats Construction: Timber frame within barn’s existing steel frame Form factor: 3.44 TFA: 262 m2 Completed: September 2021 Certified: Passivhaus, September 2021

 

We were delighted to work with private clients in Somerset to realise their dreams for a new home designed and built to the Passivhaus standard

Geoff Smith, Architect  & Certified Passive House Designer, Shu Architects

 

 

 

Ground floor 

As the existing building did not have a slab and ground conditions required a shallow solution, a new concrete slab was cast as part of an insulated raft. This detail had the advantage of allowing a continuous insulation line around the house.

Roof & walls 

I-joists with blown Warmcel insulation were selected for the walls and roof. To simplify the fixing of the roof the 60mm outer layer of woodfibre was omitted and replaced with a 12mm Panelvent racking board and breather membrane. I-joists were selected over solid or trussed systems due to their smaller volume of material usage.

Floor   Reinforced concrete slab on rigid insulation  U value: 0.139 W/m2K  Wall   I-joist with Warmcel insulation and wood fibre.  U-value: 0.09 W/m2K  Roof   I-joist with Warmcel insulation  U-value: 0.09 W/m2K

 

Form factor & orientation

As the project was working with an existing single-storey building, the orientation and form factor were fixed. The form factor of the building (the total heat loss surface area divided by the treated floor area) was at the high end at 3.44. The design team had to work around this by improving the building’s performance in other ways. The building was designed and built to an improved airtightness standard and achieved 0.1 ach @ 50Pa, substantially better than the required Passivhaus standard for airtightness of 0.6 air changes per hour @50 Pa.

 

Predicted energy performance

Space heating:	13 kWh/m2.yr
Heating load:	7 W/m2
Primary energy demand:	29 kWh/m2.yr
Airtightness:	0.1ach @ 50Pa

 

Heating & renewables

The hot water and space heating are provided via an air source heat pump and the hardware for a future connection to photovoltaics was also installed. Care was taken to reduce the quantity of water in the ‘dead legs’, which was assessed against the AECB’s good practice water standards.

Preventing overheating

Solar shading provided by set-back eaves and verge on the south and west elevations Window size and orientation were carefully considered. As the primary views were to the north, careful consideration of the size and position of the windows was required in order to minimise heat loss. The design was stress-tested using the Passivhaus Trust’s Overheating tool for the Passivhaus planning package (PHPP).

Embodied carbon 

The design team used PHribbon software to help model the embodied carbon in the retrofit passive house. The project managed to achieve the RIBA 2030 climate challenge levels, even without the embodied carbon benefits of retaining the existing barn shell.

 

Key team  Architect & Passivhaus Designer: Shu Architects Contractor: MAKE Structural Engineer: BUILD collective M&E Design: Greengauge Passivhaus certifier: Etude

 

Discover more Passivhaus self-build schemes here, or better still, visit one in-person to get a first-hand experience at an upcoming Open Days site visit. 

 

Further information

Passive House, Shu Architects

Passivhaus goes Personal

Passivhaus Open Days 2022: Winter Edition

Passivhaus Retrofit

Green Building Store: Passivhaus barn conversion, Somerset

26th September 2022

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