Passivhaus Barn Conversion

An agricultural farm building has been converted into a Passivhaus standard certified home by Bristol-based architects Shu Architects and building contractors MAKE Group. The barn conversion involved the creation of a ‘box within a box’ timber frame construction inside the existing building.

21°s ULTRA (GBS 98) triple glazed timber windows, entrance doors and lift & slide doors were specified for the project. 21° also supplied some Pro Clima airtightness products and Compacfoam for the project.

Design

Class Q

The house was accepted in planning as Class Q planning policy prior approval, whereby until October 2021 existing agricultural barns could be converted into residential properties. This meant that it was possible to take advantage of the barn’s original and substantial 300m2 footprint.

The box within the box

Shu Architects suggested building a new timber-framed house designed to Passivhaus standards within the existing barn, retaining the barn’s steel frame and the existing steel insulated roof. As the existing building did not have a slab, a new concrete slab was installed, although the thickness of this was reduced by placing the insulation underneath the slab and omitting the screed. This detail made it easier to have insulation in a continuous line around the house. Planning consent prevented the existing roof being removed so the design team had to consider how the new timber roof would be constructed inside the existing roof. 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. The house was clad externally with vertical British cedar cladding.

Form factor & orientation

The quantity of insulation required for a Passivhaus is dependent upon the form factor of the building (the surface area of insulation divided by the treated floor area) and building’s orientation. As this was an existing single storey building, the orientation and form factor were already largely set. Shu Architects counteracted this by designing and building to an improved airtightness standard and achieved 0.135 ach @ 50Pa, which was 4 times better than the required Passivhaus standard for airtightness of 0.6 air changes per hour @50 Pa.

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

The size and orientation of all the windows was carefully considered as part of the design. The design incorporated verandas to the south and the west to minimise unwanted solar gain. The primary views were to the north so careful consideration of the size and position of the windows on this façade 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.

Energy performance (as modelled in PHPP)

• Space heating: 13kWh/m2/year
• Heating load: 7W/m2
• Airtightness: 0.135 ach @ 50Pa

Triple glazed windows & doors

21°’s ULTRA (GBS 98) triple glazed timber windows, entrance doors and lift & slide doors were specified for the project. The windows were ULTRA inward opening tilt and turn windows. The two lift and slide doors had triangular fanlights and the entrance door was boarded in style GR01. The paint finishes for the windows and doors were RAL 9016 white inside and RAL 7016 anthracite grey outside.

21° also offered free technical support for the project by calculating Psi values for each window and doors type.

21°’s fitting team  installed the windows and doors, using Pro Clima Contega Solido SL-D airtightness tapes to seal the junction.