Tin Barn EnerPHit - From derelict farm buildings to exemplar retrofit

A Gloucestershire barn conversion project has involved the radical transformation of a set of Grade II listed derelict agricultural barns to achieve EnerPHit (Passivhaus retrofit) certification.

Situated within a conservation area the site was purchased with the intention of creating a Passivhaus retrofit and the clients explored several options before settling on the site. One of the first tasks was carrying out an energy model on the derelict barn buildings to see whether the existing planning permission on the site would enable a EnerPHit design in terms of its form factor, orientation and overall design. Following the successful energy analysis, the design began in earnest.

PHT members involved in the project team included Arbor Architects as Architect, Delta Q as Passivhaus Consultant. Build Collective as Structural Engineer, and MEAD Consulting as Passivhaus Certifier.

Key stats

Construction: Timber frame
Type: Several barns
TFA: 118 m²
Form factor: 3.86
Build start date: 2021
Completed: 2022
Certified: EnerPHit, 2023

Tin Barn EnerPHit. Image credit: Arbor Architects & Photographer Ellen Christina Hancock

Due to the heritage status of the site, planning constraints were introduced, which constrained both the overall form of the building and required retention of some of the original structural elements. The project demonstrates not only the importance of a well-thought-out and conceived building at the earliest possible stage, but the even greater importance of effective collaboration when there are external influences that necessitate change.

Dave Bryan, Passivhaus Consultant, Delta Q

Construction

The project utilised a timber frame construction 'box within a box' design within the existing barn stone walls. This helped minimise thermal bridging and optimise airtightness for the retrofit. A vaulted roof was recreated on the original Dutch barn to create an impressive first floor space. The rural location of the barns has influenced its aesthetic with agricultural materials being used, such as corrugated metal roofing, sandstone walls and larch cladding.

We were fortunate enough to have a main contractor in CRD Bath who, despite this being their first Passivhaus project, embraced the site training we provided and, in particular, rose to the challenge of constructing some of the more complex airtightness detailing.

Piers Sadler, Director, Delta Q

Embodied carbon

The Tin Barn project involved the sensitive adaptive reuse of agricultural heritage buildings, contributing to the project's low embodied carbon. Extensive use of timber sourced from sustainably managed forests helped minimise the project's embodied carbon footprint. The embodied carbon for the building was measured at 347 kg CO2e/m2 GIA (almost half of the RIBA 2030 target of 600 kg CO2e/m2 GIA).

U-values

Roof: 0.110 W/m²K

I-joist timber frame, fully filled with recycled cellulose. Metal roof.

Wall: 0.195 - 0.110 W/m²K

I-joist timber frame, fully filled with recycled cellulose. Timber cladding/ stone walls.

Floor: 0.127 W/m²K

Insulated raft slab

Building performance

Airtightness n₅₀ (≤ 1.0 ACH @ 50 Pa)	0.35 ACH @ 50 Pa
Space Heating Demand (≤ 25 kWh/m².a)	23 kWh/m².a
Heating Load	11.74 W/m²
Primary Energy Renewable (PER) Demand (≤ 60 kWh/m².a*)	51 kWh/m².a
Overheating %	2%

*+/-15 kWh/m².a allowance if offset by energy generation. See Passivhaus criteria.

Services

The project uses an air source heat pump (ASHP) for heating and hot water and a mechanical ventilation with heat recovery (MVHR) system for ventilation. The summer comfort strategy involves the use of external blinds and timber shutters for shading.

Challenges & lessons learned

Form factor: Planning constraints, due to the heritage status of the site, constrained the overall form of the building and required retention of some of the original structural elements. This effectively removed the ability to optimise the efficiency and form factor to assist with Passivhaus compliance, resulting in a form factor of 3.86. Achieving Passivhaus Classic was virtually impossible for the project and in agreement with the Certifier, the EnerPHit Standard was pursued.
Early stage design: During the design phases of the project careful consideration had to be given to any material change on the envelope areas and treated floor area (TFA), as changes were amplified because of the high form factor. This extended to any changes, however minor, needed on site to accommodate the existing structures being retained.
On site training: The Passivhaus Consultants provided on site training for the building team, as it was the first Passivhaus project they had undertaken.

Architect’s view

The energy efficiency goal was to create a high-performing home that minimises environmental impact, meeting the stringent requirements of Passivhaus certification. The initial state of the buildings posed a significant challenge, with the barns in a severe state of disrepair, one reduced to a ruin with only rudimentary wall structures standing.

Matt Hayes, Director, Arbor Architects

Tin Barn EnerPHit. Image credit: Arbor Architects & Photographer Ellen Christina Hancock