Acorn Farm: from glass house to Passivhaus

Acorn Farm is a landmark project that combines food, community, and climate action. The £6.2 million capital build programme at St Columb’s Park, near Ebrington Square allows for a unique urban growing space within the City. The development will showcase sustainable food systems with the support of Acorn Farms horticultural team. Visitors will be able to learn, grow, and prepare food on site, supported by high-quality training spaces.

Acorn Farm annotated site plan

To match this sustainability ethos, all three new buildings have been designed to the Passivhaus Standard:

Gate Lodge: certified Passivhaus (April 2025), serving as the administrative hub.
Annex Classroom: under construction, providing training and a demonstration kitchen.
Geodesic Dome: under construction, aiming to be the world’s first geodesic dome to achieve Passivhaus certification. 20m exhibition space and flagship visitor attraction, which includes a growing space on the first floor mezzanine.

Acorn Farm. Image © Paul McAlister Architects

This is about more than energy efficiency – it’s about showing our community that sustainable food and sustainable buildings go hand in hand.

Derry City & Strabane District Council

Why Passivhaus?

The Council has committed to achieving a net zero, climate-resilient city and district by 2045. Passivhaus was chosen as the most rigorous and internationally recognised route to deliver buildings that are low-energy, resilient to climate extremes, and healthy for users.

The decision was made proactively by the Council’s Green Infrastructure Team, embedding sustainability as a design driver from day one.

Construction

Across all three buildings, airtightness and thermal-bridge-free detailing are central to achieving certification, and demonstrate a range of construction methods. The Gate Lodge architecture reflects the heritage of the park. The Dome demonstrates innovation in applying Passivhaus principles to non-orthogonal forms.

Gate Lodge: Traditional masonry with slab-on-grade floor, designed in a period style but delivering 21st-century performance.
Annex Classroom: Timber frame construction, enabling lower embodied carbon and precision offsite fabrication.
Geodesic Dome: Masonry base with precast concrete roof elements, requiring bespoke detailing to manage airtightness in complex geometry.


U-Values
	Gate Lodge	Annex Classroom	Geodesic Dome
Wall	0.131	0.126	0.197
Roof	0.077	0.085	0.191
Floor	0.071	0.107	0.201
Windows & doors	0.8	0.8	0.8

Building performance

Performance Criteria	Gate Lodge (actual performance)	Annex Classroom (predicted performance)	Geodesic Dome (predicted performance)
Stage	Complete & Certified	In construction. Design Certified. Aiming for Passivhaus	In construction. Design Certified. Aiming for Passivhaus
Treated Floor Area (m²)	78.2	111.03	87.02
Heating demand (kWh/m²·yr)	15	10.78	13.78
Peak load (W/m²)	11	10.55	14.52
Primary energy demand (kWh/m²·yr)	77	96.06	96.58
Airtightness (n₅₀ ach@50 Pa)	0.5	0.6	0.6

Services

All the buildings are heated via heat pumps and include MVHR.

Acorn Farm Geodesic Dome internal view. Image © Paul McAlister Architects

Lessons Learned

As the Council’s first Passivhaus project, the Acorn Farm development inevitably came with challenges.

Demonstrate leadership: As the Council’s first Passivhaus scheme, this project proves that ambitious standards can be met in public sector buildings.
Embed performance early: Starting with Passivhaus as a design driver was critical.

Culture shift: Traditional construction approaches often prioritise upfront cost over long-term value, so there was a need to shift mindsets and show the wider benefits of investing in energy performance and quality. A whole life carbon assessment was undertaken to provide necessary data and insights.

Upskill teams: Contractors and consultants needed support and training in airtightness and thermal-bridge-free detailing.
Complex forms need innovation: The geodesic dome demanded new approaches to junction detailing.
Measure to learn: Gate Lodge provided valuable measured data to compare with predictions.
Flexibility of the standard: Achieving Passivhaus in masonry, timber frame, and dome construction shows the adaptability of the approach.

Attention to detail has been the biggest lesson – it’s what makes or breaks Passivhaus. The high quality achieved here sets a new benchmark for all our future projects.

Derry City & Strabane District Council

Key team

Client: Derry City & Strabane District Council
Architect: Paul McAlister Architects Ltd (for all buildings)
Certification / Passivhaus Consultant: Mosart (for Annex & Dome; Gate Lodge certification)
M&E / Environmental Consultants:

Williams & Shaw Ltd (Annex, Dome)

Dynamic Design (Gate Lodge)

Whole life carbon assessors: IN2 (Annex, Dome)

Structural Engineers:

Doran Consulting Ltd (Annex, Dome)

Ivan Scott Associates Ltd (Gate Lodge)

Contractors:

McKelvey Construction Ltd (Annex, Dome)

W Doherty Construction Ltd (Gate Lodge)

From plate to Passivhaus, Acorn Farm is proof that ambitious climate action can be practical, people-centred, and inspiring. By uniting sustainable food systems with Passivhaus performance, Derry City & Strabane District Council is setting a new benchmark for public sector projects in the UK and Ireland.

To discover more about this pioneering scheme, join us at the 2025 UK + IRL Passivhaus Conference in Belfast, where the project team will share their journey, challenges, and lessons learned.