Out with the (c)old, in with the new!

Nestled in the picturesque village of Bishoptone, Blackthorn House brings Passivhaus contemporary architecture to a traditional setting.

Taking the place of its EPC F rated predecessor, this new build timber frame Passivhaus with recycled newspaper insulation and powered by heat pumps, solar PV panels and low-carbon electricity, was designed with energy efficient operation and low carbon construction at its heart. Local firm Affinity Architects created a design which reflects the landscape of agricultural buildings huddled together and to open up public views to the countryside beyond, while PHT member Greengauge provided the Passivhaus detailing.

QUOTE | If we were going to build an eco house why not try and achieve the highest efficiency possible - it would have been frustrating to live in a ‘nearly‘ Passivhaus, Alan & Prue Castle

Key stats

Construction: Timber frame.
Type: Single detached
TFA: 228 m²
Form factor: 3.3
Build start date: 2021
Completed: 2022
Certified: Passivhaus Classic, 2024

Blackthorne House | Image credit: Prue Castle

Construction

The layout of the building resulted in a challenging form factor - the ratio of wall and roof area compared to the usable floor area. This meant that a significant amount of insulation would be required to meet the Passivhaus heating target. The solution was to use an insulated timber-frame construction, complemented with in an additional layer of insulation to the face of the frame.

Blackthorne House model | Image credit: Affinity Architects

Constructed on-site from I-joists, the timber frame was finished internally with 100mm wood fibre insulation and externally with airtight OSB board. Finally, the timber cavity was packed with recycled newspaper insulation and the whole wall build up, achieved a total U-value of 0.1 W/m²K – a result more than double the 2010 Building Regulations minimum standard requirements.

QUOTE | The builders - who had never built a Passivhaus before - really got the hang of it. The site foreman embraced the concept and the detailing and was rigorous in making sure that the subcontractors understood it too and didn‘t try and cut corners, Alan & Prue Castle

The insulated raft construction of the floor build up provided a continuous insulated perimeter to the building between floor and wall. This thermal-bridge free design, along with an airtightness test result of 0.3 air changes per hour enabled the home to comfortably achieve the Passivhaus heating target of 15 kWh/m

U-values

Roof: 0.1 W/m²K

Timber I-beams and insulation

Blackthorne House section | Image credit: Affinity Architects

Wall: 0.1 W/m²K

Timber frame with full fill insulation and additional 100mm internally fixed wood fibre insulation.

Building performance

Designed energy performance
Airtightness n₅₀ (≤ 0.6ACH @ 50 Pa)	0.3 @ 50 Pa
Heating Load (≤ 10 W/m²)	9 W/m²
PER Demand (≤ 60 kWh/m².a)	31 kWh/m².a
Overheating %	1%

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

Services

MVHR: Fresh air is provided via a mechanical ventilation heat recovery (MVHR) system.
Solar PV: On site solar panels, 4.3 kWp, directly generate 10kWh/m²of the in-use energy use intensity of 34 kWh/m² . A further 7 kWh/m² solar energy is exported to the grid annually.
ASHP space heating and domestic hot water: Provided via a 7kW heat pump. The measured space heating demand from the ASHP is 11 kWh/m²/year–lower than the predicted 13 kWh/m²/year.

QUOTE | The house is undoubtably lovely to live in. Not only in terms of warmth and comfort but also light and aspect. Each room is evenly warm with small variations between rooms facing North and those facing South. The house is not too hot - we still wear jumpers! It is also super quiet - both within the house and keeping out external noise, Alan & Prue Castle

Challenges & lessons learned

Ventilation: Avoiding over-ventilation in Passivhaus homes is an important design consideration. Minimum ventilation rates are based on the floor area of the dwelling, however for large homes with low occupancy this can often be too high–something which was flagged in this project via PHPP. Greengauge initially specified an MVHR unit which could be retrofitted with an enthalpy heat exchanger–a device that recovers both heat and moisture from the extracted air stream. Ultimately, a regular heat exchanger was installed, with the clients opting in the first instance to try manual means to regulate moisture levels.
Costs: Extra costs were not Passivhaus exclusive! Labour costs for fixing the cladding and other associated architectural features not related to Passivhaus accreditation drove costs up.

Key Team

Architect: Inkbox Architects / Affinity Architects
Passivhaus Designer & M&E Consultant: PHT Member Greengauge
Contractor: Maidment & Carter
Structural engineer: Cullen Timber Design
Passivhaus Certifier: PHT Patron Etude

Congratulations to Alan and Prue on their beautiful new Passivhaus home! For more self build inspiration, why not check out our Passivhaus goes Personal page, or join us in 2025 for our new How to Build a Passivhaus series.