Harvesting Energy: A Passivhaus Plus Self-Build Success

This innovative and low-cost self-build project in Kent, predominantly constructed by the owners using the PH15 system, proudly boasts Passivhaus Plus certification. Starting as a Class Q agricultural building conversion, Oakfield Farm's design maximizes solar photovoltaic electricity generation through its north-south orientation and dual-pitched roof.

Driven by a passion for ultra-low energy demand, the clients chose Passivhaus Plus certification. This required adjustments to the original Class Q barn planning approval so that the fabric achieved Passivhaus performance levels. The result? A compact, simple volume project that mirrors the aesthetic of the original barn, ensuring high airtightness and low energy demand despite a form factor of over 3.

The owners took on the challenge of building the home themselves, constructing the above-ground thermal shell with just one carpenter’s assistance. Their dedication paid off with an exceptional airtightness result of < 0.1 ach@50Pa.

Key stats

Construction: Timber frame (PH15 system)
TFA: 147 m²
Form factor: 3.1
Costs: £2300/m²including self-build labour
Build start date: 2020
Completed: 2022
Certified: Passivhaus Plus, 2023

Oakfield Farm Passivhaus Plus. Image credit: Passivhaus Homes

We were inexperienced, middle-aged, first-time self-builders on a limited budget, working alongside an experienced carpenter who had no prior knowledge of Passivhaus. We were very pleased to achieve a high level of airtightness of 0.07 ach@50Pa & the final build cost of £338K, well within our budget estimate of £390K

Construction

The house was built on the pre-existing concrete portal of the former agricultural building. The project used the timber frame PH15 build system, this can be erected without heavy machinery, suiting the self-build nature of the project. The PH15 build system required good early decision-making as the frame is pre-cut off-site, which helped prevent potential delays and cost overruns due to changes in design mid-build.

U-values

Floor: 0.13 W/m²K

Rigid phenolic insulation and recycled glass block.

Oakfield Farm Passivhaus Plus under construction. Image credit: Passivhaus Homes

Wall: 0.11 W/m²K

Timber frame PH15 I-joist

Roof: 0.10 W/m²K

Timber frame PH15 I-joist

Further information can be found at the International Passivhaus Database listing

Embodied carbon

The timber frame PH15 system with timber-based insulation relies on low embodied carbon materials. Passivhaus Homes has modelled the PH15 system in PHRibbon and states that timber frame with timber-based insulations can achieve more than a 65% reduction in upfront carbon emissions. Every PH15 shell is also carbon offset (stages A1-A4) through the Woodland Trust to protect UK ancient woodland and expand woodland habitat.

Building performance

Designed energy performance

Airtightness n₅₀ (≤ 0.6ACH @ 50 Pa)

0.1 @ 50 Pa

Space Heating Demand (≤ 15 kWh/m².a)

13 kWh/m².a

Heating Load (≤ 10 W/m²)

3 W/m²

Primary Energy Renewable (PER) Demand (≤ 60 kWh/m².a*)

52 kWh/m².a

Primary Energy Renewable Generation

81 kWh/m².a

Overheating %

< 2%

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

Actual energy performance

The house is closely monitored by the clients using specialist software. The average internal temperature is 19.8°C, and CO₂ levels range from 400-800 ppm. The total energy demand was about 26% higher than predicted in PHPP, due to extensive electric equipment, monitoring, and refrigeration for storing home-grown food. However, the solar PV array generated 18% more energy annually than predicted.

The 13.8 kWp solar array, paired with a 13.5 kWh battery, boosts on-site renewable electricity use. Overall, the home generates about £1200 more per year than it costs to run, resulting in an income of around £100 per month.

Services

MVHR in roof eaves Oakfield Farm Passivhaus Plus. Image credit: Passivhaus Homes

The clients opted against a complex heating system like an air source heat pump. Instead, the house uses all-electric energy, featuring a roof PV solar array, a hot water tank, and two 600W electric towel radiators. Space heating is done at night using a cheaper tariff, with the building's fabric retaining heat effectively. Despite keeping a bedroom window open year-round, space heat demand remains lower than PHPP predictions. On the coldest days, two small portable radiators are occasionally used, typically for no more than two weeks annually.

For shading, external electric blinds on all windows automatically open and close based on solar azimuth. This robust strategy was developed to address client concerns about managing future heatwaves.

Oakfield Farm Passivhaus Plus Agriculture. Image credit: Passivhaus homes

Challenges & Lessons learned

Combining Class Q stipulations for agricultural building conversion with Passivhaus performance standards was challenging. The team had to adapt the existing barn shape to achieve a reasonable form factor for optimal energy performance.

The all-electric M&E strategy employed was successful, requiring minimal space heating. This approach could benefit other projects. However, onsite solar export faced a limit, reduced from 13.8 kW to 5 kW. To mitigate this, a Tesla 13.5 kWh battery was installed, increasing onsite solar usage. Future plans include adding battery capacity to further reduce energy costs and grid reliance, and installing air-conditioning to combat overheating during predicted future heatwaves.

Key team

Architect & Passivhaus Designer: Passivhaus Homes
Contractor: Self build
Structural engineer: Passivhaus Homes
M&E consultant: Passivhaus Homes
Passivhaus Certifier: Zero Energy Passivhaus

Oakfield Farm Passivhaus Plus at night. Image credit Passivhaus Homes

Congratulations to the whole team for this innovative and brilliant project. Do not miss the UK Passivhaus Conference 2024 this September, to hear more in-depth information about Oakfield Farm from Passivhaus Homes!