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Fulford Passivhaus

February ’16

The following text and images were provided by the architect, Phil Bixby of constructive individuals.

house

Fulford Passivhaus+ was built for, and with, clients Karin deVries and Rob Aitken, culmination of a lengthy process of design which began with proposals to extend their existing home and finished with a completed new-build project which eliminated the compromise (and VAT) of the extension proposals.

foundations

expanded polystyrene insulation being laid on a sand blinding

Demolition of the existing poor-quality structure left a spacious site with a southerly outlook. The design approach was deliberately simple – a three-storey house of simple rectangular plan, with the upper storey largely within the pitched roof. The roof itself is offset to increase the south-facing area, which hosts a 9.8kW PV array. This contributed towards the house being certified as Passivhaus Plus – the first in the UK.

roof construction

roof structure with Smart Ply OSB 3 sheets

qwe

cavity wall with internal timber frame leaf

One of the largest challenges with the project was building to Passivhaus standards – and incorporating a substantial renewables system – while keeping as far as possible within “conventional” build costs. This led to a lengthy refinement of the construction system, working in partnership with contractor Transcore. Original proposals for a deep-section timber frame building envelope were replaced by a cheaper – but more complex – wall construction comprising cavity construction of timber frame inner leaf, rendered blockwork outer leaf and various layers of insulation. Likewise the roof construction ended up with more layers than originally anticipated due to cost constraints limiting joist depth. The construction method had knock-on effects on the substructure; originally a fairly simple-looking raft with a perimeter strip was envisaged, but the use of a very conventional timber frame meant that point loads required additional pad supports, and as a result the foundations became more complex.

window detail

meticulous taping around membranes and window / door openings helps achieve the high level of air tightness

The final main impact of the cost constraints was that the contract with Transcore was limited to the weathertight shell, and completion of the house (joinery and decoration) and service installations were undertaken on a direct labour basis by the clients. The design had always incorporated service voids inboard of the structure and airtightness layer, but there were still some issues around separation of the “contract” and “post-contract” works.

The main structure went together without major problems. The contractor worked with the client – who was very hands-on on site – to ensure cold-bridging was minimised using the multi-layered approach to insulation. Likewise airtightness was a constant focus, and despite one or two issues with three-dimensional junctions an impressive final airtightness value of 0.41/hr was achieved, helped by input from airtightness guru Paul Jennings who carried out two additional intermediate tests.

brise-soleil

the timber brise-soleil prevents overheating when the sun is high in summer but allows solar gain in winter

The house was completed and occupied in the summer of 2015. As architect I was highly chuffed to see the sun striking the windows exactly where the 3D sunpath modelling had predicted that the overhanging roof and brise-soleil would put it. The clients are still in the process of installing heating – there was none at all until the 1kW post-heater in the MVHR system was wired up in December, and it is planned to augment this with heated towel rails in the bathrooms. Monitoring is being set up to check the performance of the PV’s, DHW cylinder and immersion heaters.

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