Whole house Passivhaus retrofit of a 1950s semi-detached Cambridge home
Aesthetics and energy performance go hand in hand at the Corbel House EnerPHit project, delivered by a project team led by Rock Architects. The whole house refurbishment of a 1950s semi-detached house in Cambridge has recently culminated in the project achieving EnerPHit certification.
The project had high sustainability aspirations from the outset, but when Greengauge carried out initial energy modelling showing how close the scheme was to achieving EnerPHit, the decision was made to develop the trickier details and push for formal certification. External wall insulation was specified for the insulation strategy, with the aesthetics of the project referencing the traditional residential architecture of Cambridge as well as adding a more contemporary style. An insulated timber frame extension was added to the home’s existing garage.
Greengauge was part of the project team supporting the retrofit in the role of Passivhaus Consultant, as well as undertaking M&E Design for the project.
Strategy
Prior to the retrofit, the property suffered from poor insulation, damp and ventilation issues, which led to the decision to undertake a whole house retrofit. The retrofit involved a combination of both external wall insulation and internal wall insulation along the party wall and the addition of a new timber frame extension. Existing cavity wall insulation was topped up where possible. Corbel House was quite a natural fit for an EnerPHit project, as it was able to employ an external wall insulation strategy, which made detailing easier, and had suitable structure and heights to run MVHR ductwork without compromising floor to ceiling heights.
Project team
Architect: Rock Architects
Contractor: Arnold & Willis Construction
Passivhaus Consultant: Greengauge
M&E Design: Greengauge
Moisture Risk: Greengauge
Structural Engineer: Gawn Associates
Passivhaus certifier: Etude
Key data
“Corbel House has been a fantastic project to work on, and it has been great to help the clients achieve their goal of EnerPHit certification. A combination of good building physics and building services design has helped the project get over the line.”
James Clifford, Passivhaus Consultant, Greengauge
Key features
EWI strategy
External wall insulation was the predominant insulation strategy employed at the Corbel House EnerPHit. The existing home was cavity wall and planning requirements were such that external wall insulation (EWI) strategies were permissible. An EPS & XPS insulation was used for the EWI and was taken down below the foundations to minimise a potential thermal bridging at the wall to floor junction. Some internal wall insulation was needed to the party wall at ground floor, which them became a timber framed newbuild extension at first floor level. Careful detailing of the junctions between EWI and IWI was important. The building had previously been insulated with cavity wall insulation blown into the cavity; upon inspection it was deemed the insulation could be topped up to improve performance.
Airtightness
The airtightness strategy was very carefully designed and delivered on site, resulting in a final airtightness test result of 0.8 ach @ 50Pa. An AeroBarrier system was used to help achieve the EnerPHit airtightness requirements. AeroBarrier works by creating positive pressure inside the space, forcing air to flow from areas of higher pressure to areas of lower pressure through leaks. This airflow draws a nontoxic atomized fog of sealant into gaps, where it bonds to seal them.
Ground floor
The ground floor was completely dug up and replaced with 450mm of recycled Foamglas aggregate.
Extension
A 370 mm timber frame extension, insulated with wood fibre insulation, was added on top of the garage. The front and back garage walls had already been insulated with cavity wall insulation. This mean that continuity of insulation and thermal bridging between the two interfaces was not as challenging as it could have been.
Semi-detached
To minimise any thermal bridges associated with the neighbouring house, some EWI was added across the neighbour’s adjacent wall. However, a semi-ventilated gap between the two homes meant that this was not a huge issue.
Sliding door specification
The clients were keen on having an uncertified large glazed sliding door within the retrofit, but Greengauge was concerned that this would be detrimental to the airtightness of the project and would also require extra modelling to determine the thermal performance of the doors. Sourcing a door with good airtightness that would ensure EnerPHit certification was challenging, but the project team managed to find a solution.
Ventilation
A mechanical ventilation with heat recovery (MVHR) system was specified to deliver good internal air quality and to minimise ventilation heat loss. An essential requirement to meet EnerPHit certification requirements, the MVHR system was carefully concealed in the home. with semi-rigid ductwork concealed in the suspended timber floors and new stud walls. Initially a rigid ductwork scheme was considered with main ductwork running across the attics and dropping down, but it was discovered that the joists were running in the right directions and at sufficient spacing to accommodate a more straightforward and cost-effective semi-rigid ductwork strategy.
Waste water heat recovery
Greengauge designed a waste water heat recovery (WWHR) system for the project’s main shower to reduce energy consumption. The outgoing warm water from the master bathroom shower heats up the cold water coming into the shower. WWHR systems can sometimes be challenging space-wise, but for the Corbel House project, the shower room was directly above a workshop and plant-space, so this was relatively straightforward to accommodate.
Fossil free
The clients were keen to move away from fossil fuels, so the Greengauge team specified an air source heat pump to deliver both hot water and heating. Underfloor heating and low temperature radiators were connected with the ASHP. A separate electric cylinder was chosen for occasional use for the guest en-suite bathroom.
Minimising heat pump noise
Greengauge helped locate the heat pump to avoid any noise coming in through bedroom windows and neighbour’s windows . A solution was agreed for a new screened ASHP enclosure, with a bin store, to be located on the noisier front side of the house.
Renewables
A solar PV array on the roof was combined with battery storage to provide some electricity for the house from renewable sources and allow the house to make use of cheaper and lower carbon off peak electricity from the grid overnight.
M&E Design & EnerPHit
The mechanical services strategy contributed to the Corbel House project meeting the EnerPHit standard. The heat pump and solar PV specifications contributed to the project meeting EnerPHit’s requirements for PER carbon emissions targets and reducing the heat load requirements.
