Canterbury Passivhaus Plus

Joining a fleet of Passivhaus Plus projects throughout the UK, a once deteriorating 1950’s home has now been transformed into a pair of semi-detached powerhouses. Thanks to the fabric first approach teamed with renewables, the house now generates more energy than it consumes, and stands as an exemplar of energy-efficient construction, holding an impressive Passivhaus Plus certification.

Canterbury Passivhaus Plus, Image credit: Frances Forward

Sandwiched on two sides by a conservation area and with a public sewer intersecting the site, the project began life as a dilapidated detached house set on a sloping residential street. But with great challenges come great opportunities. Certified in July 2020 by PHT member MEAD, the site now houses, not one, but two properties which maximise the incline of the plot.

From street level, the adjoined homes offer a contemporary zinc clad and render exterior over two storeys which seemingly merge into the landscaping to the rear of the site. Only the upper ground level remains visible and accessible from the garden, with the lower ground floor now existing at basement level.

Key Stats

Completion Date: July 2018

Certification Date: July 2020

TFA: 292m²

Form Factor: 2.27

Construction: Lower floor pre-cast concrete, upper floor timber frame

Canterbury Passivhaus Plus, Image credit: Francis Forward

The in-use living experience is very positive in both halves of the semi-detached pair. As the compact unit takes care of ventilation, hot water and heating automatically, comfort levels are high and electricity bills are much lower than in conventional homes. An internet enabled monitor of each compact unit has been installed, which allows for daily interrogation of energy use and optimisation of controls. It is noticeable that household appliances such as a computer or a hair dryer use a disproportionate amount of energy compared with running the house itself.

Frances Forward, Project Architect & Passivhaus Consultant

Taking advantage of the south-facing orientation, the new houses each feature their own diagonal 15° mono pitch roof. Descending to the rear of each property, the roofs are positioned to accommodate an array of solar PV and thermal panels, enabling the homes to generate additional energy to satisfy Passivhaus Plus criteria. All windows and rooflights are triple-glazed with a g-value of 0.50, and with the total glazed area as a percentage of the TFA being 13.3%.

Canterbury Passivhaus Plus, Image credit: Francis Forward

Construction

Canterbury Passivhaus Plus Construction

As a key component of the Passivhaus design and with challenging site constraints to consider, a methodical system was required in the construction of the building envelope to ensure a high-quality build and assembly. It was determined that prefabricating the structure would allow for increased accuracy, quality, and preservation of the materials before assembling later onsite. This also meant that once all components were in place, the airtightness strategy would be executed thoroughly, ensuring no thermal bridges and air leakages.

Wall

Basement: pre-cast, pre-insulated reinforced concrete

Superstructure: laminated timber stud frame, pre-insulated and pre-glazed

U-value: 0.116 W/(m²K)

Floor

Basement/ Floor slab: Insulated in-situ concrete raft

U-value: 0.088 W/(m²K)

Roof

Pre-insulated laminated timber stud frame

U-value: 0.086 W/(m²K)

Canterbury Passivhaus Plus, Image credit: Francis Forward

Bounded on two sides by a Conservation Area and with a public sewer running across it, the site was challenging in terms of both Planning and engineering, requiring a lot of preparatory work to get the required permissions. However, these constraints helped to shape the form of the Passivhaus, both in terms of the roof shape (hiding the PVs and solar thermal panels from the street view) and in terms of dropping the lower ground floor to the same level as the sewer (to be allowed to build close enough to utilise the site).

Frances Forward, Project Architect & Passivhaus Consultant

The lower ground floor was constructed from pre-cast, pre-insulated reinforced concrete and rendered on-site, with the upper ground floor, mezzanine and roof constructed of laminated timber stud frame, pre-insulated and pre-glazed. Chosen for ease of maintenance and as a nod to the surrounding architecture of the area, shiplap cladding wraps around the upper floor façade, and is topped with a zinc panelled roof.

Canterbury Passivhaus Plus, Image credit: Francis Forward

The MVHR (Mechanical Ventilation with Heat Recovery) system circulates ventilated fresh air throughout the day and night, providing comfortable and even temperatures in winter and efficient cooling in summer. Featuring a compact unit with a mini-air source heat pump, heating is controlled primarily by an external weather sensor, with thermostats present in each room.

Shading is provided in the form of electronically operated external louvre blinds which can be programmed to prevent direct sunlight at various times of the year. Although it was initially a concern that the two large south facing windows could be an overheating risk in summer, the external blinds successfully prevent this, making internal temperatures comfortable in all seasons. All rooflights have electronically operated roller shutters also.

Monitored performance

Occupied since July 2018, Canterbury Passivhaus Plus has been praised by the residents, in particular, for the high levels of comfort and a significant reduction in energy bills. The houses have been monitored remotely every day since occupation, in order to understand the operational energy use and to further optimise the automatic controls of the compact unit. According to monitoring data, the summer internal temperature averages at 23 degrees, and the winter temparture averages at 20 degrees.

Average internal Winter temperature:

20 C

Average internal Summer temperature:

25 C

Relative humidity MVHR extract air:

26% - 30%

Predicted PER demand from PHPP:

38kWh/m2

Canterbury Passivhaus Plus pushes the boundaries in terms of energy-efficiency and comfort. Despite initial challenges, it is worth going the extra mile to create a comfortable, healthy and high-quality Passivhaus home which will continue to perform optimally for years to come.