Passivhaus goes large

Agar Grove is set to become the UK’s largest Passivhaus scheme, with its housing tenants already seeing the benefits from the first completed phase. James Woodward, architect at Hawkins\Brown, tells Roseanne Field why the practice decided to pursue the ‘gold standard’ of energy efficiency for large-scale affordable housing

Agar Grove, earmarked for refurbishment as part of Camden Council’s Community Investment Programme, was the biggest project the council would undertake within the overall scheme. Split into six phases, the project will provide 507 homes – both new build and refurbished – for new and existing tenants, and private sale, as well as new communal spaces such as gardens, play areas, shops, a cafe and a community hall. The aim was for the project to be self-financing through the sale of the private homes on this sought-after north London site.

In 2012 Hawkins\Brown put in a joint OJEU bid with architects Mae, the latter having worked with Camden Council previously. Part of the plans included a comprehensive resident consultation that would largely dictate what direction the project would take, a concept that appealed to Hawkins\Brown. “We were attracted by the prospect of creating an exemplar regeneration scheme – Camden was keen on a resident ballot to establish the principle of the development, and in 2013 this was really unusual,” explains James Woodward, architect and Passivhaus certified designer at Hawkins\Brown.

At this stage however, Passivhaus wasn’t in the brief. Woodward explains that the practice has found on “a lot of projects” that when it comes to Passivhaus, “the client needs to develop confidence that it won’t have an adverse impact on cost or programme, by which time the scheme has already developed.” It was the consultation process with residents that led to the idea that Passivhaus certification could be the target for Agar Grove. Woodward explains: “Existing residents voted in favour of comprehensive redevelopment, as opposed to infill or refurbishment options.” With a major objective of the client behind the regeneration the estate being “to minimise fuel poverty,” all new blocks in the scheme will be built to Passivhaus standards.

The estate’s housing stock, which dates back to 1966, had become “inefficient, outdated and disconnected from the wider city.” An 18 storey tower block was at the heart of the estate, with low-rise blocks surrounding it, comprising 249 social-rent homes in total. “The masterplan is built around a series of streets and squares that stitch the estate back into the surrounding streets,” explains Woodward. “Streets are lined with maisonettes and shared entrances, with bedrooms located away from the street at ground level, and each perimeter block contains a shared semi-private residents’ amenity space plus infant play space.” The central tower block will be stripped back to the concrete frame, and the space around it extended to provide amenity space. A new tower will house a community centre and tenant management offices.

The project was broken down into six phases in order to minimise disruption to existing tenants, and keep the community together. Phase 1a consisted of building 38 social rented homes on a site which previously housed garages predominantly, and four homes. This enabled, says Woodward, a “single phase decant” process for the entire estate – delivering on another of the client’s main objectives. “Camden had promised the residents that they could watch their new home being built then move just once, into their new home.” Only one family from those four houses elected to move away from the estate altogether.

A collaborative effort

A multitude of firms have been and are involved in the design and construction of Agar Grove. Hawkins\Brown have, alongside Mae, designed the masterplan and completed detailed planning for specific blocks. These were then individually taken on by different architects for their detailed design under the appointment of contractor Hill. Architype were the Passivhaus delivery architects for both Phase 1a and 1b.

Hawkins\Brown have also been appointed to deliver Phase 1c – 125 homes across two blocks – one of which they originally designed, and the other being by Mae. The Hawkins\Brown team contains three Passivhaus Certified Designers, including Woodward, and will be collaborating with Max Fordham who are also looking after the PHPP model.

Sustainability

While all new build elements are aiming for Passivhaus status, EnerPhit – the retrofit equivalent – is not currently being targeted for the refurbishment of the tower block; the team are instead aiming to achieve BREEAM Domestic Refurbishment Excellent certification. However, explains Woodward, “Until the structure is stripped back, we are unsure exactly what quality of fabric we have.” It will be taken back to the concrete frame and then extended, in order to minimise embodied energy, he says.

Designing for Passivhaus is demanding, and the typology as well as scale at Agar Grove was always going to increase the design challenges. “At the outset this was quite an unusual typology for a Passivhaus scheme,” says Woodward. Despite this, the standard wasn’t amended or revisited by the Passivhaus Institute in order for Phase 1a to receive its certification. A further advantage of the multi-phase approach is that the practice will take lessons learnt from earlier phases and apply them to upcoming ones on achieving the energy efficiency needed.

One of the main challenges encountered early on in the project was how to minimise thermal bridging, given heavyweight materials were being used for the facade (brick and stone). Minimising thermal bridging has been key, with masonry support required due to the brick cladding, explains Woodward: “This works hard to support metal balustrades and GRC window linings, so as little as possible bridges back to the primary structure.” Balconies have also been stacked where possible, and the designers “internalised some, to minimise the thermal envelope,” he adds. These internal balconies – or “external rooms,” also offer better privacy and shelter to residents.

Air tightness, another crucial element, was achieved through various design measures. “It was a mixture of concrete framing, wet plaster ‘parged’ blockwork infill, triple glazed windows and an array of airtightness tapes for various junctions,” explains Woodward. Blown mineral fibre insulation was “key to ensuring continuity of insulation, with minimal thermal bridging and avoiding a resulting performance gap,” he says. This insulation took up an additional 50 mm of space, but this was gained back thanks to the full filled external walls, which removed the need for a cavity.

One of the biggest challenges was avoiding overheating – largely from solar gain. The east and west facing facades in particular proved problematic, explains Woodward, due to the lower position of the sun in the sky. “Getting the window design right was tricky,” he says. “The proportion is key to get maximum daylighting for a given glazed area. We needed to work hard to achieve a balance, especially seeing as Passivhaus does not measure daylighting, but criteria still need to be met for planning.”

Windows and balconies on the south facade were particularly important for helping achieve these daylighting requirements and ensuring a pleasant living environment. The balconies have a “dual benefit” Woodward explains, not only providing outdoor amenity space for residents, but also shading during the summer months. “Their depth ensures that low winter sun can still penetrate deep into the plan,” Woodward adds.

The way tenants would use and live in the building was also key to maximising the building’s energy efficiency potential. Expectations of likely user behaviour were modelled in the PHPP software, with one of the “most difficult areas” being overheating, says Woodward, such as “assuming whether residents were likely to open their window if rooms felt too hot and stuffy.” All other elements were kept as simple as possible for residents, with most of the home technology “running in the background.” Nonetheless, he explains that they were given a “detailed home user guide to ensure any questions were answered.” The controls were kept as simple as possible, for example a dial in the kitchen to control the MVHR boost for cooking or bathing.

MVHR was used internally in Phase 1a, and this is the intention for all future phases. Communal plant has been installed on the roof, to provide convenient access for maintenance purposes – and because it provides a cleaner air source. Looking ahead to the rest of the development, the third phase will use an all-electric system with communal air source heat pumps, an ambient loop and a unit-based water-to-water heat pump and tank, only possible due to the recent decarbonisation of electricity in the national grid.

Further key sustainability design measures on the project include a site-wide SuDS strategy, centred around permeable paving and attenuation, and an emphasis on biodiversity. “Biodiversity targets are based on Camden’s own standards and have resulted in most roofs being either green or planted as a wildflower meadow.” Over 100 bird and bat boxes will also be installed across the estate in what Woodward describes as “suitable locations integrated into the brick facades.”

Reconnection

One of the major focuses for the masterplan was improving the layout of the estate and its connection with the wider area – a task Woodward says was an essential part of improving the estate as a place to live. “The existing estate had a perimeter fence, and no clear hierarchy between public and semi-private spaces,” he says. “Landscaping was poorly programmed, and mainly acted as a buffer space to ground floor bedrooms.”

The layout was reconfigured to make it “coherent,” connecting the estate to the city via pedestrian and cycle paths. Vehicle access has also been provided although the only parking included in the masterplan was for existing blue badge holders. If and when residents move, the plan is that the parking spaces will be given back to the landscape.

At ground level the practice included maisonettes which allowed them to lift the bedrooms previously at this level up and away from the street. A two storey plinth in the form of different textured brickwork to that above “gives a more expressed character to the elements closest to the street, with deeper window sills and juliette balustrades,” says Woodward. Residents benefit from “passive surveillance” from the waist-height kitchen windows. Double height communal entrances also sit within the plinth – these provide a direct view through to the communal garden upon entering.

The facade is predominantly brick, with reconstituted stone; both materials chosen to complement the surrounding buildings. Stone banding was included to give “depth and detail,” and “break up what could otherwise be quite a dominant facade,” says Woodward. Decorative metalwork was also included in the form of a side panel to windows, doors and balconies, further reducing the proportion of brick and, explains Woodward, “ensuring a low proportion of glazing for the north facade, minimising heat loss as there would be relatively little gain on this elevation.”

The building was designed to bring in as much natural light as possible into both flats and communal areas such as stairwells and corridors. The flats give residents a larger living area than they previously had – Woodward states the practice’s previous work “helped with producing efficient flat and core layouts.”

Moving forwards

Completion is hard to put a time on due to the single phase decant – each phase requires demolition before construction can begin, which in turn relies on the previous phase being completed so residents can move. The budget is also frequently re-appraised as the market fluctuates, dictated by the potential value of the homes to be privately sold.

With residents now living in Phase 1a, Woodward says they’re currently measuring a 70% reduction in their energy bills – it had been estimated a 90% reduction could be possible. “Residents are very happy, especially those that came from overcrowded homes, and those who have purpose-built wheelchair homes for the first time,” says Woodward.

Looking ahead to the rest of the project, Woodward says one of the biggest challenges is going to be applying the Passivhaus construction methodology that Hawkins\Brown developed to the new build tower in Phase 2b. He also anticipates the retrofit of the existing tower will “bring its own complications with extending floorplates and the compromises required when dealing with existing structures.”

Performance measurement is being carried out within the completed apartments to enable lessons to be learned for the future phases, as well as a qualitative survey of how residents are using their new homes.

Project Factfile

  • Client: LB Camden
  • Masterplan: Hawkins\Brown, Mae, Grant Associates
  • Lead designer: Hawkins\Brown
  • Architects: Hawkins\Brown, Mae
  • Delivery Architect: Hawkins\Brown (Phase 1c), Architype (Phase 1a/b)
  • Contractor: Hill Partnerships
  • Project Manager: Arcadis
  • Structural Engineer: Stantec (Phase 1a/b), Price and Myers (Phase 1c)
  • Services Engineer: Max Fordham, Robinson Associates (Phase 1a/b delivery)
  • Landscape: Grant Associates
  • Passivhaus Consultant: Max Fordham
  • Passivhaus Certifier: WARM

Agar Grove Awards

  • CIBSE Awards 2020: Project of the Year – Residential (Winner)
  • New London Awards 2019: Overall Winner; Sustainability Prize (Winner); Housing (Shortlisted)
  • London Planning Awards 2019: The Mayor’s Award for Good Growth (Winner); The Mayor’s Award for Sustainable and Environmental Planning (Winner)
  • Housing design Awards 2019: Completed (Shortlisted)
  • RTPI Awards 2019: Excellence in Planning for Homes – Small (Shortlisted)
  • The Sunday Times British Homes Awards 2018: Development of the Year – More than 100 homes (Shortlisted)
  • The Sunday Times British Homes Awards 2015: Housing Project (Commendation)
  • Housing design Awards 2015: Project Schemes (Winner)
  • Bd Architect of the Year Awards 2014: Masterplanning & Public Realm (Shortlisted)
  • Bd Architect of the Year Awards 2013: Masterplanning & Public Realm (Shortlisted)