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Willmott school targets UK first with next-level precision

In south London Willmott Dixon is having to hit exacting standards to deliver what is billed as the UK’s first Passivhaus secondary school.

Project: Sutton Secondary School
Client:
Sutton Borough Council
Contract value: £38m
Main contractor: Willmott Dixon
Groundworks subcontractor: J P Dunn Construction
Timber cladding subcontractor: NHE
Roofing subcontractor: Roofline Group
Start date: January 2018
Completion date: June 2019

In 2017, Willmott Dixon notched up a new record for UK construction when it completed work on the George Davies Centre for the University of Leicester.

The building is said to be the biggest non-residential project to be built in the country using the Passivhaus sustainable building methodology.

Now the contractor is targeting what it believes to be another first, with the delivery of a new secondary school in the London borough of Sutton. Once complete and certified, the building, which will be home to 1,275 pupils and 95 staff, will be the first secondary school in the UK to have used Passivhaus.

Developed in Germany in the early 1990s, Passivhaus claims to be the fastest-growing energy performance standard in the world. However, it is still rarely used in the UK, particularly in a non-residential environment.

So why did Sutton Borough Council choose to use the standard and what challenges has it thrown up for the construction team? 

Consultant headteacher

The project as a whole was necessitated by growing demand for school places in Sutton, a trend that has been apparent for 10 years.

As the local education authority, the council realised that it was on the cusp of exhausting the ability of existing schools to be expanded, according to the local authority’s project manager Adam Whiteley.

“We needed a consultant to act in situ as head teacher to give a school-side view on the plans as they were being developed”

Wendy Bishop, Architype

“It was basically a brief from the education team for a new school,” he says. “New schools have to be done through the Education and Skills Funding Agency, so there was then a parallel process of us working with them and also on design development and getting to planning.”

At the same time, the education department at the council set about finding a free school provider to run the new facility. It selected Architype as lead consultant architect and also appointed a specialist education consultant. The latter was made necessary owing to the fact that the free school provider had not yet been appointed, so the design team needed somebody to act as the de facto end-user.

Sutton Passivhaus 5 mid

Sutton Passivhaus 5 mid

“We needed a consultant to act in situ as head teacher to give a school-side view on the plans as they were being developed,” says Architype associate Wendy Bishop. “There were three organisations in the running to take on the school during that process, so the consultant met with all three and gathered opinions on the emerging designs. The design would work for all three, but it wasn’t tailored to any one of them.”

The decision to go for a challenging environmental standard partly reflected Sutton’s efforts to build a reputation for encouraging sustainable development. The then cutting-edge BedZed residential project, for instance, was built in the area in the mid-1990s, while the Hackbridge part of the borough has long maintained particularly testing environmental standards. “But Sutton generally has got higher emission targets than many other boroughs,” Mr Whiteley adds.

The site of the new school also falls within the masterplan for the London Cancer Hub (LCH), a new life science innovation cluster focused on cancer research and treatment located at the former Sutton Hospital site. Again, the ambition for the LCH is that it will be an exemplar of sustainable development.

“This is a masterplan that has been in place for a few years and the school, with its pressing timetable, became part of a first wave of activity,” Mr Whiteley says.

Better than BREEAM?

However, Sutton council didn’t need to go down the Passivhaus route. It could have opted for the more commonly used – and less prescriptive – BREEAM standard.

“One does have concerns about buildings that are designed to meet BREEAM. It’s a mix-and-match approach to achieve an overall score”

Adam Whiteley, Sutton Borough Council

So why was the decision made? “We took the view that going with Passvhaus design and certification would truly deliver a high-performing building,” Mr Whiteley explains. 

“One does have concerns about buildings that are designed to meet BREEAM. It’s a mix-and-match approach to achieve an overall score. It doesn’t fully determine how energy performance has to be achieved. Passivhaus focuses very much on energy performance, as well as the comfort and the quality of the internal environment. We felt that was the right kind of standard to represent the council’s wider ambitions.”

Architype describes itself as a specialist in Passivhaus. The practice has been using the standard for more than 10 years in the UK, having started out using it for the design and construction of two primary schools in Wolverhampton before going on to design a Passivhaus archive, university building and various one-off houses.

As Ms Bishop points out, however, the Sutton school is on a different scale: after the University of Leicester building, it is believed to be the largest Passivhaus project in the UK to date.

Totally different approach

Achieving Passivhaus is a very technical feat. The building envelope – the roof, walls and ground-floor slab – is highly insulated, with exceptional levels of airtightness required to prevent heat leakage. Despite Willmott Dixon’s experience in Leicester, construction manager Graham Thompson knew that he and his team would have to be meticulous in terms of planning and execution. 

“[Through] collaborative workshops with the architects, we started to understand the process,” he says. “The complexity with Passivhaus is the process compared to traditional construction. You have to completely encase the building in insulation from the foundations to the roof in order to achieve the test results that we need to achieve.” 

What that required was for Willmott Dixon to work even more closely with the project’s architects and engineers, as well as the client, than would normally be the case. “We realised early on that we needed to approach the construction in a different manner,” Mr Thompson says. “There is a lot more design work upfront. It’s not a BIM project – it’s not fully designed [from the outset]. It’s a constant design process, but the basic principles were established for Passivhaus early on.”

Sutton Passivhaus school

Sutton Passivhaus school

Mr Whiteley adds: “The amount of design time before construction began was largely due to the scale of the project and the complexities involved in a number of the packages, whether that’s structure or M&E or windows. Each of those are sophisticated packages and require design engagement by the suppliers. It’s different to an old school approach where stuff is picked out of catalogues and there isn’t really a design element involved.”

To ensure the design of the building is translated from page to construction site, Architype maintains a complicated spreadsheet that is slowly updated with live information coming in to replace theoretical assumptions. “So, it’s getting more and more accurate as you go on,” Ms Bishop explains. “You need to get information from people to ensure it’s genuinely accurate.” 

“There is no hiding behind anything; we have to be sure that it works. We go out on site with the supply chain as a partnership” 

Graham Thompson, Willmott Dixon

Getting that information is a painstaking task and requires Willmott Dixon to check that work undertaken by specialist contractors is being done to the letter. “My main role is checking the details,” says Joshua Day, management trainee at Willmott Dixon and a Passivhaus expert. “We’re checking it [for] what Passivhaus certifiers want in terms of evidence. So, before a bit of membrane gets covered it has to be checked. It’s every layer and very detailed.”

Built on trust

This approach, Mr Thompson points out, requires everyone on site to work together with a high degree of openness and trust. “Passivhaus is new for many people in the industry,” he says. “So, we have to have trust. We have to have trust in our supply chain because everything has to be right.

“There is no hiding behind anything; we have to be sure that it works. We go out on site with the supply chain as a partnership. A mistake the size of a 5p piece will affect the air test on this job, and we have to achieve that air test.”

Building that level of trust doesn’t happen by accident. To minimise the risk of anything going wrong, Willmott Dixon has put together a comprehensive education programme. “Everyone who comes through the gate goes through a standard induction with Willmott Dixon,” Mr Thompson says.

“Then we do a Passivhaus induction with Josh and the build managers to explain the principles behind it and why we’re doing it. Sometimes it’s frustrating for the supply chain that they can’t just get on with it, but they need to understand. What we’ve found is that once they understand something, they can achieve it.”

To date, the project is on track to complete in June, as per the build programme. The facades will be 70 per cent complete before Christmas; once that’s done early in the new year, the fit-out will begin. It’s obviously been a steep learning curve, but it is equally clear that Willmott Dixon and the rest of the team take enormous pride in what they are trying to achieve.

“We are going into minute detail, but that’s Passivhaus,” Mr Thompson says. “It’s taking it to the next level in terms of precision.”

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