Tilbury Douglas scores top marks for Passivhaus school

There’s one word that keeps coming up on a tour of Tilbury Douglas’ Wednesfield Technology Primary School project: meticulous.

Built for the Shireland Collegiate Multi Academy Trust (SCMAT), on behalf of the Department for Education (DfE), this is a Passivhaus-standard school project. There’s no room for ‘good enough’ on this project; to get accreditation from the Passivhaus Institute, everything must be bang on.

“This is one of the most challenging projects I have had,” says Tilbury Douglas Project Manager Kevin Miners. “It’s certainly the most meticulous I’ve ever been on.”

From the design and execution of joints and other details to the materials used to the collation of information for every piece of kit, the Passivhaus standard requires all the ‘t’s to be crossed and the ‘i’s to be dotted. For Tilbury Douglas and its supply chain, that has meant extra attention to detail – and extra time taken – particularly on the floor slab and the joints for the timber-framed system.

DfE framework

The school was tendered under the DfE’s 2017 construction framework back in 2020. “This is a flagship project for the client,” says Matt Lakin, Contracts Manager for Tilbury Douglas. “Passivhaus is not just about applying sustainable design to save the planet, it’s about saving money for the schools too.”

Wednesfield Technology Primary School is a two-form entry school which will eventually take 446 pupils. The main part of the building is a rectangular block with classrooms located over two storeys: nursery and younger children on the ground floor and older children upstairs, together with a small library and a SEND (special educational needs and disabilities) room. A smaller, one-storey wing joins the main one at right angles, housing a multi-purpose hall and kitchen.

This will be the fifth school that Tilbury Douglas has built for this trust. That gave the contractor and its designers an advantage when tendering, says James Edwards, associate architect at Corstorphine and Wright (C&W) Architects, which leads the design team.

When bidding through the framework, contractors are given a control design which has been produced by the DfE’s architects, in consultation with the school involved. Bidders then have a short time – around four weeks – to price it and propose any changes.

The trick is to deliver both the generic requirements for DfE and the specific requirements of the trust or school, explains Lakin. Bids are then assessed on cost, quality and programme.

Tilbury Douglas had delivered other technology schools for the trust. The idea of these schools is that the children work with digital equipment and tools from the earliest stages so that they are equipped to work in a technology-driven world.

Start on site

Tilbury Douglas started on site in February 2023, demolishing the one-storey Edward the Elder School building, which had been vacant since 2007. Most of the demolition waste has been used to level up the site – which required up to 2.5m of fill in places – and some of the bricks were retained.

Once planning permission was granted, Tilbury Douglas faced its first Passivhaus hurdle. The building sits on a raft foundation, with a double layer of insulation under the slab. The joints of the insulation are staggered, to prevent heat loss, and the insulation extends all the way under the upstand and around the edge of the slab.

“All the pieces of insulation were hot wire cut so that they were dimensioned exactly to the millimetre,” says Lakin.

Like every detail on this school, it had to be modelled, explains Edwards. “We had a Passivhaus designer who modelled every detail in 3D,” he says.

Each joint has to be individually assessed on site, with evidence – including photos – supplied to the Passivhaus designer and then on to the Passivhaus institute. Tilbury Douglas’ Passivhaus champion on this project was site manager Ben Howman, whose job it was to scrutinise every element of the build.

Achieving airtightness

The need to prevent cold bridging and achieve airtightness at junctions caused a few extra issues when it came to assembling the timber frame, a system called Larsen Truss which was supplied by Timber Innovations, a company located just 25 miles from the site. The system consists of twin walls of timber frames braced together, boarded with OSB internally, cement board externally, then infilled with 380mm of mineral insulation – some in the factory and some on site.

The panels are more usually used for housing projects and the way that adjacent panels are normally joined to each other was not good enough for the school Passivhaus requirements. To overcome this challenge, Timber Innovations delivered panels that had short, unfinished sections at either end, so that the insulation and boards could be installed on site, ensuring there was no cold bridging.

“It took us until January to get the building watertight because it took us a lot longer to do the structure than we had expected,” says Miners. “But taking that extra time to make sure paid off in the end when we had such good airtightness results.”

For Passivhaus accreditation, two airtightness tests must be undertaken, the first when the structure is complete but before fit out. The first one delivered outstanding results, far surpassing the minimum requirement: air changes per hour were 0.225 air changes per hour (ACH) at 50 pascals, well below the required 0.6 ACH. The second airtightness test, carried out when the building was nearing completion, also came in well under the requirement at 0.48 ACH.

Meticulous timing

The building is brick clad – for its entire height on the wall facing the road, with render on the upper parts of the other walls. The brick was a requirement from the planners, explains Edwards, echoing the brick of the school that had stood on the site and of the Vine Inn, a Grade II-listed pub which sits directly opposite, on the other side of the main road.

The brick lodge building for the original school also remains on site. Tilbury Douglas has repointed it, replaced damaged roof tiles and given it new facias, guttering and drainpipes. The school will refurbish it internally, with plans to make it into a community space.

To the back of the school site, which runs along the Wyrley & Essington Canal, there will be a huge outdoor space, enhanced by existing woodlands – whose preservation was a strict part of the brief. The wet weather this year means that Miners is not as advanced with the outdoor elements as he would like, but they are shaping up now.

Tilbury Douglas handed over the new school during the school summer holidays, in time for the autumn term. It appears that the meticulous Passivhaus approach has paid off.

This article was first featured in Construction Management Magazine.