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Major Willmott Dixon Passivhaus assessment takes shape in Camden

Incredibly high attention to detail, new techniques and methods, and a re-education process for site workers are all contributing to the delivery of a high-specification housing development.

Project: Chester Balmore residential scheme
Client: Camden Council
Region: London
Main contractor: Willmott Dixon
Lead designer: Architype
Sustainability and M&E consultant: Mott MacDonald
Concrete frame subcontractor: J Reddington
Start date: January 2012
Completion date: July 2013

“This isn’t a box in a field like you get on Grand Designs,” Camden Council senior development manager Ivan Christmas says. “We expect this development to demonstrate that Passivhaus has crossed over into the mainstream.”

He is talking about the Chester Balmore residential scheme currently being built on a tight urban plot in north London.

The development, located in the Dartmouth Park Conservation Area of Highgate, comprises 23 homes for social rent, 26 for market sale and four for intermediate shared ownership. There will also be 500 sq m of commercial floor space, which will become local amenities such as a pharmacy and a GP surgery.

It is snowing hard when CN visits the site, and it is hard to believe the only heating in the homes will be for towel rails.

Passivhaus implemented for free

Yet Willmott Dixon was awarded the contract to build the project after submitting a proposal that made no extra charges for Passivhaus principles.

“We expected a 10 to 15 per cent cost difference, but Willmott Dixon came back with a proposal that did not appear to apply a premium at all”

Ivan Christmas, Camden Council

“We ran two cost plans in parallel,” Mr Christmas says. “One was Code for Sustainable Homes Level 4 and the other was Passivhaus.

“We expected a 10 to 15 per cent cost difference, but Willmott Dixon came back with a proposal that did not appear to apply a premium at all.”

Residents of existing housing on the site were decanted and the contractor started a six-month period of enabling works in June 2011. A specialist subcontractor was needed to clear asbestos, and utility disconnection took some time. In the first few days of 2012, the main build got under way.

Cutting principles

“The principle of Passivhaus is cutting out air leakage and heat loss,” Willmott Dixon operations director Jeremy Graham explains. “We need to reach a constant 16 deg C internal temperature all year round. This then goes up to 20 to 21 with people, televisions and ovens, which is considered a comfortable temperature.”

An airtight envelope had to be created and it is critical that this is not breached – meaning every possible cold bridge had to be identified and eliminated. “The main thrust of the design development was detailing the outside of the unit to stop air and heat loss,” Mr Graham says.

“A building settles and moves slightly over the first year so we had to look at all the junctions, such as windows and doors, as well as any connections between the inside and the outside, such as balconies and wall ties.”

Extra attention to thermal detail

An extremely high attention to detail is required to keep the warm air in and the cold air out. Just 0.4 air changes per hour were permitted on this design – a tenth of the airflow Willmott Dixon usually designs to.

“Architype specified reinforced nylon wall ties – the first time I’ve used the product in 26 years in construction”

Jeremy Graham, Willmott Dixon

Concrete walls were packed with 250 mm of high-spec air-blown cavity insulation, which includes a resin to stop it drifting out of place at any point. The roof and the ground floor were insulated as well. However, the stainless steel ties typically used to bond the two concrete wall skins together would breach this barrier.

“Architype specified reinforced nylon wall ties – the first time I’ve used the product in 26 years in construction,” Mr Graham says.

The process for creating the frame took some time to arrive at. It was essential that air-tightness was maintained by other trades once it was created. “We put up blockwork for the front and back walls, followed by triple-glazed windows going in; then we used a 6 mm parge coat inside the blockwork, and finally air-tightness tapes around all the junctions where different materials met,” Mr Graham says.

German and Austrian input

German firm Pro Clima supplied the tape, and Austrian manufacturer Internorm, the windows. Both firms were heavily involved in the drawing up of the minutia of the processes to install them. “For example, you have to expose half of the tape when fixing it to the window, but you can’t get dust on it before you fix it down,” Mr Graham says. “It has to stand for the lifetime of the building.”

“Everyone that walks through the door has to change their mindset – they can’t compromise the fabric of the building at all”

Jeremy Graham, Willmott Dixon

The construction programme for the project has 1,600 activities on it. “It describes in infinite detail how you get from the start of the job to the end and which activities are critical to the programme.”

One cold bridge that cannot be eliminated entirely is through the foundations. Steel piles were needed and could not be done without. “We insulated the foundations to minimise heat loss,” Mr Graham says.

Once the envelope was tested and showed to be airtight, work began on the services. High-performance insulation is used on pipes to ensure the temperature does not drop between leaving the boiler and reaching the taps.

Different methods of service installation

Service installation had to be done very differently to normal. “Our internal services sit on the wall rather than within them,” Mr Graham explains. “This required an educational process. Everyone that walks through the door has to change their mindset – they can’t compromise the fabric of the building at all.”

As well as a safety induction, a Passivhaus induction was carried out for all site workers. “When people turn up here, 99 times out of 100 it is their first Passivhaus job,” he says. “We need to ensure they don’t do what they’ve done throughout their career.”

Willmott Dixon appointed an air-tightness champion, trainee James Warren. “His only job is to check every day that we are not doing something fundamentally wrong on Passivhaus,” Mr Graham says. “Are we doing anything out there that could be a cold bridge? We’ve had several details where we’ve had to rework them with the supplier and the architect.”

“As air is pushed outside, the heat is stripped from it and applied to the air brought in”

Jeremy Graham, Willmott Dixon

All data from the ever-changing design of the building are inputted into a Passivhaus modelling system that highlights cold bridges and air loss. Exact dimensions can be worked out for the smallest details of the project. Ironically, one challenge with creating such an airtight environment is providing fresh air for residents. Willmott Dixon used mechanical heat/vent recovery systems.

“As air is pushed outside, the heat is stripped from it and applied to the air brought in,” Mr Graham explains. After all the services are installed, another series of air-tightness tests will be carried out before the second fix can begin. Willmott Dixon will carry out its own final tests on the building before decorating and undergoing an external evaluation.

Winter weather hampers construction

While the winter weather will not affect residents in years to come, it has had an impact on the construction of the project. A crane is critical to lift many of the heavy materials – such as triple-glazed windows – but can’t operate in high winds. Brickwork cladding grinds to a halt in the cold.

“We might get more time but we have to get the quality right,” Mr Graham says. “Clients won’t come back if it’s wrong.”

Nevertheless, completion remains scheduled for July, with the first residents expected to move into their homes by the end of the summer. They may just be getting a glimpse of the future. “In 10 years’ time the majority of what we are building will be Passivhaus.”

Get the handover right

Camden Council and Willmott Dixon are spending a huge amount of time and effort creating a Passivhaus development – but how will it be protected when they hand over the keys?

A handover workshop will be held with the client and subcontractors to ensure the parties have covered as many possibilities as possible. “We need to ensure we keep control of the quality,” Mr Graham says.

Mr Christmas adds: “We are talking about having a Passivhaus induction process for residents. We will pass the knowledge on and give them numbers for handymen who can tell them how everything works.

“There is also the issue of second lettings – who moves in six months’ later and how do we make sure they know how to use the building? It is not uncommon in London for people to buy and decide to rearrange it all. We can draw up special leases but you can’t rely on people following the terms of the lease.”

Post-occupancy research will be conducted, and the council is using the project to test the viability of further Passivhaus schemes.


Overcoming logistical challenges

The high density required to make the central London project worthwhile also made it very difficult logistically. “The building covered most of the site,” Mr Graham says.

“We put the site offices on the public highway – the road had to be closed, so it helped that our client was the council. There was a process to be followed but it is short-term pain to provide a landmark building.”

Deliveries had to be managed with military precision. “We have weekly meetings with subcontractors to discuss deliveries,” he says. “We need to know exact slots so the crane can be booked. If a lorry is late it has to rescheduled. The workings of an inner London site have to be like clockwork.”

Materials come in palletised ready to be distributed by a crane at the centre of the site as close to the point of fixing as possible.

“We are currently using the concrete roofs for storage, but with the roofer about to start, we’ve had to clear one of those and are trying to complete an area of groundworks for storage,” Mr Graham explains.

The site opens on Saturdays purely for materials movement. “When people come in on a Monday, everything is ready to go.”

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