"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 being built on a tight urban plot in north London.

The development 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 floorspace, 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.

Yet Willmott Dixon was awarded the contract to build the project after submitting a proposal that made no extra charges for Passivhaus principles. "We ran two cost plans in parallel;' Mr Christmas says." One was Code for Sustainable Homes Level4 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."


"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."


Airtight aim

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 Joss;' 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."

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.

 Concrete walls were packed with 250 mm of high-spec airblown 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.

"When people turn up here, 99 times out of 100 it is their first Passivhaus job. We need to ensure they don't do what they've done throughout their career"



In the frame

The process for creating the frame took some time to arrive at. It was essential that airtightness 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; we used a 6 mm parge coat inside the blockwork, and finally airtightness tapes around all the junctions where different materials met," Mr Graham says.

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. 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."

 A Passivhaus induction was given to 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."

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 says. After all the services are installed, another series of airtightness 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 woes

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 matetials- 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."

Completion remains scheduled for July, with the first residents expected to move in 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 build will be Passivhaus."



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.