City North – a case study on innovative ways of cooling buildings

Roger Olsen, Associate Director & Farrah Hassan-Hardwick, Business Development Leader, Building Engineering, Arup

Recent extreme hot weather experienced in the UK may start to occur more frequently due to climate change and, as a result, major cities around the world, including London, face potentially significant risks to people and properties.

Hot weather can cause more than just discomfort. As temperatures rise in cities, exacerbated by the urban heat island effect, so do the number of heat-related illnesses, emergency call-outs and, in some cases, deaths.

Increasingly, Local Authorities in London, across the UK and similarly temperate urban areas are restricting mechanical cooling in residential units. City North, a mixed use development just a stone’s throw away from Finsbury Park Tube Station in London, presented such a challenge to our engineers. Comprising two twenty-three story residential towers, linked by a 12 storey block of flats, with the usual mix of uses including retail and leisure, Arup has pioneered the use of an innovative technology to mitigate against overheating residential apartments.

Typically, contemporary residential buildings are often heavily glazed structures with timber and stud wall partitions offering little thermal mass. Lighter weight buildings have less thermal mass compared to those traditionally built with masonry and concrete. These lighter weight buildings are unable to absorb direct solar energy and re-emit to the atmosphere. The speed at which heat moves through a material or a wall is dependent on that material’s conductivity.

Heavyweight buildings, constructed with concrete or brick, absorb heat from the sun slowly. A substantial amount of absorbed heat is reflected back to the atmosphere, directly away from the building’s interior. In lighter weight buildings, absorbed heat from the sun passes quickly through exterior walls, and into the building interior. When combined with large areas of glazing for daylight and views, heat gains can be substantial and unsurprisingly apartments can often overheat. Arup was appointed at a late stage in the design process. Our review determined that the proposed buildings were at risk of overheating.

Additional thermal mass could not be added to the design without significant structural re-design and this approach would take up additional floor space. This was not feasible as flats had already been sold off plan. Ambitiously, the Design Team replaced traditional plasterboard with Phase Change Material to compensate. PCM plasterboard is impregnated with material that is “tuned” to change from solid to liquid phase at approximately 27°C, just below the Part L mandated internal temperature limit. The resulting cooling is released into the overheating rooms when required. In effect, the PCM plasterboard provides the same beneficial cooling effects of 80mm of dense concrete, without the weight. Notwithstanding costs, PCM plasterboard offers; long term reliability, consumes no energy and is easy to install and maintain. Significantly, at 12.5mm thick, it has virtually no impact on the flat floor area.

Clever thinking and intuitive use of new technology has enabled a comfortable living environment to be realised in the City North example above. However, as more and more modern high-rises start to grace the skylines of our major cities, more needs to be done to factor in crucial thermodynamic considerations from the start of the design process. Our climate is changing and our buildings must be able to adapt.