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CIBSE TM49 – How London Avoids Overheating

31 Jan 2017 at 9:42 AM

The Greater London Authority is concerned that our climate is changing. Writing for CIBSE, Strategy Manager for Climate Change Adaptation Alex Nickson sets out the weather events that have demonstrated this most clearly to him.

In recent memory, the South East has experienced the hottest summer (2006), the highest peak temperature (2003), the coldest December (2010), the wettest summer (2012), the driest two winter period (2010–12), and what became the wettest winter in over one hundred years (2013–14).

Complicating things further is the urban heat island effect. The same phenomenon that keeps cities warmer in the winter can also raise temperatures to extremes during the summer months. Studies estimate that in Manchester and Birmingham, urban density typically raises temperatures by between three and five degrees higher than the surrounding countryside. On a hot summer’s evening, parts of central London can be eight degrees warmer than the green belt, rising to ten degrees during a heat wave.

This can have life and death implications. Some estimates claim that between 35,000 and 70,000 heat related fatalities were the direct result of the 2003 European heat wave, with urban centers most acutely affected. Such was the severity of the situation during August of that year that a refrigerated warehouse outside of Paris was used by undertakers, as they did not have enough space in their own facilities.

“We clearly cannot and should not be using the past as a guide to the future — not if we want to reduce our carbon emissions and ensure that our buildings are comfortable to live in and affordable to run.” – Alex Nickson, Greater London Authority

It is into this climate uncertainty and increased exposure to overheating risks that cities must plan for the future. This is challenging enough given that 80% of the buildings we will occupy in 2050 are believed to already have been built. The Greater London Authority have dubbed this a ‘retrofit hangover’ of homes and offices that are likely to require new intensive mechanical cooling or air conditioning to remain habitable.

To avoid adding to this workload and repeating the mistakes of the past, the Greater London Authority have commissioned CIBSE to produce research and guidance on designing out the overheating risk. This was published in 2014 as CIBSE TM49: Design Summer Years for London. It is a requirement of the London Plan that all major new developments in the capital follow this guidance.

What is CIBSE TM49?

A Design Summer Year (DSY) is a weather dataset used during dynamic thermal modelling, to assess overheating and cooling requirements for new buildings. This can impact decisions during design and specification to avoid excessive overheating risk.

The TM49 Design Summer Years for London introduce the concept of incorporating the urban heat island effect, as well as the severity of hot events in the design of buildings. Whereas previously a single location for London was used (using observed data from Heathrow), three are now available to better represent urban, semi-urban, and rural areas and for three years of varying severity of hot events.

How is Overheating in New Buildings Measured?

Dynamic Simulation Modelling (DSM) is expected of all new major schemes by the Greater London Authority, and occasionally of minor developments by the local borough authorities. This is a simulation, where a virtual model of a building is created to account for built form, local conditions, building specification and heating and cooling equipment.

The two most common software packages used for this purpose are IES VE and EDSL TAS. For London, the CIBSE TM49 Design Summer Years are used to assess risk that incorporates the urban heat island effect and the location within the capital.

The Rules for Low Risk Design

The London Plan (5.9) targets the urban heat island effect and reliance on air conditioning systems through the design and construction of new buildings. All major developments must be created using a list of design rules known as ‘the cooling hierarchy’. These are:

  • Minimising internal heat generation through energy efficient design: For example, heat distribution infrastructure within buildings should be designed to minimise pipe lengths, particularly lateral pipework in corridors of apartment blocks, and adopting pipe configurations which minimise heat loss e.g. twin pipes
  • Reducing the amount of heat entering the building in summer: For example, through use of carefully designed shading measures, including balconies, louvres, internal or external blinds, shutters, trees and vegetation
  • Use of thermal mass and high ceilings to manage the heat within the building: Increasing the amount of exposed thermal mass can help to absorb excess heat within the building
  • Passive ventilation: For example, through the use of openable windows, shallow floorplates, dual aspect units, designing in the ‘stack effect’
  • Mechanical ventilation: Mechanical ventilation can be used to make use of ‘free cooling’ where the outside air temperature is below that in the building during summer months. This will require a by-pass on the heat recovery system for summer mode operation

These measures may only influence new buildings, but in the long run can ensure that the homes and workplaces we produce now will still be useful in the decades to come.

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