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Policy SI4 Managing heat risk


  1. Development proposals should minimise internal heat gain and the impacts of the urban heat island through design, layout, orientation and materials.
  2. Major development proposals should demonstrate through an energy strategy how they will reduce the potential for overheating and reliance on air conditioning systems in accordance with the following cooling hierarchy:
    1. minimise internal heat generation through energy efficient design
    2. reduce the amount of heat entering a building through orientation, shading, albedo, fenestration, insulation and the provision of green roofs and walls
    3. manage the heat within the building through exposed internal thermal mass and high ceilings
    4. provide passive ventilation
    5. provide mechanical ventilation
    6. provide active cooling systems.

Climate change means London is already experiencing higher than historic average temperatures and more severe hot weather events. This, combined with a growing population and the urban heat island effect, means that London must manage heat risk in new developments, using the cooling hierarchy set out above.

In managing heat risk, new developments in London face two challenges - the need to ensure London does not overheat (the urban heat island effect) and the need to ensure that individual buildings do not overheat. The urban heat island effect is caused by the extensive built up area absorbing and retaining heat during the day and night leading to parts of London being several degrees warmer than the surrounding area. This can become problematic on the hottest days of the year as daytime temperatures can reach well over 30˚C and not drop below 18˚C at night. These circumstances can lead many people to feel too hot or not be able to sleep, but for those with certain health conditions, and some young or elderly Londoners, the effects can be potentially lethal. Green roofs can provide some mitigation of this effect by shading roof surfaces and through evapotranspiration.

Many aspects of building design can lead to increases in overheating risk, including high proportions of glazing and an increase in the air tightness of buildings. There are a number of low-energy-intensive measures that can mitigate overheating risk. These include solar shading, building orientation and solar-controlled glazing.

The increased use of air conditioning systems is not desirable as these have significant energy requirements and, under conventional operation, expel hot air, thereby adding to the urban heat island effect. Therefore, passive ventilation should be prioritised. If active cooling systems, such as air conditioning systems, are unavoidable, these should be designed to reuse the waste heat they produce. Future district heating networks are expected to be supplied with heat from waste heat sources such as building cooling systems.

The Chartered Institution of Building Services Engineers (CIBSE) has produced guidance on assessing and mitigating overheating risk in new developments, which can also be applied to refurbishment projects. TM 59 should be used for domestic developments and TM 52 should be used for non-domestic developments. In addition, TM 49 guidance and datasets should also be used to ensure that all new development is designed for the climate it will experience over its design life. The GLA’s Energy Planning Guidance provides further information on how these guidance documents and datasets should be used.