Policy CR4
Managing risks of overheating
All developments must be designed to maximise the use of passive methods wherever possible to control overheating risks and minimise the need to provide active cooling. In order to achieve this, developments must do the following:
a) Assess ventilation provision and overheating risks and include passive mitigation measures where necessary, using site layout and building design to improve ventilation and reduce the risk of overheating. Unless prevented by issues such as poor air quality or noise, natural ventilation and cooling should be prioritised. Major developments in particular will be required to carry out overheating modelling.
b) The London Plan "Cooling Hierarchy" should be followed to demonstrate that the design ensures that the potential for internal overheating is managed using passive design measures.
c) Use soft landscaping and green infrastructure to help provide shading and reduce surface temperatures. As well as ground level planting, features such as green roof s and walls can also be used to provide other benefits e.g. increase biodiversity levels and reduce flood risk.
d) Include other passive measures such as blinds and shades or painting surfaces white (e.g. roofs) where appropriate have been considered and used before reverting to proposing energy intensive active air conditioning systems.
e) The use of active air conditioning systems should be avoided wherever possible but where their inclusion is demonstrated as necessary, they must include heat recovery measures.
The overheating assessment and details on mitigation measures can be provided either separately or as part of the Energy Assessment if one is submitted.
5.30 Overheating is one of the main impacts caused by climate change. Extreme weather such as heatwaves are expected to increase in frequency and severity and could have serious impacts, including causing increases in death rates, other health and quality of life impacts and causing damage to and preventing efficient operation of vital infrastructure such as transport and energy networks. Heatwaves also create higher demand for drinking water supplies.
5.31 As a densely built inner London borough, H&F is particularly vulnerable to 'Urban Heat Island' (UHI) effects, in which urban areas can be up to 10°C warmer than surrounding areas. New developments need to be designed and built to provide adequate ventilation and avoid causing overheating in the summer, particularly to protect those most at risk from heat related health impacts. It is important that use of higher insulation levels and more stringent air tightness standards do not lead to undesirable impacts such as overheating. This can be avoided through good design and construction practices. For existing buildings, there are retrofitting measures that can be considered to help improve air flow and keep over-heating to a minimum.
5.33 Approved Document O of the Building Regulations provides details of mitigation measures to help manage overheating in new residential buildings. PAS 2035 is also a useful reference document for domestic retrofit projects.
5.34 Passive measures that can be used to manage overheating risks include:
- Orientation and shading
- Prioritise passive ventilation where external factors such as noise levels and air quality allow.
- Use of high albedo materials
- Inclusion of windows with low solar gains
- Use of insulation which can help retain heat in the winter but prevent heat retention in the summer.
- Use green infrastructure to provide shading and cooling
5.35 Inclusion of green infrastructure such as green vegetation barriers, walls and roofs as well as tree planting etc are particularly supported as they provide wide-ranging climate resilience and benefits in addition to helping manage overheating and can improve local microclimates through cooling, sustainable drainage, and improved local air quality as well providing Biodiversity Net Gain and boosting the Urban Greening Factor of a scheme. Protection of existing and provision of new 'blue' infrastructure such as raingardens, lakes and ponds and other water features can also provide cooling benefits. The guidance laid out in the GLA's Energy Assessment Guidance[1] should be followed to show how the Cooling Hierarchy has been used to guide development.
[1] Energy Planning Guidance | London City Hall