Part One: Spatial Strategy and Policies
Managing Heat Risk
12.23 As part of the Plan's proactive approach towards mitigating and adapting to climate change, Policy DLP43 sets out the requirements for managing heat risk within new development proposals.
Policy DLP43 Managing Heat Risk
- Development proposals12 should minimise both internal heat gain and the impacts of urban heat islands13 by using appropriate design, layout, orientation and materials.
- Development proposals will be expected to demonstrate how their potential for overheating and reliance on artificial cooling systems will be reduced, in accordance with the following cooling hierarchy:
- minimise internal heat generation through energy-efficient design;
- reduce the amount of heat entering a building through orientation, shading, albedo14, fenestration, insulation and the provision of green roofs and walls (see also Policy DLP39 Design);
- manage heat within a building through exposed internal thermal mass15 and high ceilings;
- provide passive ventilation;
- provide mechanical ventilation;
- provide active cooling systems16.
Justification
12.24 Global temperatures are rising, and this has been paralleled by changes in the weather in the UK. The Met Office published a document17 in 2019 outlining current trends and predictions in the UK, including the following:
12.25 The average temperature over the most recent decade (2009-2018) has been on average 0.3°C warmer than the 1981-2010 average and 0.9°C warmer than the 1961-1990 average. All the top ten warmest years for the UK, in the series from 1884, have occurred since 2002. (paragraph 2.1)
12.26 The urban heat island effect18 is caused by extensive built-up areas absorbing and retaining heat during the day and night, leading to those areas being several degrees warmer than their surroundings. With higher temperatures across the country, the likelihood of heat being trapped in this way is very likely to increase.
12.27 Retained heat can become problematic, to the point where circumstances can lead to physical discomfort and disruption, but for those with certain health conditions, the very young or the elderly, the effects can be serious. The use of green roofs and / or walls can provide some mitigation by shading roof surfaces and through the mechanism of evapotranspiration.
12.28 The concept of thermal inequity19 will also have relevance in areas of the borough, whereby because of uneven social geographies, urban heating effects impact disproportionately on poorer / marginalised communities living in urban environments. This is exacerbated by a planning policy approach that concentrates development in urban areas, at higher densities and in taller forms. The removal of urban greening and trees to facilitate increased development densities will have further adverse effects on ambient temperatures in the vicinity.
12.29 Certain aspects of building design intended to increase energy efficiency and reduce heat demand, such as increased glazing and airtightness, can also exacerbate heat risk and cause uncomfortable living conditions. Design solutions can be found in the hierarchy proposed in Policy DLP43.
12.30 Means of minimising heat risk may include, though not be limited to, inclusion of mitigation measures such as: -
- solar shading, for instance through landscaping or brise-soleil20,
- using appropriate materials in areas exposed to direct sunlight,
- using landscaping and permeable surfaces to mitigate against flooding / run-off, counter poor air quality and allow for heat absorption.
12.31 Mechanical air conditioning will utilise more energy and generate significant amounts of additional greenhouse gases and thus should be avoided where possible.
12.32 As addressed in Policy DLP33, the use of trees in landscaping schemes can generate significant natural shading. The layout and orientation of new houses should also be considered carefully, to avoid existing or newly planted trees creating excessive shading during cooler, darker times of the year.
Evidence
- UK Climate Projections: Headline Findings September 2019
- Research into Overheating in New Homes MHCLG September 2019
- Overheating in Dwellings BRE Guidance Document 2016
- Historic England Energy Efficiency and Historic Buildings
Delivery
- Development Management and Building Regulations processes.
