BRE IP19/10 PDF

INTRODUCTION

In 2007, the number of people living in the world's urban areas exceeded the number living in the countryside. The proportion of city-dwellers was only 13% a century ago, a figure which had more than doubled by the 1950s. Such rapid development places many demands upon urban environments. Of particular concern is a type of local climate modification which can result from higher building densities – the urban heat island effect.

The morphology and materials of the built environment are not only more effective than natural surfaces at absorbing solar radiation, but they also retain a greater proportion of this thermal energy later into the night. The resulting increased night-time air temperatures can be as much as 5°C higher than in neighbouring rural areas, giving the characteristic appearance on a thermal map of a hot urban ‘island' in a ‘sea' of cooler countryside. Predicted global warming is expected to exacerbate these issues so an understanding of the heat transfer processes from urban streets and buildings is becoming ever more important. By selecting street geometries that maximise convective heat exchange, the layouts of towns and cities can be optimised to take advantage of the cooling effects of the wind. However, these heat transfer effects cannot be considered in isolation when designing urban areas as other factors must also be taken into account including:

• providing shelter from the wind for pedestrians

• dispersing pollutants (eg traffic fumes) from the streets

• maximising solar access into the street and especially to windows

• street aesthetics and compatibility with existing buildings

• creating a comfortable sense of enclosure for pedestrians

• the usage of the street (eg vehicular access)

• site constraints (eg size and topography).

The relative significance of these factors will vary between towns and cities[2], and between neighbourhoods in the same urban area, depending upon the local climate, the nature of the existing buildings and other such considerations. To take account of these often conflicting requirements, it is usually necessary to employ computer models to evaluate aspects such as daylighting or thermal performance. Design options can be simulated to optimise the arrangement of the buildings. Such a modelling exercise may not always be feasible, but some basic ‘rules of thumb' are available to the urban designer. However, these rules have not previously included any guidance on maximising the passive convective cooling of streets by the wind. Convective exchange is the least well understood of the heat transfer processes that take place from buildings (the others being radiation and conduction), but the rate of convection can have a significant impact on the energy balance both of individual buildings and the wider urban area. Better determination of the rates of convective exchange from various street arrangements has therefore been one of the topics addressed by recent research at BRE[3], which forms the basis of this Information Paper.