to the urban heat island. Lower surface temperatures reduce the radiant field, significantly improving comfort, as well as producing less convected heat. External comfort can be improved by the use of high-mass materials in the shade to reduce operative temperatures in hot conditions – for example, stone colonnades are a common device to produce cooler external routes. High albedo (reflective) materials reduce
the amount of solar radiation absorbed into both the surfaces and the buildings within them. These materials include white paint and plaster, light-coloured stone and shiny aluminium. Surfaces should also be diffusive, avoiding any specular or glossy finishes to prevent direct reflection and glare.
A view over Dubai , United Arab Emirates, which claims to be incorporating sustainable development into the city’s rapid growth
will require shading for both pedestrians and buildings. Low ventilation rates in narrow streets are idea for cool shaded pedestrian routes, which may utilise other heat sinks to improve conditions further.
Natural ventilation Reduced anthropogenic heat gain and quieter streets will improve the viability of natural ventilation of buildings to provide comfortable working and living environments. Resulting improvements in air quality can reduce urban heat island impacts even further than the direct heat emissions. The provision of more comfortable external environments for pedestrians will reinforce walking, cycling and use of public transport. The potential for natural ventilation
Urban-scale passive design Building-scale passive design District systems Building systems District renewables
Building-integrated renewables Figure 1: A revised hierarchy of low carbon design
can be improved by the use of building geometry to provide high pressure differentials for cross flow ventilation. Differential building heights or wind catching structures can improve the wind environment available to ventilate buildings.
Materials Material selection has a significant effect on the urban environment, influencing surface energy balances and visual fields. Heavy materials provide a dynamic element to the urban environment, acting as heat stores and leading to elevated night time temperatures. In very hot climates this can have a
detrimental impact on evening comfort (and may be of benefit in cooler climates). This thermal-storage effect is a contributor
26 CIBSE Journal August 2011
Vegetation Planted surfaces tend to have significantly lower surface temperatures than hard surfaces. This reduces the radiant field and can cool air blowing over the surface. Vegetation can create a cooling effect not only within its own environment, but also upon its surrounding area. This can lead to a direct reduction of cooling load and greater use of the outdoors. The cooling effect of vegetated surfaces
is partly due to evapotranspiration from leaves, which occurs mainly at night. The upper part of a tree’s leaf canopy loses heat to the sky by transpiration and the leaves cool the air around them. This can be very useful in cultures where external areas are largely used at night. Solar radiation is mostly absorbed in
the leaves of plants, so that the reflected radiation is low and leaves can intercept solar radiation without producing undesirable reflection. The use of ground planting adjacent to shaded areas or windows can reduce reflected radiation. Green roofs and walls provide a heat sink as well as reduced convective and radiative heat fluxes compared with concrete. This effect is pronounced in enclosed zones in hot, dry climates. The benefits of vegetation should be achieved within appropriate water use.
Water The use of water in external environments provides cooler surfaces, reducing the mean radiant field, and evaporation can provide cooler air temperatures. At most angles (less than 80 degrees from the normal), water has very low reflectivity,
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