Urban Heat

Following Britain’s unusually warm July, we focus here on summer overheating. According to the Met Office statistics, July 2014 has been one of the ten warmest and sunniest since 1910. It is becoming more and more common for the UK to witness hot weather in the summer months, with an increasing frequency during the past few years.

Climate change is causing an increase in the number and the severity of the heat waves expected in temperate European countries. This phenomenon is actually more intense in urban environments, due to human activities and degradation. In 2013, 80% of the UK’s population lived in urban areas, while in 2010 half of the people on a global level lived in cities. It is therefore important to address summer overheating especially in urban environments.

On a city level, air movement through and around buildings can prevent overheating, as it hinders the build-up of heat and pollution. Green and blue areas are also significant for maintaining temperatures at lower levels. The Met Office calculated that doubling green areas in London would decrease the average urban heat island by 35%, equal to 0.7˚C, while the implementation of green roofs and the use of reflective roof materials are also expected to be highly beneficial. It has been shown that in a city like Tokyo, implementing green roofs on a large scale can reduce temperatures by several degrees.  Finally, the changing building shapes and surfaces has the potential to reduce the urban heat island effect by up to 1˚C if this change is implemented across entire cities.

On the other hand, due to the increased number of projects aimed at building retrofits, it is crucial to understand the causes and the actual risks of summer overheating on a building level, allowing us to make informed decisions regarding building retrofit and transforming our current building stock. Doing so can have a significant impact on city-wide temperatures as research has demonstrated that the effect of building characteristics is more significant than that of the urban heat island. For example, buildings with very low or very high levels of insulation and those that cannot limit their solar heat gains, are the ones where higher temperatures are observed during the summer.

CIBSE Guide A: Environmental Design explains that the thermal performance of buildings is compared to a benchmark that should not be exceeded for more than 1% of the building’s occupied hours annually. Exceeding the specified temperatures will cause overheating. The summer indoor thermal comfort temperature for non-air-conditioned buildings is 25˚C for offices. Dwellings have different requirements, with the temperature limit being again 25˚C for living areas, but 23˚C for bedrooms. CIBSE Guide A warns that sleep may be negatively influenced by temperatures higher than 24˚C.

Overheating in dwellings can cause significant health problems for their occupants.  Research suggests that at its worst, overheating is responsible for approximately 2,000 deaths annually. Due to climate change and the expectation of rising temperatures, it is feared that this number could reach 5,000 in the 2080s’ unless action is taken. Therefore, it is crucial to address this issue on an urban and on a building scale.

One irony of new, more efficiently insulated homes is that they may be have a higher risk of overheating compared to older, less insulated ones.  Overheating in dwellings is a potential risk due to high insulation and air tightness levels which are common characteristics of modern constructions. These high levels of insulation and air tightness do not allow free air movement through the home and this lack of movement can lead to dangerously high temperatures.

Having said that, it is not impossible to build a modern, well-insulated home that does not overheat.  The position of insulation materials can play a significant in reducing the risk of overheating. For example, external rather than internal insulation is thought to minimise the risk of summer overheating. Reducing this summer overheating is particularly important in light of attempts to reduce carbon emissions as lowering summer temperatures will reduce the use of AC and, as a result, lower demand for electricity generation.

New homes are only part of the solution in avoiding overheating however.  Retrofits of existing homes will also be required.  Simulations have shown that living rooms of 19th century terraced houses could avoid overheating under 2080s’ scenarios with the necessary retrofit projects.  These include external wall insulation, use of solar reflective paint externally, shading and intelligent use of ventilation strategies. Some of these options, such as insulation, are actually beneficial in saving energy used for heating in the winter. Nevertheless, when fixed shading or solar reflective paints are used, there is a possibility that they could offset the benefits of additional solar heat in the winter and thus more energy is required for space heating. Therefore, careful and informed decisions are needed to balance the energy requirements for the whole year.

It is worth highlighting the role that building occupants have in avoiding overheating. As mentioned above, appropriate ventilation strategies can be a factor towards the avoidance of summer overheating, as well as shading. For example, making sure shading is installed and operated properly, as well as ensuring adequate ventilation at the times that it is most needed, can achieve considerable decreases in internal temperatures. These are simple measures but they are very dependent upon the occupants implementing them.

Finally, personal choices on activity and clothing levels are very important in ensuring personal thermal comfort and in avoiding the adverse effects of summer overheating. Reducing the clothing level as appropriate and lowering activity levels during the hours of the day when overheating is most likely to occur, are simple ways of ensuring better personal thermal comfort levels.  Combining these simple steps with more substantial building changes will ensure the adverse effects of summer heat can be avoided even as the temperature continues to rise.

Image Credit: Lamiot via Wikimedia Commons

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