White roofs are widespread in Bermuda, where they help keep buildings cool under the hot sun. Photo: Acroterion / Wikimedia Commons.
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Green or white? Planted or painted roofs can cool buildings
15th April 2015
As the world inexorably warms, Roger Kemp shows how we can help to keep our buildings cool with roof gardens - or just with white paint. And if enough people do it, entire cities will become more cooler, more pleasant places to be when hot weather hits ...
A cool roof was most effective at reducing air temperature during the day, when solar energy is greatest, whereas a green roof reduced air temperatures mostly during the evening.
It's getting hot in the city, and our overheated cities are only going toget hotter still as more people pile in and development and energy use intensifies.
But planting away the problem could be a surprisingly low-cost solution to create cool roofs that will reduce office temperatures and improve working conditions for millions.
One means to counteract this heat and cool things down is through planting various types of vegetation to form a green roof. Another option is to paint roof surfaces white, as is common in the Mediterranean, known as a cool roofs.
These simple measures can help reduce the urban heat island effect, where built-up areas experience higher air temperatures than rural areas due to higher-density buildings that trap heat.
As you'd expect, this leads to more overheating and energy use (from use of air conditioning) within buildings during the summer, and less energy needed for heating during winter.
The roofs of office buildings within central London are often flat and unused, offering up plenty of space for either plants or an easy white paint job. In our study we examined just how effective these measures could be in a London office, typical of any UK city.
Cool passive options
We found that the amount of heat felt by an office worker can be reduced by installing a green or cool roof, without the need for additional air conditioning. To explain the results in further detail, we need to understand how these roofs work.
A green, planted roof decreases the amount of solar energy that enters the building and provides added insulation. The air directly above the layer of plants is cooled, and this varies depending on whether the vegetation is sufficiently watered. Dried-out roof plants provide less cooling than irrigated ones, which may be a disadvantage in possible drier summers of the future.
Cool roofs are typically painted white because lighter surfaces reflect more light than darker ones. This lowers the air and surface temperature and less heat energy finds its way into the building.
How well a white-painted roof works depends on weather conditions. For example a building in Boston, US, which experiences warm summers and cold winters, saved more energy from heating and cooling by doubling the insulation (13%) than from planting a green roof (12%).
But a similar building in Lisbon, Portugal, with hot summers and warm winters made energy savings of 26% with a green roof, whereas insulation made almost no difference (0.01%).
Green and white cool at different times
Trying to establish just how much the air directly above the plants or white-painted roof was cooled, we examined a particular area around Victoria Station, London - an area that has been highlighted as a potential site for green and cool roofs.
Our findings showed that a cool roof was most effective at reducing air temperature during the day, when solar energy is greatest, whereas a green roof reduced air temperatures mostly during the evening.
The cooling effect of a green roof lasts longer, but both will have a cooling effect on the local environment and so reduce the urban heat island effect.
It's crucial that we fully understand the net effects of green and cool roofs, so we can predict the how they will change urban temperatures.
A hot future
Using a simulation, we also looked for the point of overheating - the level at which the building's occupants felt uncomfortably hot - in typical office buildings under two possible climate scenarios, one in today's climate and one in the projected climate of 2040-2069.
For the present day model, without the addition of a green or cool roof, the office building overheated for 8% of occupied hours over the summer period.
After adding a green or cool roof, this was reduced to less than 3% of occupied hours, with the cool roof being most effective. In the future climate scenario, the building overheated around 25% of the time - a significant amount.
But with the addition of a green or cool roof, overheating was reduced to 14% and 11% of occupied hours respectively.
By 2050, expect uncomfortable office heat
So our study shows that without adding some measure to counteract the heat, office buildings are set to become uncomfortably hot during the summer during projected 2050s temperatures.
Because of this, work is currently being completed at UCL that will show exactly how urban climate impacts will effect buildings, this research will also develop guidance on how to combat these issues.
This and other related studies highlight how in a changing climate, new and innovative designs will have to be considered to ensure comfortable temperatures without excessive energy use, with ongoing projects at UCL continuing to explore the nature of changes to the urban climate and sustainable ways of tackling the problem.
Gurdane Virk is an EngD student at University College London. He receives funding from the Engineering and Physical Sciences Research Council.
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