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Green or white? Planted or painted roofs can cool buildings

It’s getting hot in the city, and our overheated cities are only going to get hotter still as more people pile in and development and energy use intensifies. But planting away the problem could be a surprisingly…

Growing a solution to the growing problem of urban warming. Alison Hancock/Shutterstock

It’s getting hot in the city, and our overheated cities are only going to get 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.

As more people move into urban areas and the effects of climate change steadily increase, it’s not surprising that urban warming is projected to worsen in the future.

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%).

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.

It’s getting hot in here… so consider planting shrubs. Gurdane Virk

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.

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.

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4 Comments sorted by

  1. Lorna Jarrett

    PhD, science educator and science advocate

    Hi Gurdane,
    "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%)."
    Can you please explain the connection between doubling the insulation and painting the roof white?
    As I understand it, a white roof works by reflecting visible light - whereas insulation reduces conduction of heat from the roof surface to the inside.

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    1. Gurdane Virk

      EngD student at University College London

      In reply to Lorna Jarrett

      Hi Lorna,

      The linked study analysed the annual energy balance for different city and roofing scenarios.

      Boston's energy balance will be dominated by heating loads due to its cold winters. The green roof reduced both heating and cooling energy, with greater savings in the summer. This is due to its cooling potential in the summer and the effect of the soil insulating the building in the winter.

      The cool roof reduced the cooling energy by a greater extent, but also increased the winter…

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  2. Dennis Anthony

    logged in via Facebook

    I can't remember now where I got this information but it stated that having a white car/van does will not be cooler inside compared with a black vehicle.

    True or not I wonder.

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    1. Gurdane Virk

      EngD student at University College London

      In reply to Dennis Anthony

      Hi Dennis,

      That is an interesting question. I suppose it really depends on the balance of physical interactions of the painted surfaces, the windows and the interior.

      The heat capacity would not be a major factor, as the materials are so thin, so there would be not be a large time lag involved.

      The white painted surface would reflect more incoming solar radiation and would be cooler than the black surface. The amount of radiation absorption and conduction would then be a factor.

      The…

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