What is the effectiveness of white roofs to reduce the temperature of buildings and houses in summer

It has long been known that painting the roof of a building in white reflects the sun's rays and reduces its temperature.

In a recent interview with the BBC, the former secretary general of the

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Ban Ki-moon, suggested that this reduction can reach 30 ° C and that the internal temperature of the building can be reduced up to 7 ° C.

In how many degrees does the heat actually decrease when you paint a ceiling in white and what are the disadvantages of this technique?

Ban Ki-moon was talking about a pilot project in the city of Ahmedabad, in the state of Gujarat, in western India, where summer temperatures can reach 50 ° C.

Known as "cold roof", this process is designed to reduce the absorption of solar radiation, which reduces the heat transferred to the building's interior.

In 2017, more than 3,000 roofs in the city were painted with white lime and a special reflective coating.

Known as "cold roof", this process is designed to reduce the absorption of solar radiation, which in turn reduces the heat transferred to the building's interior.

Cold roofs also relieve some of the heat that buildings normally keep, which makes them even cooler.

The planning document for the Gujarat project indicates that a reflective roof "can help lower the roof temperature up to 30 ° C and that of the indoor temperature from 3 ° C to 7 ° C".

"Depending on the configuration, cold roofs can help keep indoor temperatures between 2ºC and 5ºC lower than traditional roofs," said Anjali Jaiswal of the US Natural Resources Defense Council. UU., Who oversaw the Ahmedabad project.

In another pilot project in Hyderabad, southern India, a cold roofing membrane was used, which reduced the internal temperature by an average of 2 ° C.

These ranges are slightly lower than those given by Ban Ki-Moon, but they remain a significant drop.

30ºC colder?

But how is it true that the roof temperature itself can drop to about 30 degrees Celsius, as Ban Ki-Moon said?

A study done by the Berkeley laboratory in California can give indications.

According to research, a white roof that reflects 80% of sunlight remains about 31ºC colder one summer afternoon.

But California's conditions are very different from those in India, where over 60% of the roofs are metal, asbestos and concrete, which retain heat inside buildings, even treated with a white coating.

However, the cities of Ahmedabad and Hyderabad have had enough success with their pilot projects and have launched broader programs on cold roofing in 2019.

So, why are not there more cities with white painted ceilings?

Global cooling

Of course, the idea of ​​cold roofs is not new: roofs and white walls are a typical image for centuries in the countries of southern Europe and northern Africa.

Recently, New York City has painted nearly one square kilometer of white roofs.

Other places, like California, have updated building codes to promote cold roofs, considered an important way to save energy.

Recently, New York City painted nearly one square kilometer of white ceilings

A cold roof can reduce air conditioning costs by 40%.

An experiment in the city of Bhopal, central India, found that solar reflective paint used in low-rise buildings reduced energy expenditure by 303 kilowatt-hours during the hottest periods of the summer. .

There are even estimates of the potential reduction in global carbon emissions if cooling paint is used on the roofs of all major cities in the world.

The Berkeley Lab says that the use of reflective roofs around the world could produce an equivalent global cooling effect that would stop producing 24 gigatons of carbon dioxide or remove 300 million cars from runways for 20 years.

disadvantages

Without a doubt, cold roofs are an inexpensive option, especially in the poorest countries.

The differences can be considerable in terms of personal comfort and energy saving for cooling.

For cities with colder winters, reflective roofs can increase the demand for heating and the condensation of moisture on the ceiling creates a risk of mold.

But "the political will plays an important role," says Jaiswal of the Natural Resources Defense Council. In addition, there are possible disadvantages to consider.

For cities with colder winters, reflective roofs can increase the demand for heating and the condensation of moisture on the roof creates a risk of mold.

This is the reason why a team from University College London, UK, decided not to use white paint for a real estate project in New Delhi. But there are more reasons.

"The locals were also opposed to painting their white roofs because they were using them for other purposes," said Renu Khosla of the Delhi-based Center for Urban and Regional Excellence.

The highly reflective paint makes it difficult to climb ceilings for warehouses and daily housework.

The Reality Check Team – BBC News