Does whitewashing the roof reduce heat?

It is generally believed that whitewashing the roof of a building causes the sun’s heat to fall back on it, which is the best way to reduce the temperature.But how effective is this method and what are its negative effects?
Former UN Secretary-General Ban Ki-moon recently told the BBC in an interview that it could drop to 30 degrees Celsius and indoor temperatures could drop to 7 degrees Celsius.
Where do these data come from and what research proves them correct?
Ban Ki-moon was talking about an ongoing pilot project in Ahmedabad, Gujarat, India. In Ahmedabad, the summer temperature reaches 50 degrees Celsius.
In 2017, more than 3,000 roofs across the city were painted with special white paint.
This is a roof cooling process that reduces the heat generated by the sun so that the minimum heat reaches the inside of the building.
As much heat as the building absorbs, this process of cooling the roof also helps to reduce the temperature by expelling that heat.
The project documents state that this reflective roof layer can reduce the temperature of the roof by 30 degrees Celsius while the indoor temperature can be reduced by three to seven degrees.

Two to five degree drop in temperature

Anjali Jaiswal, of the US-based Natural Resources Conservation Agency, says after inspecting the Ahmedabad project, “It depends on the layout, but the process of cooling the roof, compared to normal homes, can increase the indoor temperature by two to five degrees.” Helps to reduce to.
This is slightly lower than Ban Ki-moon’s estimate but still important.
Another pilot project in the southern Indian state of Hyderabad used a membrane sheet to cool the roof, which found the indoor temperature to be less than two degrees lower than the outside.
As far as the temperature drop to 30 degrees is concerned, the Gujarat pilot project did not find any answer but such results have come to light in the Berkeley laboratory in the US state of California.
Research has shown that up to 80% of the light from a white roof hits and returns, and on hot summer days this roof can reduce the temperature by up to 31 degrees.
Of course, the situation in California will be different from that in India. Metal, asbestos and concrete are used in the construction of more than 60% of the roofs in which heat is present inside the building despite the layer of white paint.
However, pilot projects in Indian cities such as Ahmedabad and Hyderabad have been successful.
This is not a new idea
So why aren’t more and more city roofs being painted white?
The idea is not new, white roofs and walls have been seen in southern Europe and North African countries for centuries.
New York City’s 10 million square feet of roofs have recently been painted white.
Building codes have been updated to promote cold roofs in places like California. This is considered an important way to save energy.
Cool roofs can also reduce your air conditioning bill by up to 40%.
Low cost and savings too
An experiment in Bhopal found that sunlight-reflecting paint can save up to 303 kW of energy in buildings with low altitudes.
According to one estimate, if the roof cooling paint is applied to every roof around the world, it is likely to reduce global carbon emissions.
The Berkeley Laboratory says a concrete roof that reflects sunlight can cool up to 24 gigatonnes of carbon dioxide.
It’s like removing 300 million vehicles from the roads for 20 years.
Adverse effects
“Yes, there are some downsides,” says Jaiswal.
In cities where it is very cold or where the weather is cold, the roofs may need to be heated, but in doing so there is a natural risk of pressure from the roofs.
That’s why the University College London team did not use white paint in the restoration project in New Delhi.
Reno Khosla, at the New Delhi Centre’s Center for Urban and Regional Authority, says, “People here are against painting their roofs white because their roofs are used for many other purposes.”