07.08.2024

Green or white roofs: Which are more effective for cooling cities?

Green and white roofs are designed to lower city temperatures and cool people, but how exactly do they work – and which is more efficient? Green roofs involve planting vegetation on top, while white roofs involve painting. The purpose is to help lower the air temperature.

In the "urban heat island" effect, temperatures can often be 10 degrees hotter in cities than in rural areas, as tall buildings trap heat and materials such as asphalt and concrete absorb it.

Plants help alleviate this effect by storing heat, which is then released when they evaporate, it says euronews.com. Not only do they help lower daytime temperatures, but they bring numerous benefits to biodiversity.

Researchers from London found that white roofs are more efficient

Researchers from University College London (UCL) analyzed the temperatures of different roofs and environments in London during two hot summer days in 2018. Their study found that while green roofs were effective at lowering temperatures during the day, they actually increased ambient temperatures at night .

"Through radiation - that is, through transpiration, plants give off more heat in this form of latent heat (the heat you can't absorb), so basically it's water vapour," explains Oscar Bruce, Senior Research Fellow in Urban Climatology and Health at UCL .

Green roofs reduce air temperature, he adds, but because they then heat up an area at night, you get "zero average drop" in temperature. The findings suggest that white roofs may be a more effective roof modification system.

How do white roofs cool?

"Cool roofs" are sprayed with reflective properties, such as white paint.

"The principle is that the energy that comes from the sun is reflected back right into the atmosphere and into space, so that energy that comes in the first place just doesn't accumulate in the city, in the buildings, on the streets, etc. Bruce explains.

White roofs have the added benefit of being much lighter than green gardens, which in some cases can be prohibitively heavy for a building.

“The problem with whitewashing roofs is the fact that over time they get dirty and so their reflective capacity decreases and you have to make sure you always keep it as white as possible. This can be an expensive part of maintenance," adds Oscar Bruce.