We are seeing more and more publications and videos dedicated to high albedo coatings, whether they are “Cool Roof” type paints designed for roofs or light-colored floor coverings.
Albedo makes it possible to measure the quantity of solar energy that is reflected by a surface: the higher it is, the clearer the surface is and reflects a significant fraction of solar radiation, like a mirror.
Conversely, the lower the albedo, the more solar energy is absorbed by the surface and warms it.
We intuitively understand the benefit of a high albedo to limit the heating of mineral surfaces and combat the urban heat island phenomenon.
However, resorting to a high albedo is not a miracle solution to combat thermal discomfort in summer and painting all our cities white is far from desirable.
In addition to requiring regular maintenance in order to remain clear, floors and ground floors of facades with a high albedo can generate thermal stress for pedestrians during the day: in addition to direct radiation, they receive the radiation solar reflected by surrounding light surfaces, which can increase the thermal discomfort felt in summer.
A study published in 2014 showed that thermal stress felt by pedestrians was highest in streets whose ground treatment and facades benefit from a high albedo (0.7) compared to those which benefit from a medium or low albedo, independently of their morphology, their orientation and the climate.
This study did not take into account glare and visual discomfort which can also be produced by clear coverings (stabilized, stone slab, etc.).
Thus, the use of a high albedo for squares, sidewalks or roof terraces exposed to direct solar radiation is not desirable and must be carefully studied by specialists.
If “albedo is an essential concept to strengthen our adaptation to climate change at low cost” as Raphaël Ménard rightly reminds us, it is important to master its use well so as not to aggravate the discomfort of certain public spaces in periods summer.
There is no miracle solution for cooling our cities!
Sources: E. Erell, D. Pearlmutter, D. Boneh, P. B. Kutiel.
Image: University of Paris 1, ENVCAL