The role of materials in heat accumulation
A low albedo(1), i.e. a low reflective power of a physical body, determines excessive thermal accumulation, with continuous release from surfaces directly exposed to the sun; if these are made of dark-coloured, poorly reflective and low-inertia materials, a large part of the solar radiation is absorbed and transformed into sensible heat, rather than being rejected. In this case, the surfaces of buildings and road surfaces, especially if made of asphalt, emit a large amount of heat.
This phenomenon is accentuated in the presence of materials consisting of a low thermal capacity, such as concrete (1000 j/kgK) or the asphalt of driveway surfaces (920 j/kgK) or low-thickness brick bricks (from 1000 to 1050 j/kgK), which absorb daytime heat and slowly release it at night, effectively preventing the cooling of the casing of the surfaces exposed directly to the sun’s rays during daylight hours.
Urban greenery, building density and radiation trap
The absence of evapotranspiration in cities is common where only asphalt and concrete exist, while urban greenery and water are absent. In these urban spaces, solar energy is converted into an increase in temperature, producing a phenomenon that is the exact opposite of what happens in cultivated, planted and lawn areas, where the fluid contained in the soil and plant essences is released, cooling the environment.
Then there is another phenomenon that involves high-density inhabited centers, especially the peripheral ones where, from the Second World War to today, intensive volumes of public housing have been created, limiting the distances between them within the legal minimum and where the presence of public green spaces was often relegated to a few poorly maintained flowerbeds. This is the “radiative trap”, a phenomenon of thermal stasis caused by the limited distance between buildings which, by counteracting each other in the release of thermal accumulation, effectively hinders the dispersion of the accumulated heat, favoring the rebound of infrared radiation between the buildings.
The contribution of human activities to urban warming
Last but not least, the anthropic effect must be mentioned, i.e. the heat produced by human activities, such as heat engines and accumulations of the body of motor vehicles, the thermal releases of air conditioners and cooling units and industrial machinery, to name the main ones, which generate further waste heat which contributes to the thermal accumulation of the air.
The above concerns phenomena that are unfortunately common to urban agglomerations and which have a negative impact on the liveability of common spaces and homes, also with important repercussions on public health. In fact, overheating increases body dehydration and affects breathing, especially in frail people, many of whom suffer from asthma problems.
Excessive urban heat favors an increase in ozone in the air and increases the deposition on the ground of fine dust emanating from fossil combustion, just as the increase in air temperature requires greater consumption for the start-up of cooling systems, with excessive peaks in electrical energy demand and consequent greenhouse gas emissions.
Strategies to mitigate the heat island
What solutions can be made to avoid or mitigate these effects? The main solution, frequently named by the various studies and experiences of urban projects carried out in Italy and abroad, most often takes into account mitigating and not radically resolving works, such as the increase and distribution of free areas intended for lawn, tree-lined avenues, the expansion of parks, the insertion of green roofs, vegetal facades (or worse, planted balconies such as the Vertical Forest), as well as fountains, lakes, flood areas of rivers and streams, or by implementing a greater distance between buildings where building speculation can be avoided. These are all praiseworthy and also necessary works, useful not only to counteract the action of summer heat and increase evapotranspiration, but also to contribute to greater well-being, in terms of liveability, of the inhabitants.
Unfortunately, however, a very simple resolution action is missing, which should be considered the most important of all and which historical experience has taught us: changing the materials, replacing those with low inertia and low specific heat with more performing ones.
Learning from historic centers to design the cities of the future
At the beginning of this text I mentioned the problem of the heat island which would exclude historic centres. With this note I was referring to the centers of ancient formation, especially those of medieval origin, made up of a road network with a curvilinear layout, but also to those with a road network of Roman formation, with rectilinear streets determined by the original castrum, but with an alternation of squares, even planted squares, and frequent widenings, as well as narrowings in the alleys(2), where the wall surfaces and pavements were built from stone, high-quality materials. inertia and of high thickness, which do not absorb daytime heat, places where air circulates freely and summer breezes were created.
Therefore, learning from our past, we should first eliminate asphalt roads from urban centers, which in summer melt under the incident heat and deform, replacing them with reflective and hygroscopic materials. The presence of asphalt for road paving could remain only along the extra-urban connecting roads, those that pass through the rural countryside and through the woods, where the action of the heat is largely mitigated by the presence of plants. Let us remember that asphalt is a direct vehicle of pollution of the aquifers, where the polluting vector is rainwater which, flowing on the surface, upon disposal brings with it traces of the hydrocarbons with which the paving material is made and of the heavy metals deposited on the surface.
In new constructions we therefore learn to manage house projects with simple shapes and to use appropriate construction and insulating materials, of natural origin, but, above all, of a renewable type, with high inertia, with a high specific heat, which counteract the summer heat wave, with reflective external surfaces and which at the end of their life can be reused, without constituting useless waste.
Notes
(1) The albedo is the reflective power of a surface which translates into the quantity of solar radiation that is reflected by a body compared to the total incident radiation.
(2) When an air current encounters a forced narrowing, such as a road alley between tall buildings, due to the Venturi effect it tends to accelerate to keep the flow rate constant.
