Cities absorb more heat than their surrounding rural areas, resulting in higher temperatures. This phenomenon is known as the urban heat island effect.
Building materials play a major role in the city’s thermal environment, especially when they are used to construct buildings. This article will discuss how the use of different types of construction materials can reduce the impact of the urban heat island effect.
Roofs
Rooftops account for a significant percentage of the area in cities, and can have a huge impact on local temperatures. Using cool or green roofs can have a positive impact on air quality, building energy consumption, and the urban heat island effect.
The basic concept behind these strategies is to reflect and absorb incoming solar radiation, or energy. The surface of most conventional roofs has a low so-called albedo, meaning that it reflects very little solar radiation. A white surface has a high albedo, while black asphalt has a very low one.
Green roofs can reduce the thermal environment by shading buildings, absorbing and releasing moisture, and deflecting sunlight. They can also provide a habitat for wildlife and help protect soil, water, and building structures from excessive heat. Moreover, they can help lower building energy consumption, thereby reducing carbon emissions and peak demand for air conditioning.
Several studies have explored the effects of greening rooftops on the thermal environment. One study used a 3D microclimate modeling program, ENVI-met, to compare the effects of five different scenarios that varied the packaging material and green coverage ratio. The results showed that green roofs with a high green cover reduced the ambient temperature by up to 2°C. Another study found that green rooftops can increase the comfort level of low-income residents by lowering their energy costs.
Pavements
The pavements of urban areas absorb and emit a great deal of solar energy, especially in the dark. This causes the city to become a heat island.
An important way to reduce the heat island effect is to cover streets, sidewalks and parking lots with a white or gray coating. This makes the surface much less reflective, and it also helps to reduce the amount of sunlight absorbed by the material.
In addition, using cool pavement materials can greatly improve a pavement’s albedo and emissivity. The emissivity of an object, which ranges from 0 to 1, determines how efficiently it radiates thermal energy. An ideal pavement material would have a low emissivity and high albedo, so it can reflect the majority of sunlight. Cool pavements can be made by replacing traditional asphalt with blast furnace slag, recycled aggregate or epoxy resin.
Urban green spaces can also help to mitigate the urban heat island effect. The types of trees used, their geometry and leaf area play an important role in reducing air temperatures. Developing a database on the native tree species, vegetative cover and evapotranspiration in cities can aid in optimizing the use of natural resources in cooling urban landscapes.
The urban heat island effect can have a number of negative effects on the health and quality of life of city residents. This is especially true in historically disadvantaged neighborhoods, where hotter conditions intersect with environmental inequities and social risk factors. The resulting higher energy bills from air conditioning can further stress households with lower incomes.
Walls
Researchers at UNSW Sydney have developed new building materials that reflect rather than absorb solar energy. These cool roofs, pavements and coatings can reduce peak urban temperatures by up to four degrees and save lives, according to Scientia Professor Mattheos Santamouris. These materials are designed to decrease the need for air conditioning and to make buildings cooler without it. This will also increase occupant comfort, as the temperature of the interiors can be controlled.
Walls are a major contributor to the UHI effect because they absorb and radiate thermal radiation. Buildings with dark walls have a greater impact on UHI than do buildings with light-coloured roofs. This is because the darker colour of a wall absorbs more solar radiation and emits more thermal infrared radiation than does a light-coloured surface.
Each building material has a unique thermal characteristic. Block walls, brick walls and aerated concrete walls have different density and specific heat values. The density of these materials determines how quickly a wall surface rises in temperature, and their specific heat value indicates how much thermal radiation the wall can absorb and store.
The cooling efficiency of cities depends on the presence of vegetation and trees that intercept the sunlight and provide shade, as well as by evapotranspiration. This process helps to cool the atmosphere by absorbing carbon dioxide, which is then used for photosynthesis. As areas dominated by paved surfaces lose their trees, the cooling capacity of these regions goes down dramatically. This is why it is important to use green materials in building construction, especially for streets and pavements.
Floors
The urban heat island effect can have serious health and environmental consequences. Cities and their surrounding areas tend to be hotter than surrounding rural regions because of the concentration of heat-absorbing buildings, pavements, and parking lots, as well as the lack of cooling green spaces like trees and grass. These neighborhoods are disproportionately impacted by high air temperatures, which lead to higher energy costs and harmful GHG emissions as residents attempt to cool their homes or businesses.
The negative effects of the urban heat island effect are especially troubling for historically disadvantaged neighborhoods, as they often have higher rates of pre-existing conditions and poverty that make them more vulnerable to extreme heat. Furthermore, the economic burden of high energy costs is particularly difficult for low-income households.
The good news is that there are a number of ways to mitigate the urban heat island effect, including using sustainable construction materials and adopting innovative road and street design. For example, the use of cool pavements that reflect rather than absorb sunlight and phase-change coatings that change color based on ambient temperature can help reduce urban surface temperatures. Another important strategy is increasing the amount of vegetation and green space in urban areas. Trees and vegetation help lower air temperatures by providing shade, releasing water through their leaves, and cooling the atmosphere through evaporation.