The urban heat island effect is an increasingly worrying reality, the result of unsustainable construction and massive urban development without environmental considerations. For this reason, sustainable certifications such as LEED or VERDE include the reduction of this overheating with dedicated credits.
In this article, we explore what this effect is, how to reduce it and how to justify these actions in order to obtain the corresponding credits in sustainability certifications, as well as presenting a case study of the application of sustainable measures.
What is the heat island effect?
The heat island effect is the excessive warming that occurs on the surface of a city. This phenomenon is based on the increase in temperature due to the reflection of the sun on construction materials such as asphalt. In other words, cities, developed with low-reflectance materials, retain heat, leading to overheating of the urban environment.
This effect is exacerbated by a number of common urban planning practices:
- High density of buildings: Urban areas with a high concentration of buildings have less space for vegetation, which reduces the amount of shade, a crucial factor in cooling the environment.
- Impermeable surfaces: Streets, pavements and roofs are typically made of impermeable materials that absorb and retain heat. These surfaces prevent the absorption of water into the soil, which could help to cool the environment through evaporation.
- Sparse vegetation: The lack of green areas and vegetation in general contributes to the rise in temperature.
In short, urban design and urban development play a key role in creating and magnifying the heat island effect. Addressing these problems through sustainable urbanism strategies is essential to mitigate the impact of urban warming.
Heat island effect in the city of Barcelona. Source: National Geographic
Urban heat island reduction strategies
There are various strategies to reduce the heat island effect, which vary depending on whether they are applied in parts of buildings or in open areas.
In the buildings:
- High solar reflective materials: Use materials with high solar reflectance, especially on the roofs of buildings. An example is the use of paints or films with a high Solar Reflectance Index (SRI). The reference value for a roof is an SRI of 82. These lighter-colored materials reflect sunlight more effectively, thereby reducing heat buildup in buildings.
- Green roofs: In addition to reflective roofs, green roofs are also an excellent option. These not only help to reduce the heat island effect, but also bring additional benefits such as biodiversity enhancement and rainwater retention.
- Vegetated options in low-lying areas: In lower areas and walkways, where the use of reflective materials is not feasible due to the risk of glare, it is preferable to opt for vegetated solutions that add freshness to the environment, such as vertical gardens or surfaces covered with plants.
In open spaces:
- Shadow generation: Create shade in open spaces, such as car parks, by planting trees or installing shade structures such as canopies. Vegetation not only provides shade, but also improves air quality and urban aesthetics.
- Pavement selection: Choose suitable materials for paving in open spaces. For example, the use of light concrete that reflects sunlight or draining paving (turf) that allows heat dissipation and improves storm drainage.
How do sustainability certifications address the heat island effect?
LEED certification
LEED certification includes the "Heat Island Reduction" credit, which seeks to minimise its effects on microclimates and human and wildlife habitats. This credit can be obtained through two options:
Option 1: Strategies for roofs and non-roofed surfaces
This credit gives 2 points, except for health care projects, which get 1 point.
In order to obtain this option, a number of sustainable measures must be applied to the roof and non-roof surfaces of the building. Several of these measures have already been discussed in the previous section.
Measurements on roofs:
- Highly reflective materials: Use roofing materials with a minimum Solar Reflectance Index (SRI) according to the roof slope.
- Vegetated roof
Measurements on non-roofed surfaces:
- Vegetation: Use plants that provide shade over paved areas.
- Power generation structures: Provide shade with structures covered by energy systems such as solar panels.
- Architectural devices: Use devices with an aged solar reflectance (SR) value of at least 0.28 or an initial SR of at least 0.33 if aged data is not available.
- Vegetated structures: Provide shade using vegetation-covered structures.
- Paving materials: Use materials with an aged SR of at least 0.28 or a starting SR of at least 0.33.
- Open grid flooring systems.
Option 2: Covered parking
This credit gives 1 point. To meet the credit, at least 75% of the parking spaces must be covered. Roofs must have an aged SRI of at least 32 or an initial SRI of at least 39, be vegetated or covered by energy generation systems such as solar panels.
VERDE Certification
The VERDE certification also includes a credit dedicated to the reduction of the heat island effect, focusing on strategies that promote a cooler and healthier urban environment. This is the PE 08 Heat Island Effect credit. To meet this credit, projects must integrate landscaped surfaces with at least 5 cm of topsoil, permeable paving that allows water absorption, light-coloured finishes that reflect solar radiation and strategically placed shading elements.
The assessment process involves calculating and documenting the plot and roof areas that meet these requirements, as well as the east, south and west facades treated to reduce the heat island effect. It is crucial to provide detailed plans, calculation reports, construction details and sunlight studies to justify compliance with each indicator.
Case study on the implementation of measures
An example of a strategy for the application of measures to reduce the heat island effect is the new student residences of Grupo Moraval located in Vicálvaro and operated from September 2024 by Nodis.
In all these residences, which are eligible for LEED certification, the same strategy is followed to ensure not only compliance with the aforementioned credit, but also a real reduction of the heat island effect. In general, the strategy is based on minimising paved areas, maximising green areas and using light-coloured materials in areas such as the roof.
Its strategy is based on the following points:
- On the ground floor, the paved area is minimised as much as possible. In areas to be paved, materials with a high Solar Reflectance Index (SRI) are used in combination with turf. The entire paved area complies with the Non-Roof Measures requirement discussed above.
- Green areas are maximised and native species with low water requirements are planted
- On the ground floor, walls with an SRI greater than 33 are added, which helps to reflect solar radiation and reduce heat build-up.
- On the roofs, light-coloured gravel is added, which minimises the heat island effect due to its high reflectance.
These measures not only contribute to the credits required for LEED certification, but also significantly improve the thermal comfort and sustainability of the urban environment.
In conclusion, the urban heat island effect is a significant challenge that can be mitigated through sustainable strategies promoted by sustainable certifications such as LEED and VERDE. The use of highly reflective materials, green roofs and the maximisation of green areas are essential to reduce this phenomenon.
