Urban heat islands: challenges and green roofs

There is no doubt about it, the effects of global warming are all too palpable, with longer and more intense heat waves becoming the norm in the summer. The European State of the Climate report shows that Europe has been warming faster than any other continent in recent decades, with temperatures increasing at twice the global average rate.

Urban Heat Islands (also called UHIs) are therefore a pressing environmental challenge faced by many cities across the world. As urban areas expand and intensify, the concentration of buildings, parking lots, roads, and other infrastructure significantly alters the local climate, leading to higher temperatures. This leads in turn to increased use of air conditioning, which exacerbates energy consumption and air pollution.

Several factors play a role, including heat generated from human activities, urban geometry, and climate conditions. As cities develop, natural elements such as grass, shrubs, trees, and bodies of water are replaced with pavement, asphalt roads and buildings which are usually made from dry, impervious surfaces that accumulate heat. This results in surface daytime temperatures that can be 10 to 15°C hotter.

Apart from a city's efforts to increase its green infrastructure, the urban heat island effect can be lessened by the implementation of green roofs. These are basically roofs covered with a layer of vegetation which act through shading and evapotranspiration.

The growing medium and vegetation on a green roof block sunlight from reaching the underlying roofing surface and, as a result, this lowers the absorbed heat. Vegetation absorbs water through their roots and releases it as vapor through their leaves, a process called ‘evapotranspiration’. This cools the air and reduces ambient temperatures.

Besides helping reduce the urban heat-island effect, added benefits of green roofs include:

• Added thermal insulation: green roofs provide natural insulation, reducing the amount of heat that penetrates building interiors and decreasing the need for air conditioning.
• Improved stormwater management: green roofs absorb rainwater, reducing runoff and mitigating the risk of urban flooding.
• Improved air quality: plants filter pollutants from the air, improving overall air quality.
• Enhanced biodiversity: green roofs create habitats for various species of plants, insects, and birds.
  

The European Union has been proactive in formulating policies to encourage the adoption of green infrastructure. The European Green Deal, launched in December 2019, is a comprehensive policy aimed at making the EU climate-neutral by 2050. The deal emphasizes the role of nature-based solutions, including green roofs, in enhancing urban resilience and reducing greenhouse gas emissions.

Several European cities have since recognized the benefits of green roofs and have already implemented policies and initiatives to encourage their adoption. Examples include Germany, which has integrated green roofs into its urban planning policies. Cities like Stuttgart and Berlin offer financial incentives and subsidies to promote green roof installation.

In Switzerland, the city of Zurich has implemented building codes that require all new flat roofs to be green. In France, a policy implemented in 2023 states that at least 30% of the roof surface of all new non-residential buildings must have a green or solar installation.

The widespread adoption of green roofs still faces several challenges. The upfront cost of installing a green roof can be higher than a traditional one, although this can be offset by long-term energy savings and environmental benefits. In addition, not all buildings can support the additional weight of a green roof, and this may require structural assessments and potential reinforcements. Also, some green roofs (intensive) may require regular maintenance, including watering, weeding, and plant replacement, which can be a barrier for some property owners.

Holcim is committed to supporting the roofing industry with Elevate sustainable solutions to build a safer and more sustainable future. Reach out to a sales representative near you and learn how your roof can help reduce the urban heat island effect.