Urban Design Solutions

Mitigating the effect of urban heat islands

Green Roofs

Increasing the number of green roofs in San Francisco could provide additional insulation for buildings -- reducing the amount of heat entering buildings during summer and retaining heat during winter, leading to less reliance on heating and cooling systems. This reduction in energy consumption directly helps in reducing the urban heat island effect. Additionally, the increased evapotranspiration from green roof vegetation compared to concrete has a cooling effect. Studies show potential cooling effects at roof levels between .1 and 5 °C for extensive green roofs and .2 and 2.1 °C for intensive green roofs. This translates to a .5-1.7 °C cooling effect at street level.2

Cool Pavements

Permeable and water holding pavements or, cool pavements, could be widely implemented in San Francisco to provide cooling effects after rain or irrigation. While both pavements would aid in heat reductions, studies have shown that water holding blocks can reduce surface temperatures by 10 °C greater than permeable pavements with effects lasting for 5 days.3

Trees

Trees significantly contribute to urban cooling and environmental improvement. Strategically placing them in parks, urban canyons, and alongside roads enhances shading, helps disperse pollutants, and maintains ventilation.

Studies have demonstrated that trees in urban settings reduce Mean Radiant Temperature (MRT) and Physiological Equivalent Temperature (PET). Treed street canyons, for instance, show a notable reduction of 7.0 and 4.6 °C in MRT and PET respectively compared to treeless ones.2

By increasing tree count along sidewalks or in new parks, San Francisco can aim to mitigate the effects of urban heat islands. Notably, increasing tree and park count in historically redlined or marginalized areas could be one way to start to alleviate the current levels of disproportionate risk.

Sources

Wang, Xun, et al. "The effectiveness of cool and green roofs in mitigating urban heat island and improving human thermal comfort." Building and Environment 217 (2022): 109082.
Elliott, Helen, et al. “Improving City Vitality through Urban Heat Reduction with Green Infrastructure and Design Solutions: A Systematic Literature Review.” Buildings, vol. 10, no. 12, Nov. 2020, p. 219. DOI.org (Crossref), https://doi.org/10.3390/buildings10120219.
Nakayama, T.; Hashimoto, S. Analysis of the ability of water resources to reduce the urban heat island in the Tokyo megalopolis. Environ. Pollut. 2011, 159, 2164–2173.

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