Urban Heat Island effect (UHI) is one of the most challenging issues we face as architects, working towards mitigating the effects of climate change and urban heating. The UHI effect causes heat bubbles to envelope urban areas and cities, as vegetation is replaced by grey infrastructure and dark dense surfaces (particularly roads and building surfaces). This is a major issue for cities where grey infrastructure overpowers all natural landscapes and lacks greenery. The choice of building surface materials mitigates the excess radiant heat from infrastructure, lowering the higher temperatures which bubble cities.
As roofs are the primary surfaces on buildings which face the sun, material choices are very important for reducing the surface temperature and surroundings. At Noho we consider all aspects of the roof material. A lighter colour roof will have a lower temperature than a darker roof, but there can be issues with glare. Impermeable materials cause much greater surface temperatures, but buildings must be waterproof and durable. The challenge for our projects is to balance these aspects with the location and other needs of the building, as well as budget.
As roofs are the primary surfaces on buildings which face the sun, material choices are very important for reducing the surface temperature and surroundings. At Noho we consider all aspects of the roof material. A lighter colour roof will have a lower temperature than a darker roof, but there can be issues with glare. Impermeable materials cause much greater surface temperatures, but buildings must be waterproof and durable. The challenge for our projects is to balance these aspects with the location and other needs of the building, as well as budget.
Two different roof types can reduce urban heat, known as cool roofs and green roofs. Cool roofs are made of impermeable materials with a higher reflectivity, due to additives to the base material. Green roofs are permeable and grow vegetation. Cool roofs are often made of colorbond, but can be other materials, which are very effective at cooling surfaces and surrounding temperatures, due to their high albedo (ability to reflect sunlight). Their high reflectivity mitigates stored heat radiation, resulting in maximum reductions in urban heat island effects and energy savings (less need for air conditioning).
Green roofs have great cooling effects, with further thermal insulation benefits all year round, making it a great energy saver. Soil and vegetation on green roofs absorb radiant heat. In addition, plants provide further cooling through transpiration, with the added benefit of improved air quality. At Noho we encourage the use of native plants, which has added heat reduction benefits.
Our project Zyan house uses two green roofs to reduce surface and surrounding heat temperatures, as well as increase thermal envelope and the energy costs of the building. The result is a thermally comfortable home for the occupants, and leads as an example of sustainable houses which are designed to mitigate the effects of urban heat island effect. Creating cooler urban areas is crucial as we prepare for our inevitably hotter future. Designing with nature and thoughtful materials; cool roofs and green roofs, can mitigate radiant heat from building infrastructure.
References:
Aguiar, A.C., 2012. Urban Heat Islands: differentiating between the benefits and drawbacks of using native or exotic vegetation in mitigating climate. University of Wollongong.
Ziter, C., Pedersen, E., Kucharik, C. and Turner, M., 2019. Scale-dependent interactions between tree canopy cover and impervious surfaces reduce daytime urban heat during summer. Proceed¬ings of the National Academy of Sciences, 116(15), pp.7575-7580.
Zinzi, M., & Agnoli, S. (2012). Cool and green roofs. An energy and comfort comparison between passive cooling and mitigation urban heat island techniques for residential buildings in the Mediterranean region. Energy and Buildings, 55, 66-76.
Dwivedi, A., & Mohan, B. K. (2018). Impact of green roof on micro climate to reduce Urban Heat Island. Remote Sensing Applications: Society and Environment, 10, 56-69.
Imran, H. M., Kala, J., Ng, A. W. M., & Muthukumaran, S. (2018). Effectiveness of green and cool roofs in mitigating urban heat island effects during a heatwave event in the city of Melbourne in southeast Australia. Journal of Cleaner Production, 197, 393-405.