The heat island phenomenon raises the temperature of cities, increases the energy demand for cooling and deteriorates comfort conditions in the urban environment. To counterbalance the impact of the phenomenon, important mitigation techniques have been proposed and developed. Absorption of solar radiation by the materials of the building envelope and the city structure contribute to increasing urban overheating. Use of solar reflective materials, known as cool materials, helps to decrease the surface temperature by up to 20 °C and the ambient urban temperature by up to 2 °C (1). In parallel, it can contribute to decreasing the cooling load of buildings by up to 40 %, the peak indoor summer temperature by up to 5-6 °C, the concentration of harmful pollutants and in particular of ground level ozone, and the heat related mortality and morbidity, all the while improving indoor and outdoor thermal comfort (2).
Reflective, cool materials present a high reflectance to solar radiation combined with a high emissivity factor, thus these materials do not absorb much and can easily lose the stored energy. White materials present a high reflectivity that can reach 0.95 both in the visual and infrared solar spectrum; however, also coloured reflective materials exist that present a high reflectance in the infrared part of the solar spectrum, to cater to different users’ needs and local architectural requirements (3).
Cool materials can be used either on the envelope of the buildings, as cool roofs or in the urban structures as cool pavements and cool streets. When applied on roofs, materials of very high reflectivity can be used. However, when used on vertical surfaces, pavements and streets, reflectivity should not exceed 50 %, to avoid problems of glare and contrast. In Europe, most of the cool materials are accredited and their performance is certified by the European Cool Rood Council. A list of the certified materials can be found in (4). The various types of existing cool materials for buildings and pavements can be found in (1,5). Examples of successful projects, as well as data on the accredited materials and other useful information on reflective materials can be found in the final report of the Cool roofs council project of EU (6).
Surface temperature differences before and after the cool roof application as depicted by an infrared camera for the Athens case study.
Visual and thermal images of the roof of an industrial building in the Netherlandsbefore the cool material application (7)
Visual and thermal images of the roof of an industrial building in the Netherlands after the cool material application, (7)
LONGEVITY and REUSABILITY
Cool materials present similar mechanical properties as the conventional ones. Optical ageing of cool materials can be a significant problem especially for the non-certified products. Information on the long-term reflectance of cool materials is provided for all certified products by the European Cool Roof Council (4). The use of materials presenting a minimum loss of solar reflectance is highly recommended. When paints or coatings are used, water-based materials containing zero or minimum VOCs have to be selected.
Surface temperature of the Cool shaded, (AR01), the conventional non-shaded, (AR02), and the cool non-shaded pavement, (AR03) In the Floisvos urban Rehabilitation project. .
Reflective Cool asphalt in Athens. Source (9)
RECYCLABILITY and NEW MATERIALS
Ceramic, asphalt, or concrete based cool materials can be recycled as any other conventional material of the same nature. Especially concrete pavements should be fully recycled and reused as a filling material for new pavements or other building processes. Most of the used cool membranes can be also fully recycled.
Cool Roof Technologies and Cool pavement products are available and can be used on a commercial basis.
Advanced super cool materials presenting sub ambient surface temperature will be available for commercial deployment by 2023.