To decrease the heat gains of buildings, reflective or cool materials can be applied on the roof or the facades of buildings (1-3). Reflective materials are characterized by high solar reflectance (SR) combined with a high thermal emittance value (4).
Numerous reflective white or light-colored materials are currently commercially available for buildings presenting solar reflectance values ranging from 0.4 to 0.9, and emissivity values close to 0.9. Reflective materials present a much lower surface temperature than conventional materials of dark color. For example, under solar conditions of about 1000 W/m2, an insulated black surface with solar reflectance of 0.05 and under low wind speed conditions, presents a surface temperature up to 50 °C higher than ambient air temperature, while for a white surface with solar reflectance of 0.8, the temperature rise is about 10 °C (5).
Cool Materials
Figure 1: Visible and infrared images of cool [1,2] and standard [3,4] black coatings applied on concrete tiles, (15)
Reflective coatings can contribute to reducing the surface temperature of a concrete tile by 7.5 °C, and it can be 15 °C cooler than a silver-grey coating [6,7].
Parallel to the development of white reflective materials, a new technology of colored infrared reflective materials has been developed and is commercially available, (8,9). Use of infrared reflective materials increases the solar reflectance of the commercially available dark products from 0.05–0.25 to 0.30–0.45, (9).
Recently, the development of daytime radiative photonic cooling technologies has permitted to decrease the surface temperature of the building materials at sub ambient levels, (10). Photonic materials coolers exhibiting an extraordinary solar reflectance combined with a high value of emissivity in the atmospheric window, can operate at sub ambient surface temperatures, (11). Sub-ambient photonic materials are already available for building applications. A review of the developments and recent achievements in the field of daytime radiative cooling technologies is given in Ref. [12].
Cool Roofs
Figure 5: A cool roof on an industrial building in the Netherlands. Visual and infrared image after the implementation of the cool roof, (17).
Use of reflecting materials contributes to lowering the surface temperature of the building materials since solar radiation is reflected rather than absorbed. As a result, the heat penetrating into the building is considerably decreasing, indoor temperatures are lower and the need for air conditioning is significantly reduced.
The use of reflective materials in vertical facades may create a visual discomfort and an energy surplus to the neighboring buildings as lighting and solar radiation is reflected to them. To overcome this problem, retroreflective (RR) materials have been recently developed as an effective solution for vertical facades, (13).
Retroreflectivity refers to the ability of a specially engineered surface to preferentially reflect incident radiation back towards its source regardless of the direction of incidence, (14). Numerous retroreflective products are commercially available and can be implemented on vertical facades to increase the reflectance of the surfaces without creating any optical and energy burden to the surrounding buildings.
Reflective materials suffer from ageing problems and their optical characteristics worsen as a function of time. Regular cleaning of the roofs is necessary.
The present document aims to provide knowledge, information, and recommendations on the use reflective materials on roofs and facades of buildings.
Retroreflective Materials
Figure 6: A commercial retroreflective material, (14).
Figure 7: A retroreflective façade. Credit (18).
MATURITY:
The technology of cool roofs and cool facades is very mature, almost all products are rated and certified by the European Cool Roof Council, (21), and can be used to provide indoor comfort and decrease the cooling needs in buildings. The use of retroreflective materials is not so common in buildings, however, the existing commercial products are of very high quality.
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