Soiling of building envelope surfaces and its effect on solar reflectance – Part II: Development of an accelerated aging method for roofing materials
Introduction
Highly reflective roofs can decrease the energy required for building air conditioning, help mitigate the urban heat island effect, and slow global warming. Replacing a conventional dark gray roof with a solar-reflective white roof can reduce a commercial building's annual conditioning energy use in the US by about 20% [1]. Similar effects have been reported for different European climates [2], [3], [4]. Widespread use of cool roofs can lessen the urban heat island effect by lowering outside air temperatures. This can further decrease conditioning energy use by another 10%, and reduce the temperature-dependent rate of smog formation [5]. Replacing 100 m2 of a dark roof (solar reflectance, or “albedo,” 0.20) with an aged white roof (albedo 0.60) induces a negative radiative forcing in the global atmospheric energy balance sufficient to offset the emission of 28 t of CO2, worth US$700 at US$25/t CO2. This is equivalent to a 7 kg m−2 CO2 offset per 0.01 increase of surface albedo. Increasing the albedo of the roofs for all cities between latitudes 45°S and 45°N by an average of 0.25 could offset about 90 Gt of CO2, equal to about 3 years of global CO2 emissions [6], [7].
The initially high albedo of a "cool" roof can be reduced by soiling and weathering [1], [8]. Soiling processes include deposition of atmospheric black carbon, dust, and organic and inorganic particulate matter, as well as microbiological growth. Weathering includes exposure to moisture, ultraviolet (UV) radiation, and diurnal temperature cycles. Installed roofing products may take several years to reach a quasi-steady reflectance, modulated by seasonal variabilities in precipitation, dust deposition, and air pollution, as well as by physical and chemical changes to the exposed materials. These weathering processes have been documented by Berdahl et al. [9].
Part I of our current series on the aging of roofing materials [10] analyzed the initial and aged radiative properties of hundreds of products rated by the Cool Roof Rating Council (CRRC) or by the Energy Star program of the US Environmental Protection Agency (EPA). Part II (this article) details the chemical, physical and microbiological nature of soiling agents, and the transformations that occur on building envelope surfaces. It then describes the application of this information to develop a laboratory method that replicates 3 years of natural exposure within a few days.
Section snippets
Composition of soiling present on urban surfaces
Deposition of atmospheric particulate matter is the dominant source of soiling agents accumulating on exposed building surfaces. Thus, the composition of material deposited on buildings reflects the main constituents of atmospheric particulate matter [11], [12]. Glass substrates exposed in six European cities collected four major constituents of particulate matter emitted from human activities, generated by natural processes, and formed in the atmosphere: dust minerals (28–66 wt%), black carbon
Rating of aged roofing materials
The CRRC product rating program [40] and the US EPA's Energy Star labeling program [41] each report values of solar reflectance and thermal emittance of products in new condition and after 3 years of natural outdoor exposure. While a roofing product is undergoing this three-year exposure, the California Energy Commission's “Title 24” building energy efficiency standard [42] calculates for compliance a provisional value of aged solar reflectance based on initial solar reflectance. Part I of this
Selection of tested roofing materials
Initially, 19 roofing products in seven categories (asphalt shingle, tile, single-ply membrane, metal, modified bitumen cap sheet, factory-applied coating, and field-applied coating) were used to develop the accelerated aging method. Table 1 lists the range of colors and initial solar reflectances of the tested products. These products were selected from more than 100 roofing materials provided by 40 US and international roofing manufacturers. Selection criteria included diversity of products
Conclusions
The high solar reflectance of cool roofing materials typically decreases with natural exposure. This study provided an overview of natural soiling and presented a novel laboratory accelerated aging method that combines soiling and weathering and simulates 3 years of outdoor exposure. Several features of the proposed method make it a valuable tool to help industry develop better-performing cool roofing materials, and help code bodies implement cool roof requirements.
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It is accurate and widely
Acknowledgments
This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Building Technologies Office of the US Department of Energy under Contract No. DE-AC02-05CH11231. The authors thank Marc LaFrance, Karma Sawyer, Patrick Phelan and Alexis Abramson of the Department of Energy (Office of Energy Efficiency and Renewable Energy, Building Technologies Office) for program management and support; and Riccardo Paolini (Politecnico de Milano) and George Ban-Weiss (University
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2021, Urban ClimateCitation Excerpt :The term solar reflectance, or albedo (Karimi et al., 2020; Tran et al., 2009; Turner and Parisi, 2018), refers to the total reflection of a specific system, considering the hemispherical reflectance of radiation, and integrated over the solar spectrum. It is defined as the specular and diffuse reflectance (Sleiman et al., 2014) related to infrared emittance (Gentle et al., 2011; Pisello, 2017). Moreover, when the material has high albedo in urban areas, the high infrared of material can reradiate previously absorbed heat compared to a black body working at the same temperature.