Soiling of building envelope surfaces and its effect on solar reflectance – Part II: Development of an accelerated aging method for roofing materials

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Highlights

  • Developed accelerated aging method for a wide range of roofing products.

  • Method incorporates features of soiling and weathering.

  • Sprays a calibrated mixture of black carbon, salts, dust and organic surrogates.

  • Conditioning and weathering performed with a commercial weatherometer.

  • Calibrated to albedos measured in CRRC natural exposure program.

Abstract

Highly reflective roofs can decrease the energy required for building air conditioning, help mitigate the urban heat island effect, and slow global warming. However, these benefits are diminished by soiling and weathering processes that reduce the solar reflectance of most roofing materials. Soiling results from the deposition of atmospheric particulate matter and the growth of microorganisms, each of which absorb sunlight. Weathering of materials occurs with exposure to water, sunlight, and high temperatures. This study developed an accelerated aging method that incorporates features of soiling and weathering. The method sprays a calibrated aqueous soiling mixture of dust minerals, black carbon, humic acid, and salts onto preconditioned coupons of roofing materials, then subjects the soiled coupons to cycles of ultraviolet radiation, heat and water in a commercial weatherometer. Three soiling mixtures were optimized to reproduce the site-specific solar spectral reflectance features of roofing products exposed for 3 years in a hot and humid climate (Miami, Florida); a hot and dry climate (Phoenix, Arizona); and a polluted atmosphere in a temperate climate (Cleveland, Ohio). A fourth mixture was designed to reproduce the three-site average values of solar reflectance and thermal emittance attained after 3 years of natural exposure, which the Cool Roof Rating Council (CRRC) uses to rate roofing products sold in the US. This accelerated aging method was applied to 25 products–single ply membranes, factory and field applied coatings, tiles, modified bitumen cap sheets, and asphalt shingles–and reproduced in 3 days the CRRC's 3-year aged values of solar reflectance. This accelerated aging method can be used to speed the evaluation and rating of new cool 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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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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