Assessment of long term solar reflectance performance of roof coverings measured in laboratory and in field

doi:10.1016/j.buildenv.2011.09.004

Building and Environment

Volume 48, February 2012, Pages 164-172

https://doi.org/10.1016/j.buildenv.2011.09.004 Get rights and content

Abstract

Heat island refers to the temperature increase in urban areas compared to rural settings, intensifying the cooling energy consumption of buildings. Roofs constitute a major part in urban areas, hence cool roof coverings contribute to mitigate heat island. Heat island is an important issue for cities located in western and southern parts of Turkey, however cool roof coverings are not a common application. This study presents the assessment of long term solar reflectance performance of commonly used roof coverings in Turkey. Assessment is based on the data obtained from laboratory measurements conducted on new and 1-year aged several test specimens and field monitoring of red clay tile and red shingle using pyranometers for duration of one year. Laboratory results revealed that standing-seam profiled metal coverings and red colored mineral coated modified bituminous membrane required surface treatments to serve as cool roof coverings. Field results indicated that both the red clay tile and the red shingle showed a slight decrease in the solar reflectance due to weathering. Comparison of the new and 1-year aged solar reflectance values of red clay tile and red shingle measured both in laboratory and in field showed that there was no significant difference between the values.

Highlights

► Some metal coverings and red modified bituminous membrane require cool coatings. ► Red clay tile is a cool roof covering while red shingle is not. ► Field-laboratory results of new-aged clay tile and shingle were nearly the same.

Introduction

Heat island, which is related to higher temperatures in city centers compared to the surrounding suburban or rural areas, is a climatic phenomenon that has an important energy and environmental impact in the urban environment. Increased urban temperatures intensify cooling energy consumption, increase peak electricity demand and amount of greenhouse gases in the atmosphere [1], [2], [3]. Heat island occurs mainly due to the use of high solar radiation absorbing materials in urban areas in which there is high density of buildings and urban structures and the lack of green surfaces. In urban areas, pavements and roofs constitute a major part of urban surfaces [4], therefore it is best to use cool roof coverings, which have high solar reflectance and high infrared emittance to reduce cooling energy consumption [5], [6], [7], [8], [9] and to mitigate the heat island effect [10].

Solar reflectance and emittance of conventional and cool roof coverings have been studied by many researchers [10], [11], [12], [13] and reported in several databases such as US Cool Roof Rating Council, Energy Star Roof Products program, EU Project Cool Roofs and LBNL cool roofing materials database. Main roof coverings include single-ply membranes, modified bitumen membranes, coatings, built-up roofs, metal roofs, shingles and tiles. Based on the given studies and databases, Table 1 summarizes representative values of initial solar reflectance and infrared emittance of some conventional and cool roof coverings.

Several test methods can be used for measuring solar reflectance of roof coverings. ASTM E903-1996 [14], CIE 130-1998 [15] and ASHRAE 74-1988 [16] are laboratory test methods, which use a spectrometer with an integrating sphere to measure the solar reflectance of an area approximately 0.1-cm² of a flat and uniform test specimen. ASTM discontinued E903-96 test method in August 2005 in accordance with Section 10.5.3.1 of the Regulations Governing ASTM Technical Committees, which requires that standards shall be updated by the end of the eighth year since the last approval date. However, this method continues to be widely used by industries and researchers. ASTM C1549-2004 [17] uses a portable reflectometer to measure the solar reflectance of a flat and uniform surface of an area approximately 5-cm². In Europe, the use of portable reflectometer for measuring solar reflectance is not widespread apart from their use in the measurement of color [18]. ASTM E1918-2006 [19] uses a pyranometer to measure the solar reflectance of various horizontal and low-sloped surfaces with an area approximately 10-m², and is best applied to large surfaces that may also be rough and/or non uniform. In general, Standards C1549-2002 and E1918-1997 are used for measurement of solar reflectance of roof coverings in field, while E903 provides a laboratory method to validate the field methods [20].

Roof coverings are exposed to environmental agents, namely wind, sunlight, rain, hail, snow, atmospheric pollution, and temperature variations and consequently degrade over time. Even the most durable materials are modified by deposition of ambient dust and debris, and may provide an opportunity for colonization by biological organisms such as fungi and algae [21]. Aging and weathering can reduce the solar reflectance of roof coverings. Hence, for assessment of the long term performance of the coverings, not only the initial values but changes in solar reflectance and emittance over time must be taken into consideration. There are several widely used building energy-efficiency standards and programs, which define initial and/or aged solar reflectance and emittance values as cool roof requirements. ASHRAE Standard 90.1–2010: Energy Standard for Buildings Except Low-Rise Residential Buildings requires a minimum three-year aged solar reflectance of 0.55 and a minimum thermal emittance of 0.75. ASHRAE Standard 90.2–2007: Energy Efficient Design of Low-Rise Residential Buildings requires a minimum total solar reflectance of 0.65 and a minimum thermal emittance of 0.75. Environmental Protection Agency specifies initial and 3-years aged solar reflectance values that roof coverings shall meet as a criterion (low-slope roofs ≤ 2/12: initial solar reflectance > 0.65 and aged solar reflectance > 0.50, for steep-slope roofs > 2/12: initial solar reflectance > 0.25 and aged solar reflectance > 0.15). However, these requirements have two weaknesses. First, by specifying neither a minimum thermal emittance nor a minimum solar reflectance index, they permit the use of bare-metal roofs with high solar reflectance but low thermal emittance. Second, the minimum three-year aged solar reflectance required for a steep-sloped roof (0.15) excludes only the hottest of roofing materials, such as granule-surfaced fiberglass asphalt shingles colored with conventional dark pigments [22]. Studies have been conducted for measuring the solar reflectance of aged roof coverings. A field study measuring the effects of aging and weathering on 10 California roofs found that the reflectance of cool materials can decrease by as much as 0.15, mostly within the first year of service [10]. Another study demonstrated that an initial solar reflectance of 0.8 of a white roof membrane can be decreased by deposition of soot, dust, and/or biomass to about 0.6 [23].

The heat island effect has been documented in many cities worldwide [24], [25], [26], [27] and is an important issue for cities located in Turkey, as well. Studies based on historical temperature data of cities located at western and southern part of Turkey indicate that temperature increased as a result of urbanization notably in summer [28], [29], [30], [31]. Future climate projections demonstrate that these cities will experience temperature increases up to 6 °C [32]. Electricity consumption in residential buildings already constitutes 24% of the total electricity consumption while industry accounts for 46%, other sectors (services, agriculture, and forestry) 29%, and transport less than 1% [33]. Electricity production from combustible fuels holds the highest share followed by hydro and wind, solar energy source [34]. The use of combustible fuels for electricity production in Turkey has high greenhouse impacts. Turkey signed Kyoto Protocol in 2009 to undertake the obligations to reduce greenhouse gas emissions. Since the given date, Turkey has been promoting the use of energy efficient technologies in buildings to reduce energy consumption and new policies to ensure energy is supplied in the most sustainable way possible. Cool roof coverings are one of the environmentally friendly and energy efficient technologies, however their application is limited to a few examples in Turkey. Turkish designers, roof covering manufacturers and roof contractors lack knowledge on the benefits of cool roof and the solar reflectance performance of new and aged conventional roof coverings produced and commonly used in Turkey.

This study presents the long term solar reflectance performance of commonly used roof coverings in Turkey. Long term solar reflectance performance was assessed based on the data obtained from the laboratory measurements conducted on new and aged test specimens and field monitoring of two test specimens using two pyranometers acting as an albedometer for duration of one year.

Section snippets

Method

Roof covering types that would be used as test specimens in laboratory and in field measurements were determined based on a research, which presented the market share distribution of the roof coverings in Turkey for the year of 2007 [35]. According to the research, nearly 100 million m² of roof coverings were applied in Turkey every year and mostly clay tile was preferred followed by shingle and mineral coated modified bituminous membranes. Other commonly used roof coverings were galvanized

Laboratory measurements

The total solar reflectance rates and the solar reflectance rates at each spectral range of 12 new and 1-year aged 6 test specimens are provided in Table 3. Spectral range is given as ultraviolet (UV), visible (VIS), infrared (IR) and total. For example, the new red clay tile has solar reflectance rates of 0.06, 0.15, 0.62 and 0.36 in UV, VIS, IR ranges and in total spectrum, respectively. 1-year aged red clay tile has solar reflectance rates of 0.07, 0.16, 0.62 and 0.37 in UV, VIS, IR ranges

Discussion and conclusions

This study presents the solar reflectance of new and aged roof coverings commonly used in Turkey. Solar reflectances of several roof coverings were measured in laboratory in accordance with ASTM E903-96 [14] and in field in accordance with ASTM E 1918-06 [19], with the exception that the solar reflectance values were monitored continuously instead of being measured at specific times.

ASTM E903-96 [14] is a laboratory test method, which provides a general measure of solar reflectance for flat and

Acknowledgments

Authors gratefully acknowledge to Istanbul Technical University (ITU) and Kılıçoğlu for funding the research project. Authors express their appreciation to members of Glass Technology Group of Sisecam and Prof. Dr. Omer Lütfi Sen (Asst. Director of Eurasia Institute of Earth Sciences in ITU) for their technical expertise and support.

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Assessment of long term solar reflectance performance of roof coverings measured in laboratory and in field

Abstract

Highlights

Introduction

Section snippets

Method

Laboratory measurements

Discussion and conclusions

Acknowledgments

Energ Build

Solar Energ

Landscape Urban Plann

Energ Build

Energy

Energ Build

Energ Build

Energy Policy

Energ Build

Constr Build Mater

J Atmos Environ

Sci Total Environ

Atmos Environ, Part B: Urb Atmos

Solar Energ

Estimating the ecological footprint of the heat island effect over Athens, Greece

Climatic Change

Demonstration of cooling savings of light colored roof surfacing in Florida commercial buildings: retail strip mall

Measured energy savings and demand reduction from a reflective roof membrane on a large retail store in Austin

Literature review of uncertainty of analysis methods (cool roofs)

Standard test method for solar absorptance, reflectance, and transmittance of materials using integrating spheres