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Title
Solar Reflectance Testing of Steep-slope Roof Systems in the Field
Solar Reflectance Testing of Steep-slope Roof Systems in the Field
Date
9/2011
9/2011
Author(s)
Wade Vorley
Wade Vorley
Page(s)
Keyword(s)
reflective; solar reflectance; ASTM E-1918; cool roof; pyranometer; slate; cleaing; historic preservation; sustainable
reflective; solar reflectance; ASTM E-1918; cool roof; pyranometer; slate; cleaing; historic preservation; sustainable
Abstract
This study is an evaluation of methods, techniques and standards for testing of roof surfaces for solar reflectance. Steep-slope roofing is highlighted and highly reflective low-slope roofing also is evaluated for validation of standards and cleaning of roof surfaces to restore reflectance. The evaluation of proposed ASTM-1918A, “Procedure for Measuring the Solar Reflectance of Flat or Curved Roofing Assemblies,” is central to this study. The importance of reflective roof surfaces to provide enhanced energy performance of buildings and for heat island mitigation is well-accepted in the roofing industry and construction industry as a sustainable design strategy. Reflectance testing in the field for existing roof surfaces is important for monitoring performance of reflective surfaces and the evaluation of existing surfaces during building renovations and historic preservation projects. Currently, many testing standards used to evaluate reflectance for existing roof surfaces have been withdrawn or are under review. This paper provides an evaluation of current and past roof reflectance testing standards, evaluates a proposed revision to ASTM International E-1918-06, “Standard Test Method for Measuring Solar Reflectance of Horizontal Low Sloped Surfaces in the Field,” and summarizes testing conducted at two test locations in Seattle. A review of papers written by others indicates that the proposed alternate test standard E-1918A is an acceptable method for roof slopes of up to 5:12 (23 degrees) with a standard deviation for reflectance of less than 0.01 and slopes between 5:12 (23 degrees) and 12:12 (45 degrees) with a standard deviation for reflectance of about 0.02. E-1918A also is reported to be acceptable for incident angles (defined as the sun angle to the normal from a surface) of up to 60 degrees. This study demonstrates that the proposed alternate standard E-1918A is an acceptable testing method for low-slope roofs (2:12 or less) at incident angles of as high as 60 degrees with a standard deviation in reflectance of 0.013, which represents about ± 2 percent of the reflectance value for highly reflective surfaces. For extreme slopes on steep-slope roofs greater than a 12:12 (45- degree) slope, it was determined the test method does not meet acceptable margins of error and should not be used.
This study is an evaluation of methods, techniques and standards for testing of roof surfaces for solar reflectance. Steep-slope roofing is highlighted and highly reflective low-slope roofing also is evaluated for validation of standards and cleaning of roof surfaces to restore reflectance. The evaluation of proposed ASTM-1918A, “Procedure for Measuring the Solar Reflectance of Flat or Curved Roofing Assemblies,” is central to this study. The importance of reflective roof surfaces to provide enhanced energy performance of buildings and for heat island mitigation is well-accepted in the roofing industry and construction industry as a sustainable design strategy. Reflectance testing in the field for existing roof surfaces is important for monitoring performance of reflective surfaces and the evaluation of existing surfaces during building renovations and historic preservation projects. Currently, many testing standards used to evaluate reflectance for existing roof surfaces have been withdrawn or are under review. This paper provides an evaluation of current and past roof reflectance testing standards, evaluates a proposed revision to ASTM International E-1918-06, “Standard Test Method for Measuring Solar Reflectance of Horizontal Low Sloped Surfaces in the Field,” and summarizes testing conducted at two test locations in Seattle. A review of papers written by others indicates that the proposed alternate test standard E-1918A is an acceptable method for roof slopes of up to 5:12 (23 degrees) with a standard deviation for reflectance of less than 0.01 and slopes between 5:12 (23 degrees) and 12:12 (45 degrees) with a standard deviation for reflectance of about 0.02. E-1918A also is reported to be acceptable for incident angles (defined as the sun angle to the normal from a surface) of up to 60 degrees. This study demonstrates that the proposed alternate standard E-1918A is an acceptable testing method for low-slope roofs (2:12 or less) at incident angles of as high as 60 degrees with a standard deviation in reflectance of 0.013, which represents about ± 2 percent of the reflectance value for highly reflective surfaces. For extreme slopes on steep-slope roofs greater than a 12:12 (45- degree) slope, it was determined the test method does not meet acceptable margins of error and should not be used.