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Study on roof system reflectivity and near surface air temperatures in Chicago Illinois


Cool Roof Proceedings


5/2005


reflectivity; thermal resistance; Chicago Energy Code; near surface air temperature; urban heat

In June 2001, the City of Chicago revised its energy code. As a result, all low-slope roof systems constructed were prescribed to have a minimum thermal resistance along with an initial solar reflectivity of 0.65. After three years, the roof reflectivity was to achieve a minimum of 0.50 (washed or unwashed). A Roofing Industry Alliance was formed and a study team was established to evaluate the on-site reflectivity values of low slope roof systems in Chicago. Further work helped developed solar reflectivity criteria for using light colored roof aggregate. This effort led the City of chicago to change the solar reflectivity elvel in its energy code to a minimum (aged) value of 0.25 until 2009, pending a study of the impact of different roof systems' solar reflectance on urban air temperatures. This paper outlines the practical findings of Voogt and Krayenhoff regarding near surface air temperatures in Chicago, as part of the Roofing Industry Alliance effort. It describes the modeling of roof systems and walls in Chicago's downtown loop area and in a dense, urban, residental area. The findings indicate that if all roof systems were changed from black (solar reflectivity of 0.06) to white (solar reflectivity of 0.65 or greater), a decrease in average annual daily temperatures of less than 1C would occue in the residential area. In the downtown loop area, advection (horizontal air movement) significantly reduces the effects of sumertime daily average and maximum air temperatures. This paper also addresses theoretical temperature changes resulting from changing roof systems' reflectivity in both downtown loop and dense, urban residential areas.


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