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Abstract
It is well known that the mass of a ballasted roof can reduce peak roof temperatures and delay the heat flow into a building. Although ballasted roofs perform these “cool” functions, they do not meet the traditional requirement of high solar reflectance. This is one of the criteria set out by the Environmental Protection Agency (EPA) and other organizations in order for a roof to be “cool.” To address whether ballasted roofing systems offer energy efficiency benefits similar to cool roofs, a project to perform side-by-side experiments was initiated. Different loadings of stone-ballasted roofs and an uncoated paver-ballasted roof were compared to roofs under exposed black and white membranes. The six test sections were constructed and installed on a test building at the Oak Ridge National laboratory and monitored for energy performance for thirty-six months. One year into the project, systems with two loadings of pavers coated with a white coating were added. They were monitored along with the other six systems for the rest of the thirty-six month period. Data collection included continuous monitoring of temperatures, heat flows and weather conditions as well as periodic verification of the solar reflectance of each surface. These data answer what impact a ballasted roof has on heat flow into a building and on roof surface temperature. Furthermore, comparisons between the ballasted and unballasted membranes allow for an assessment of whether ballasted systems perform as well as white membranes and are deserving of “cool roof” status within the codes. The cooling loads show that the heaviest and medium stone-ballasted and the uncoated paver-ballasted systems perform as well as the white system. The coated paver systems outperform it. This report also describes the modeling of the energy performance of all systems with the Simplified Transient Analysis Roofs (STAR) program. STAR does well for light weight roofs with exposed membranes. Reasonable values of effective specific heat and thermal conductivity were sought for the ballasted systems in order to get good agreement between the predicted and measured membrane temperatures and insulation heat fluxes on several clear and sunny days during the project. These properties were used to predict annual cooling and heating loads for comparison to the measured loads for all three years of the project. Modeling of ballasted roofs was desired so that the experimental results from east Tennessee could be generalized to more typical R-values than the low R-values selected to maximize experimental responses and to other climates. The modeling verified that the measured trends persisted with more typical roof R-values and in locations with more severe cooling requirements than during the tests.
It is well known that the mass of a ballasted roof can reduce peak roof temperatures and delay the heat flow into a building. Although ballasted roofs perform these “cool” functions, they do not meet the traditional requirement of high solar reflectance. This is one of the criteria set out by the Environmental Protection Agency (EPA) and other organizations in order for a roof to be “cool.” To address whether ballasted roofing systems offer energy efficiency benefits similar to cool roofs, a project to perform side-by-side experiments was initiated. Different loadings of stone-ballasted roofs and an uncoated paver-ballasted roof were compared to roofs under exposed black and white membranes. The six test sections were constructed and installed on a test building at the Oak Ridge National laboratory and monitored for energy performance for thirty-six months. One year into the project, systems with two loadings of pavers coated with a white coating were added. They were monitored along with the other six systems for the rest of the thirty-six month period. Data collection included continuous monitoring of temperatures, heat flows and weather conditions as well as periodic verification of the solar reflectance of each surface. These data answer what impact a ballasted roof has on heat flow into a building and on roof surface temperature. Furthermore, comparisons between the ballasted and unballasted membranes allow for an assessment of whether ballasted systems perform as well as white membranes and are deserving of “cool roof” status within the codes. The cooling loads show that the heaviest and medium stone-ballasted and the uncoated paver-ballasted systems perform as well as the white system. The coated paver systems outperform it. This report also describes the modeling of the energy performance of all systems with the Simplified Transient Analysis Roofs (STAR) program. STAR does well for light weight roofs with exposed membranes. Reasonable values of effective specific heat and thermal conductivity were sought for the ballasted systems in order to get good agreement between the predicted and measured membrane temperatures and insulation heat fluxes on several clear and sunny days during the project. These properties were used to predict annual cooling and heating loads for comparison to the measured loads for all three years of the project. Modeling of ballasted roofs was desired so that the experimental results from east Tennessee could be generalized to more typical R-values than the low R-values selected to maximize experimental responses and to other climates. The modeling verified that the measured trends persisted with more typical roof R-values and in locations with more severe cooling requirements than during the tests.
Date
4/2008
4/2008
Author(s)
A Desjarlais; T Petrie; J Atchley; R Gillenwater; D Roodvoets
A Desjarlais; T Petrie; J Atchley; R Gillenwater; D Roodvoets
Page(s)
31
31
Source
Oak Ridge National Lab
Oak Ridge National Lab
Keyword(s)
energy performance; ballast; solar reflectance; STAR program
energy performance; ballast; solar reflectance; STAR program