Energy Performance Aspects of a Florida Green Roof Part 2

Abstract

Green roof installation in the United States is growing at a significant rate. There are a number of reasons for this growth including rainwater runoff reduction and aesthetic benefits. Energy performance evaluations of green roofs, the subject of this study, are also becoming available. This monitored study is an evaluation of summer and winter energy performance aspects of a green roof on a 2-story central Florida university building addition that was completed in 2005. An earlier report on this study was published through the 2006 Symposium on Improving Building Systems in Hot and Humid Climates. This report reviews these earlier results and provides second-summer results which show significant performance improvements for the green roof compared with the first summer results. One half of the two-story project building’s 3,300 square foot project roof is a light-colored, conventional flat membrane roof, the other half being the same membrane roof covered with 6” to 8” of plant media and a variety of primarily native Florida vegetation up to approximately 2 feet in height to create an extensive green roof. Analysis of 2005 summer data from the first year the green roof was installed indicates significantly lower peak roof surface temperatures for the green roof compared with the conventional roof and a significant shift in when the peak green roof temperature occurs compared to the conventional roof. Data analysis of the same 2005 period also shows lower heat fluxes for the green roof. Calculations show the green roof to have an average heat flux of 0.39 Btu/ft2•hr or 18.3% less than the conventional roof’s average heat fluxrate of 0.48 Btu/ft2•hr. Analysis of 2006 summer data when the green roof was more established and conventional roof somewhat darker, shows even greater temperature and heat flux differences between the two roofs. The weighted average heat flux rate over the 2006 summer period for the green roof is 0.34 Btu/ft2•hr or 44.1% less than the conventional roof’s average heat flux rate of 0.60 Btu/ft2•hr. An additional heat flux analysis was performed for an April 1st 2006 through October 31st 2006 monitoring period to provide an estimate of heat flux for an extended cooling season. The weighted average heat flux rate over the period for the green roof is 0.25 Btu/ft2•hr or 45.7% less than the conventional roof’s average heat flux rate of 0.46 Btu/ft2•hr. Winter data again show substantially lower peak roof surface temperatures, higher nighttime surface temperatures and significantly lower heat flux rates for the green roof compared with the conventional roof. For periods during which the ambient air temperature was less than 55oF, the weighted average winter heat flux rate for the green roof is -0.40 Btu/ft2•hr or 49.5% less than the conventional roof’s average heat flux rate of -0.79 Btu/ft2•hr. Because of air conditioning zoning limitations, an extensive energy savings analysis was not possible for this project. However, an energy savings analysis was performed using the roof heat flux results and equipment efficiency assumptions. Based on this analysis the total estimated cooling and heating season savings for the green roof compared with the conventional roof, if the entire 3,300 square foot project roof were green, would be approximately 489 kWhr/yr.