Dynamic U-value estimation and energy simulation for green roofs

Green roofs cooling and thermal insulating features have been demonstrated in many research projects. However, all efforts mostly have assessed green roof thermal properties under steady state conditions or by computational modeling. The present study evaluated green roof thermal performance in terms of the thermal transmittance coefficient, in real scale and under dynamic conditions. For the study's purposes, five semi-intensive green roof systems were constructed on the roof of an outdoor test cell. It was found that the green roof with 8 cm thick rock wool substrate with 2 cm sod on top had a very low U-value. For the same level of substrate moisture content, the other two green roof systems made of 8 cm deep coarse aggregate substrates with 2 cm sod on top provided higher U-values. In contrast, deeper amounts of same substrates (20 cm) reduced the U-value. The relation between the estimated thermal transmittance and the substrate moisture content was investigated and found to be linear. The green roof systems were also simulated for a single-storey residential building in order to quantify their possible energy savings. The results from the simulation showed that shallow substrates conserve building energy mainly during the summer period of the year. Rock wool and deeper substrates showed significant cooling and thermal insulating features. (c) 2011 Elsevier B.V. All rights reserved.