Effects of shading and composition on green roof media temperature and moisture.

Three of the primary functions of green roofs in urban areas are to delay rainwater runoff, moderate building temperatures, and ameliorate the urban heat island (UHI) effect. A major impediment to the survival of plants on an unirrigated extensive green roof (EGR) is the harsh rooftop environment, including high temperatures and limited water during dry periods. Factors that influence EGR thermal and hydrologic performance include the albedo (reflectivity) of the roof and the composition of the green roof substrate (growing media). In this study we used white, reflective shading structures and three different media formulations to evaluate EGR thermal and hydrologic performance in the Pacific Northwest, USA. Shading significantly reduced daytime mean and maximum EGR media temperatures and significantly increased nighttime mean and minimum temperatures, which may provide energy benefits to buildings. Mean media moisture was greater in shaded trays than in exposed (unshaded) trays but differences were not statistically significant. Warmer nighttime media temperatures and lack of dew formation in shaded trays may have partially compensated for greater daytime evaporation from exposed trays. Media composition did not significantly influence media temperature or moisture. Results of this study suggest that adding shade structures to green roofs will combine thermal, hydrologic, and ecological benefits, and help achieve temperature and light regimes that allow for greater plant diversity on EGRs.

A framework for optimizing hydrologic performance of green roof media.

One of the primary functions of green roofs in urban areas is to moderate rainwater runoff, and one of the major impediments to the survival of plants on an extensive green roof (EGR) is a lack of available water during dry periods. Runoff moderation and water storage are both influenced by the composition of the growing media. Here we present a framework for evaluating the hydrologic performance of EGR growing media and also provide hydrologic attribute data for several commonly used EGR media constituents. In this three-phase study, we: 1) measured hydrologic attributes of individual EGR media constituents, 2) predicted attributes of media mixtures using individual constituent data, and 3) tested the seven top-ranking mixtures to evaluate hydrologic performance. Hydrologic attributes included wet weight and water held at maximum retentive capacity, long-term water retention, and hydraulic conductivity. Because perlite was light in weight yet held the greatest amount of water both at its maximum retentive capacity and in the long term, media mixtures dominated by perlite were predicted to have the best overall hydrologic performance. Mixtures dominated by pumice were also predicted to perform relatively well but were heavier. Despite the slightly greater weight and slightly lower performance, pumice may be a preferred alternative to perlite because perlite is a processed constituent with greater estimated embodied energy. Results indicate that performance of mixtures can be adequately predicted using performance of individual constituents for wet weight, water held, and long-term water retention. Hydraulic conductivity was less predictable because the pore volume in mixtures can be unrelated to the pore volume of the individual constituents. The framework presented here can be used to evaluate the performance of other EGR media, and the media attribute data can be used in formulating EGR media mixtures for specific applications. In addition, the attribute data can serve as a benchmark for evaluating other EGR media. Our results underscore the need for standardization of methods for more effective comparisons of EGR substrates, and also reinforce the need to evaluate EGR components using real-world scenarios.