©2009 ASHRAE. THIS PREPRINT MAY NOT BE DISTRIBUTED IN PAPER OR DIGITAL FORM IN WHOLE OR IN PART. IT IS FOR DISCUSSION PURPOSES ONLY AT THE 2009 ASHRAE ANNUAL MEETING. The archival version of this paper along with comments and author responses will be published in ASHRAE Transactions, Volume 115, Part 2. ASHRAE must receive written questions or comments regarding this paper by August 3, 2009, if they are to be included in Transactions. ABSTRACT An interesting approach to reduce building energy consumption is to use green roofs as a part of building enve- lope. However, many building designers ignore this opportu- nity as it is quite difficult to estimate the resulting energy saving. This paper provides results from an ongoing experi- mental research project that focuses on the thermal perfor- mance of extensive green roofs when buildings are in the cooling mode. The paper discusses the importance of green roofs and reviews previous research studies. In particular, this paper focuses on the role of plants for the heat flux reduction through the roof structure. The performance of the plant mate- rial was assessed in an environmental chamber by experiments with two samples, one with the plant material, and another one without the plant material. Overall, plants reduced the measured heat flux through the green roof sample by 40-50% compared to the roof sample without plants. In conclusion, plants have an important role in reducing the heat flux by regu- lating: (1) latent heat flux through better water management and additional water storage in the plant leaves/roots, and (2) sensible heat flux through additional shading provided by the plant leaves. Based on these results, future research will focus on thermal modeling of green roof including the role of plants. INTRODUCTION Green roofs are an emerging sustainable technology that is becoming more popular in North America (Miller et al., 2005). As a definition, green roofs are “specialized roofing systems that support plant growth on rooftops” (Liu et al., 2004). From top to bottom, a typical green roof consists of several layers: (1) vegetation, (2) substrate, (3) filter membrane, and (4) drainage layer. Plants used for extensive green roofs are typically drought tolerant, and selected from the group of native or Sedum plants. Substrate is the soil-like layer where plants grow, and it has to be porous, retain mois- ture and nutrients, and support plant growth (Snodgrass et al., 2006). The filter membrane prevents drainage clogging by containing the substrate and roots. The drainage layer trans- ports the rainfall water runoff to the roof drainage (Peck, 2002; Snodgrass et al., 2006). There are basically two types of green roofs: extensive and intensive green roofs. Extensive green roofs have lower weight, lower capital cost, minimal maintenance, and a substrate depth between 2 and 6 inches (5 and 15 cm). Inten- sive green roofs have greater weight, higher capital costs, wider planting selection, higher maintenance requirements, and a substrate depth between 8 and 24 inches (20 and 60 cm). However, intensive green roofs are less cost-effective than extensive and required more structural support (Peck et al., 1999; Tanner, 2004). Moreover, extensive green roofs repre- sent about 2/3 of the total green roof square footage installed in North America (Johnston, 2007). Therefore, this research project focuses on summer thermal performance of extensive green roofs as a more economically viable solution to be adopted in the building industry. The popularity of green roofs is increasing due to their potential benefits. In general, green roofs have a potential to (Liu et al., 2004): • reduce energy demand on space conditioning • reduce storm water runoff • improve air quality, and • reduce the urban heat island effect in cities. The Role of Plants in the Reduction of Heat Flux through Green Roofs: Laboratory Experiments Paulo Cesar Tabares-Velasco Jelena Srebric, PhD Student Member ASHRAE Member ASHRAE Paulo Cesar Tabares-Velasco is a graduate student and Jelena Srebric is an associate professor at the Department of Architectural Engineering, The Pennsylvania State University, University Park, PA. LO-09-076 Authors may request permission to reprint or post on their personal or company Web site once the final version of the article has been published. A reprint permission form may be found at www.ashrae.org.