43 Comparing the Microclimatic Role of Horizontal and Vertical Vegetation to Improving the Thermal Comfort of Outdoor Spaces Between Buildings: A Case study (Faculty of Agriculture, I.K.I University), Qazvin. Fatemeh Sarhadi a , Sasan Moradi a, * a Department of Architecture, Faculty of Architecture and Urban Planning, Qazvin Branch, Islamic Azad University, Qazvin, Iran Received:04 February 2019 - Accepted: 13 June 2019 Abstract Vegetation moderates a microclimate by casting shadows, increasing solar reflection and evapotranspiration, and modifying wind patterns. The present study aims to investigate the microclimatic role of vegetated surfaces and structures in improving thermal comfort in outdoor spaces between buildings. The main research question is which green system—horizontal vegetation (green floor) or vertical vegetation (green façade)—is more effective in moderating a microclimate and improving thermal comfort in outdoor spaces? To find the answer, an academic building with vertical and horizontal walls facing an outdoor space (yard) was selected for the case study. Data were analyzed via numerical modeling (ENVI-met) and the RayMan software model. The Indices of PMV (predicted mean vote), PET (physiological equivalent temperature), Tmrt (total mean radiant temperature), and RH (relative humidity) were computed and analyzed to identify and analyze thermal comfort levels in outdoor spaces. The analysis results indicate that vegetation significantly affects thermal comfort in outdoor spaces between buildings in warm seasons by reducing PMV, PET, and Tmrt, while increasing RH. In all analytical models based on the indices of thermal comfort in outdoor spaces, the green floor performed better than the green façade due to a more extensive tree coverage on horizontal surfaces. The trees planted on horizontal surfaces and the ground improve thermal comfort in outdoor spaces by shading and blocking direct sunlight. Further, the results indicate that compared to vegetation, blocking direct solar radiation and providing shading on surfaces are much more effective in improving thermal comfort in outdoor spaces. Keywords: Horizontal greenery, Vertical greenery, Green cover, Thermal comfort, Spaces between building, ENVI-met software 1. Introduction Buildings significantly contribute to global and local climate change (Lassandro & Di Turi, 2017:183). Studies show that increasing the ratio of vegetation and high- albedo materials in urban areas can potentially reduce UHI (urban heat island) effects in cities (Imran et al., 2018: 2). Thermal comfort analysis, especially for outdoor spaces, is a great challenge due to a large number of environmental and personal (physiological and psychological) factors influencing it (Ahmadpour Kolahrodi et al., 2017: 60-63). Such analysis requires understanding the microclimate of an environment. Studying urban microclimates has heightened our knowledge about the thermal behavior of such environments, which, if incorporated in sound design strategies, can contribute greatly to improving living conditions in urban areas (Hatami, 2016: 3). Vegetation reduces the temperature of a microclimate by shading, increased reflection and evapotranspiration, and changing wind patterns (Karimian et al., 2014: 682). The façade of a building is one of the most influential components of the building’s energy consumption (Mohamed Farid et al., 2016: 174). The material used in the façade of a building greatly affects the consumption of non-renewable energies. Therefore, increased UHI not only increases urban temperature, but it also reduces the thermal comfort of persons and users of urban outdoor spaces. Consequently, vertical and horizontal vegetation systems can significantly improve thermal comfort in outdoor spaces. The study aims to investigate the microclimatic role of vegetated surfaces and structures in improving thermal comfort in the outdoor spaces between buildings. The main research question is which of the vegetation systems, including horizontal (green floor) and vertical (green façade), will more effectively moderate a microclimate and improve thermal comfort outdoors. Accordingly, an academic building in the Imam Khomeini International University of Qazvin, Iran, which features a central yard with vertical and horizontal walls facing outdoors, was used as the case for modeling and analysis. Numerical modeling (ENVI-met) and software modeling (RayMan) were used to identify and analyze thermal comfort and the respective affecting factors of the studied case. 2. Literature Review There have been numerous studies on vegetation in recent years. Herath et al., 2018 In a study in Colombo, Sri Lanka, the impact of vegetation on urban facades have been investigated by using ENVI software. The results indicate that vegetation can affect the weather conditions in a city and be used as a strategy to reduce the UHI *Corresponding author Email address: ssn.arc.95@gmail.com