1 Citation: Shamshiri R., Ahmad, D.B., Wan Ismail, W.I., Hasfalina, C. M., Zakaria A.J., Beveren, P. Van., Yamin M. 2016. Comparative Evaluation of Naturally ventilated Screenhouse and Evaporative Cooled Greenhouse based on Optimal Vapor Pressure Deficit. Paper Number: 2454215. In proceeding of: ASABE 2016 Annual International Meeting, Orlando, Florida, USA, July, 17-20, 2016. Citation: Shamshiri R., Ahmad, D.B., Wan Ismail, W.I., Hasfalina, C. M., Zakaria A.J., Beveren, P. Van., Yamin M. 2016. Comparative Evaluation of Naturally ventilated Screenhouse and Evaporative Cooled Greenhouse based on Optimal Vapor Pressure Deficit. Paper Number: 2454215. In proceeding of: ASABE 2016 Annual International Meeting, Orlando, Florida, USA, July, 17-20, 2016. Comparative Evaluation of Naturally ventilated Screenhouse and Evaporative Cooled Greenhouse based on Optimal Vapor Pressure Deficit Ramin Shamshiri 1,2* , Desa Ahmad 1 , Wan Ishak Wan Ismail 1 , Hasfalin Che Man 1 , AbdJamil Zakaria 2 , Peter van Beveren 3 , Muhammad Yamin 1 1 Department of Biological and Agricultural Engineering, Universiti Putra Malaysia, Serdang, 43400, Malaysia 2 Institute AgroPolis, Universiti Sultan Zainal Abidin, Campus Tembila, Kuala Terengganu, Malaysia 3 Farm Technology Group, Wageningen University, P.O. Box 16, NL-6700AH Wageningen, The Netherlands *Corresponding author: ramin.sh@ufl.edu Abstract The objective of this study was to compare two closed-field plant production environments for tomato cultivation based on optimal vapor pressure deficit (VPD). Experiment was carried out in tropical lowlands of Malaysia by collecting 11 days of sample data during March (2014), from an evaporative cooled Polycarbonate Panel (PP) covered greenhouse and a naturally ventilated Screenhouse (SH). A computer application was designed and used for VPD calculation and data processing with respect to three light conditions (night, sun and cloud). The average and maximum VPD were respectively equal to 0.97 and 3.81 kPa for SH and 1.19 and 5.1 kPa for PP. The largest differences in the VPD of the two environments were between 2.9 and 3.1 kPa and were observed between hours of 12:30 and 17:30 at sun conditions. Results did not show significant differences in the two environments between hours of 00:00 and 8:00, when inside air temperature was between 24 to 26°C, and relative humidity was near 90%. The hypothesis that the PP, compared to SH, provides VPD closer to the optimal range was rejected. Further analysis of the results showed that linear correlations with R 2 >0.9 exist between daily averaged VPD of each greenhouses. It was concluded that VPD in the SH was closer to the optimal range in the entire days of experiment. The outcome of this study contributes to knowledge-based information for greenhouse growers by addressing questions about trends in VPD data, peak-hours and light conditions associated with maximum and minimum values. Keywords: Greenhouse, Screenhouse, Vapor pressure deficit, Optimal, Evaporative cooling, Natural ventilation, Tomato 1. Introduction Tropical lowlands of Malaysia have specific hot and humid climate with averaged ambient air temperature (T) and relative humidity (rH), respectively between 25.8-33°C and 82- 91%. High cost of cooling systems in these regions has motivated researchers to investigate possible techniques for shifting from energy consuming to energy neutral greenhouses. It is therefore necessary to study the effects of greenhouse design, covering materials and cooling systems on the resulting microclimate, for calculating plant’s reference evapotranspiration (ETₒ) and simulating growth responses. Evaluating microclimate parameters and monitoring conditions that cause evaporation stress is the first step toward this objective. Producing site-specific and knowledge-based information can provide grounds for developing and validating optimal greenhouse design and climate controllers with minimal cooling energy demands (Beveren et al., 2015a; Beveren et al., 2015b) in tropical condition. Protected cultivation of solanaceous crops such as tomatoes and peppers by means of Screenhouses operating on natural ventilation is commonly practiced in tropical lowlands to reduce insect migration on the crop and subsequent crop damage, reduce risk of damaged by high rainfall, extreme solar radiation and high wind speeds. Reports indicate that external and internal shading nets reduced horizontal and vertical gradients in air temperature