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This paper presents a comprehensive numerical study of the performance of a capacitive humidity sensor for heating, ventilation, and air conditioning (HVAC) applications. The proposed sensor comprises a sensing layer sandwiched between an array of top and bottom electrodes. A combination of both parallel plate and interdigitated electrode arrangements is considered to achieve their distinctive advantages. Polyimide is used as the humidity sensing material due to good sensing characteristics and aluminum is used as the electrode material because of the ease of fabrication. A layer of polyimide covers the top electrodes to provide protection from atmospheric contamination thus improving durability. The influence of relative humidity on the dielectric constant of the sensing layer is determined theoretically using the models of Looyenga and Shibata The model is validated by comparing model predictions with experimentally measured data for a previously reported capacitive humidity sensor. The model is then used to simulate and predict the performance of the proposed humidity sensor. The effects of design configuration, sensing layer thickness, electrode polarity, electrode width and thickness, and electrode gap are studied. The influence of operating conditions including relative humidity, temperature and voltage is investigated. Based on the simulation results, the optimum design configuration is identified.