A concept for a greenhouse dehumidifier has been designed. Important constraints for the design are low-energy consumption and homogeneous greenhouse climate. From a survey of dehumidifying methods, condensation to a cooled surface was selected as most promising. Low-energy demand is achieved by natural air circulation through the system and by recovering sensible heat. A homogeneous climate can be realized by decentralized local dehumidification in the greenhouse. Applying a computational fluid dynamics (CFD) program to simulate fluid flow and heat exchange enabled great improvements in the design. A vertical geometry was chosen first in a double-chimney approach to exploit the vertical distance between inlet and cold surface and that between cold and hot surface for natural convection air circulation. However, CFD calculations indicated stagnating flow in this vertically oriented system. Orienting the system horizontally greatly enhanced the systems performance. A separate model for condensation has been created to complement the CFD programme that did not include condensation. A prototype of the designed dehumidifier was built and tested. Calculations and experiments were in fair agreement and demonstrated the potential for practical application.