Independent temperature and humidity control may be required for a variety of reasons. One application under study at the NASA Johnson Space Center is the environmental control of completely sealed plant growth chambers. The chambers are used to optimize plant growth and to develop engineering prototypes of future plant growth chamber modules for long duration space travel. One chamber at the Johnson Space Center which is part of the Early Human Test Initiative was rebuilt and upgraded during 1994. Requirements called for a thermal control system which could supply the plants with a wide range of air temperatures and independently control humidity.A math model was developed using G189 thermal/environmental modeling software to simulate the internal environment of the plant growth chamber. The model was used in the design of the chamber thermal control system. Temperature and humidity in the chamber are controlled with heaters, condensing heat exchangers which remove both sensible and latent heat, coldplates which remove only sensible heat and atomizing sprayers which inject moisture if required. The G189 model was used to investigate various schemes for maintaining temperature and humidity levels over a range of plant growth conditions. Parameters such as heat exchanger coolant flow, coolant temperature, blower speed and water addition were all investigated in various configurations to maintain the optimum chamber conditions and optimize the energy efficiency of the chamber.Previous plant growth experiments had used a control scheme in which the chamber dew-point temperature was maintained during a crop cycle by controlling the condensing heat exchanger coolant flowrate at a fixed coolant temperature. Early in the crop cycle there was little moisture addition from the plants requiring removal by the condensing heat exchanger. This caused the controller to reduce the coolant flow to the heat exchanger. This maintained the dew-point within the chamber at an acceptable level, however the low coolant flowrate was not sufficient to remove the sensible heat load in the chamber and the chamber dry bulb temperature limit was exceeded. The new design adjusts coolant flowrate to control dry bulb temperature. Incorporation of a coldplate in the light box to remove some sensible heat allows the condensing heat exchanger to run warmer in order to achieve higher dew-points. Additional moisture can also be added by atomizing water sprayers if required.Analysis and tests have shown that the new temperature and humidity control system allows for chamber operation over a large operating envelope with minimal power consumption.