Typical cooling methods for greenhouses include natural and mechanical ventilation, shading and evaporative cooling. For natural ventilation systems, larger vent openings often improve the ventilation performance. Open-roof greenhouses have particularly large vent openings. The effects of vent design on natural ventilation and the resulting temperature difference between the inside and outside environments of a conventional and two different open-roof greenhouse designs were studied experimentally during the summer and autumn. The roof vent designs investigated were: a conventional arched-roof vent type (AVT), roof sections hinging at the gutters and opening at the ridge (RV), and roof sections sliding on a lattice beam and opening at the gutter (GV). When the roof vents were fully opened in the RV and GV, the inside air temperatures were consistently significantly lower compared to the fully opened roof vent of the AVT. In addition, three different light conditions for open-roof greenhouses (GV) were investigated using no shading (Control), 50% shading and 100% shading (using typical internal and movable greenhouse shading screen materials). When the roof vents were fully open under either 50 or 100% shading, the resulting inside air temperatures were higher than the Control. Moreover, the wet-bulb globe temperature (WBGT, environmental heat stress index on humans) under 100% shading was significantly higher than under the 50% shading and Control. These results confirmed that the restricted ventilation due to the 100% shading was ineffective in reducing the air temperature in an open-roof greenhouse during high light and air temperature conditions. In addition, two cooling methods were examined: Control and 100% shading combined with fogging. For the latter, the resulting air temperature was lower and relative humidity was higher than the Control. Future work will hopefully result in an environmental control system for solar radiation, temperature, relative humidity, and carbon dioxide concentrations inside open-roof greenhouses.