A low-energy ventilation system is often incorporated as one of the major energy saving measures in sustainable building design. These systems often employ a hybrid strategy in which mechanical equipment, governed by a computer controlled building management system, is used to assist or manage a naturally-driven airflow - the latter occurring due to the density difference between warm air inside and cooler air outside the room. Hybrid ventilation flows are poorly understood and the principal aim of the research was to enhance our understanding of the fluid mechanics through complementary theoretical and experimental modelling. The research begins by comparing solely natural and solely mechanical ventilation of a room. The hybrid ventilation of a room is then considered under the combined effect of naturally occurring and mechanically imposed pressure differences, in which a mechanical fan imposes a fixed airflow rate through one vent, thereby altering the natural pressure distribution. Simplified theoretical models, to describe the ventilation airflow rate through a room and the resulting mean air temperature, were developed for solely natural ventila- tion, solely mechanical ventilation and, finally, hybrid ventilation. At each stage the theoretical model was compared with results from small-scale experiments, and good agreement was demonstrated. From the theoretical investigation, the neutral pressure level emerged as a key pa- rameter in determining the characteristics of the ventilation airflow. Moreover, it was found that the airflow rate through an open vent can be controlled remotely by managing the position of the neutral pressure level, and that this can be achieved by varying the magnitude of the mechanically imposed airflow rate. Experimental investigations revealed that, as the neutral pressure level approached the plane of a vent, quasi-steady pulsing flows and bi-directional or exchange flows were observed. The complicated fluid dynamics involved in these flows provides inspiration for significant future work.