An animal house ventilation system (1) of the invention comprises a roof (2) having a medium level pitch and vents (3) in walls of the building. The roof (2) supports exhaust chimneys, including some chimneys (4) having both baffles (6) and exhaust fans (7), and chimneys (5) having only baffles (8), without fans. A computer-based controller (10) is linked with sensors including temperature, pressure, ammonia (NH3) and carbon dioxide (C02) sensors, and at its output controls the vents (3), the exhaust fans (7), and the exhaust baffles (6, 8). The controller (10) causes the system (1) to operate as a natural ventilation system utilizing the vents (3) and all of the chimneys (4, 5) in the roof as openings for ventilation until a point is reached where there is not enough air movement through the house under natural conditions to ensure the comfort of the animals. At this stage the fans are started at a low speed and, when this happens, the baffles (8) in the chimneys S (those without fans) are closed by their baffles to ensure that there is no downdraught. The inlet vents (3) can then continue to open and at the same time the fans are gradually increased in speed, and continue to increase in speed after the inlet vents (3) have reached a position of fully open to keep the temperature in the room at the required level. <；Figure 4>；
