A system for optimising one or more environmental parameters of an enclosed space comprising first and second relatively enclosed regions, such as a living space and a roof space within a building, is disclosed. The system comprises an internal sensor, in the first region, and an external sensor, external to the enclosed space, for measuring respective environmental parameters. The system also comprises a controller and a diverter valve. The controller is configured to specify the first region's ideal internal environmental parameter, and compare this specified parameter with the measured first region's internal parameter and the measured external parameter to determine whether the diverter valve directs air from the first or second enclosed region to exhaust through the ventilator.