Fan Seal

The Emerald Planet Fan Seal is an innovative way to block unwanted air flow between the ceiling space and your home through ceiling exhaust fans, when they are switched off.
 
The simple one-way vents allow normal airflow when the fan is on, but seal closed as soon as the fan is turned off.
 
Can be installed from inside the house - no ceiling access required. More info: https://www.emeraldplanet.com.au/2012-07-19-12-08-45/fan-seal