Back draft shutters are extremely useful within ventilation systems. If you use a single fan for your ventilation back draft shutters can be installed to allow air into the grow space when the extractor fan is running, when the fan shuts off the shutter will close. They can also be installed on the extract side of your ducting to prevent any unwanted flow of air, light and smell escaping from your ventilation.
They are especially useful when using CO2 enrichment as they help to prevent wastage. These may be needed when fans are switched off to prevent cold drafts entering the grow room.