A back draught shutter can take a couple of forms but performs the same function: They prevent air travelling back up the duct, through the impellers of the fan and into the room - particularly important in winter! Back draught shutters that are integral to the fan are as simple as a one way hinged membrane (they blow backwards when the fan is on, but cannot bend forward when it's off), or may be thermo-electrically controlled (such as the Elite range). External baffles also assist in reducing air back the ducting follow the link below for options on these.
The external baffles finishes off the installation. Baffles are fitted on the outside of the house; they prevent objects entering the duct run and can provide some protection from backdraughts. Colours available complement traditional and modern brickwork styles.