Quite simply, these devices provide important background ventilation as they can be left open to deal with ventilating an area in a trickle-like manner.
Trickle vents have been incorporated in many window designs and can be opened to provide fresh air to a reveal area. So, why do so many reveals display mould?
Quite simply they are either not utilised or, those that are, have collected dust and debris during their open period that has resulted in a restricted if not, compromised function. In addition, trickle vent performance can also be hindered by heavy net curtains that, when close to the window itself, will reduce the vents capability.
Passive vents fitted to walls serve to provide a steady flow of fresh air into a room. However, it is recognised particularly in winter periods, cold air freely enters making the room cold. In addition, this cold air will make the surrounding wall colder that could attract warm moist air when the room is used. The emphasis will always be on maintaining a balance of heat and ventilation in a home to make the installations effective.