Every house needs ventilation, after all we need fresh air to breathe and we also want to extract stale air to remove excessive moisture, smells etc. However we don’t want gales blowing through the house taking away all of our precious heat in the winter. The trick with ventilation is to control it.
In the UK we have a history of excessive amounts of uncontrolled ventilation (draughts) through poor fitting doors and windows, old chimneys, floorboards etc. so in our haste to improve our homes many people have blocked up chimneys, replaced floorboards with concrete and fitted double glazed windows and doors. On the face of things, all of this is fine. However some rooms can suffer.
Bedrooms, bathrooms and kitchens are all subject to high levels of humidity at different times of the day. Cooking, washing and breathing may be all well and good, but they do leave a house with lots of water vapour to deal with. Kitchens and bathrooms are required to have extraction systems and this can deal with most of the humidity, however bedrooms are not and hence can find it more difficult to remove excess moisture. This is where trickle vents can come into their own.
Where new windows have been fitted without trickle vents you will often see condensation starting to occur. This in turn can cause mould. Whereas, where trickle vents are built in, the fresh air that they bring in and the stale air that they let out allows the room to feel much healthier and also reduces the risk of mould.
The balance then between losing heat and gaining fresh / extracting stale air is one where energy efficiency loses. Trickle vents are therefore an important element for most houses to possess. There are of course other systems of ventilation (whole house ventilation systems etc.), but for most people this passive style of ventilation is the most common and the appropriate for their home.