Trickle vents are located in the frames of your windows and let fresh air into your home. If you are installing extract ventilation in your kitchen or bathrooms, you should also install trickle vents in your windows if they are not already present. The extract ventilation then removes stale, humid air from the wet rooms, while the trickle vents allow a supply of fresh air in to replace it.Trickle vents can simply be installed in existing window and door frames, by a competent DIYer with guidance found online, or by any skilled contractor.
If you are installing Mechanical Ventilation with Heat Recovery (MVHR) you should not install trickle vents as this type of ventilation system has it's own dedicated air inlets.