A trickle vent is a narrow, closable opening built into a window or door frame that lets a controlled amount of outdoor air into a room at all times. It provides "background ventilation" — the slow, constant air exchange that carries away moisture and pollutants between the times a window is opened.
Trickle vents matter for condensation because mechanical extract fans and positive input ventilation both need a way for replacement air to enter the property. Without trickle vents or other background openings, a sealed modern home starves those systems of make-up air and humidity climbs.
Since 15 June 2022, Approved Document F requires that replacement windows do not reduce a home's existing ventilation, which in practice means fitting trickle vents on most window replacements. They are necessary but not sufficient — background ventilation alone will not clear an established condensation problem.