A ventilation louvre is an opening protected by slats in a wall or window. It allows the ingress of fresh air to a building but excludes birds and rain. The slats may either be fixed or adjustable to suit required ventilation levels. 

Ventilation louvres are used in a wide range of buildings, often with the purpose of ensuring at least a minimum level of ventilation at all times. The higher the standard of thermal insulation used in a building, the more important this ventilation becomes. Louvres may also be used for additional purposes, such as for controlling the amount of sunlight reaching a space and for attenuating noise ingress from outside.