Adjustable air valve

The adjustable air valve (1) has a closure (7) which is relatively adjustable with respect to insert (4) such that different large flow cross sections (15) are formed between the air opening of insert and the closure. The closure is rotatably mounted relative to insert in air flow direction along central axis (11). The air opening (13) of closure is overlapped with air opening of insert by rotating the closure with respect to insert. An independent claim is included for ventilation system for building.