A liftable closure element movable between a lowered and a raised configuration, is associated with a portal structure having a compartment for storing the closure element, delimited by a front wall, a space being provided which communicates with the compartment and with the outer environment between the wall and the closure element. An air seal device for the liftable closure includes first and second sealing members adapted to reach a facing position in which they adhere by magnetic attraction to close the transverse space, one connected to the wall for supporting a ferromagnetic plate, the other consisting of a magnetic member fixed to the closure element. The first sealing member includes a base structure which can be fixed to said wall by first and second adjustable fasteners in a position exactly corresponding to the position of the sealing member fixed to the surface of the closure element.