The invention provides a flap valve for controlling the air pressure within a protected space defined by walls, the flap valve including a valve frame attachable to an opening made in the walls, a valve flap, articulated at least indirectly to the frame and being subjectable to both a sealing force which forces the valve flap against the valve frame and to an opening force which lifts a portion of the flap off the valve frame; the flap valve having a first position in which the protected space is sealed off from a contaminated environment, and a second position in which air from the protected space is allowed to escape into the environment via the opening, wherein the sealing force is constituted by at least one spring at least indirectly attached to the valve flap and the valve frame.