Abstract: A vent device comprising at least one control member pivotably supported in an opening in a frame, for movement, in response to differential gas pressure between opposite sides of the opening, between a closed position in which the control member(s) substantially blocks the opening and an open position in which the control member(s) leaves the opening substantially unobstructed for flow of gas therethrough, wherein the or at least one of the control members has associated therewith a restraining mechanism, operable to apply a force to the control member to influence movement thereof between its open and closed positions.