A ventilation valve (1; 101) for ventilation of a space for keeping animals or plants includes an opening-defining circumferential framework (3; 103) for mounting in a wall of that space, and a movable panel (5; 105). The panel (5; 105) is movably attached to the framework (3; 103) for relative movement between a fully closed position, in which substantially no air can pass between the framework and the panel, and a fully open position, in which substantially the complete opening is available for passage of air. A profiled edge on the framework provides a non-linear relation between a passage opening and an opening position of the panel (5; 105). Also provided is a system for ventilating the space, having at least a single ventilation valve, and for periodically determining parameters of inside air in the space, and adjusting the opening position of the valve based on the parameters.