Abstract: In negative pressure ventilating system wherein atmospheric air is introduced along the walls of a building adjacent the roof, insulated, hinged valves provided within the associated airways rise automatically from their lower, fully closed position by the pressure of the incoming air as the inside pressure is reduced to below atmospheric. The bottoms of the valves and the surfaces upon which they rest, when closed by gravity, are contoured to reduce resistance to free air flow in the airways. A smooth, uninterrupted change of directional flow is effected by streamlining both the valves and the opposed surfaces, virtually eliminating air turbulence from the inlets to the discharge ends of the air ducts. By elimination of abrupt changes in the direction of flow the incoming air is properly controlled.