Abstract: An air admittance valve, particularly for use in building ventilation, to close a duct when there is at least atmospheric pressure in the building, but to open to admit atmospheric air to the building upon the occurrence of even a very small less-than-atmospheric pressure in the building. The valve has a hollow body with an upward opening to outside the building, and the low point of the body forming a basin, and a valve seat about an upward opening to inside the building. A valve member in the body above the basin opens and closes the upward opening to inside the building, and the valve member is guided in alignment with the valve seat.