An air admittance valve assembly for venting ambient air into a system while preventing the escape of gases within the system. A housing includes first and second passageways in fluid communication through a valve seat therebetween. A flat generally thin, flexible, preferably resilient free-floating sealing membrane is positioned for sealing engagement by gravity only atop the valve seat. Ambient air in the second passageway will freely flow into the first passageway and the system at system air pressure less than atmospheric pressure while air within the system is prevented from flowing outwardly therefrom through the valve assembly.