A vent assembly for use with a respiratory mask of the type used in CPAP treatment. In one embodiment, the vent is made of a thin air permeable membrane. The membrane can be made of a hydrophobic material such as expanded polytetrafluoroethylene (PTFE). An expanded PTFE membrane is mounted on a polypropylene scrim. The pores of the ePTFE membrane have a reference pore size of 10 to 15 microns. Alternatively, the vent assembly includes a stainless steel vent having holes with diameters less than about 0.2 mm. In another embodiment, the membrane has a superficial cross-sectional area of approximately 500 mm2. In further embodiments, a vent of a mesh material, e.g., an auxetic vent or a PTFE mesh, may be used as an air permeable membrane, either alone or in combination with a traditional vent structure.