A patient interface comprising: a plenum chamber pressurisable to a therapeutic pressure of at least 6cmH2O above ambient air pressure, said plenum chamber including a plenum chamber inlet port, said plenum chamber inlet port being sized and structured to receive a flow of air at the therapeutic pressure for breathing by a patient; a sealing structure constructed and arranged to have a shape to form a seal with a region of the patient's face surrounding an entrance to the patient's airways such that the flow of air at said therapeutic pressure is delivered to at least an entrance to the patient's nares, said seal being formed to prevent air exiting from the plenum chamber between the sealing structure and said region of the patient's face, the sealing structure structured and arranged to maintain said therapeutic pressure in the plenum chamber throughout the patient's respiratory cycle in use; a positioning and stabilising structure to hold the sealing structure in a therapeutically effective position on the patient's head while the patient is lying in a side sleeping position and while the patient is lying in a supine sleeping position; and a washout vent structure configured to allow a continuous vent flow from an interior of the plenum chamber to ambient whilst the pressure within the plenum chamber is positive with respect to ambient, said washout vent structure being configured such that a vent flow rate of the continuous vent flow has a magnitude sufficient to help reduce rebreathing of exhaled CO2 by the patient during both patient inhalation and patient exhalation while maintaining the therapeutic pressure in the plenum chamber in use, wherein the sealing structure comprises a sealing surface to form the seal against the patient's face in use, characterized in that the sealing structure comprises a tie; the tie either extends between a first interior surface region of the sealing structure that is opposite the sealing surface and a second interior surface region of the sealing structure or extends contiguously from the sealing structure at an edge region of the sealing structure to the second interior surface region of the sealing structure such that the tie forms an extension of the sealing surface; wherein the tie forms a substantially tube-shaped structure and resists deformation of the sealing structure to prevent blowout. a plenum chamber pressurisable to a therapeutic pressure of at least 6cmH2O above ambient air pressure, said plenum chamber including a plenum chamber inlet port, said plenum chamber inlet port being sized and structured to receive a flow of air at the therapeutic pressure for breathing by a patient; a sealing structure constructed and arranged to have a shape to form a seal with a region of the patient's face surrounding an entrance to the patient's airways such that the flow of air at said therapeutic pressure is delivered to at least an entrance to the patient's nares, said seal being formed to prevent air exiting from the plenum chamber between the sealing structure and said region of the patient's face, the sealing structure structured and arranged to maintain said therapeutic pressure in the plenum chamber throughout the patient's respiratory cycle in use; a positioning and stabilising structure to hold the sealing structure in a therapeutically effective position on the patient's head while the patient is lying in a side sleeping position and while the patient is lying in a supine sleeping position; and a washout vent structure configured to allow a continuous vent flow from an interior of the plenum chamber to ambient whilst the pressure within the plenum chamber is positive with respect to ambient, said washout vent structure being configured such that a vent flow rate of the continuous vent flow has a magnitude sufficient to help reduce rebreathing of exhaled CO2 by the patient during both patient inhalation and patient exhalation while maintaining the therapeutic pressure in the plenum chamber in use, wherein the sealing structure comprises a sealing surface to form the seal against the patient's face in use, characterized in that the sealing structure comprises a tie; the tie either extends between a first interior surface region of the sealing structure that is opposite the sealing surface and a second interior surface region of the sealing structure or extends contiguously from the sealing structure at an edge region of the sealing structure to the second interior surface region of the sealing structure such that the tie forms an extension of the sealing surface; wherein the tie forms a substantially tube-shaped structure and resists deformation of the sealing structure to prevent blowout.