A humidifier (14) for humidifying a flow of breathable gas to be delivered to a patient includes a chamber (16) configured to receive the flow of breathable gas; a tub (20) configured to contain a supply of water, the tub being configured to be inserted into the chamber; a lid (18) provided on the chamber and being movable between an open position and a closed position; and a seal (19) provided on the lid, the seal being configured to seal the chamber such that the flow of breathable gas pressurizes the chamber. The humidifier may be connectable to a flow generator (12) configured to generate the flow of breathable gas. A tube for delivering the humidified flow may be connected to the humidifier. The tube may be a heated or non-heated tube. A respiratory apparatus for delivering a flow of breathable gas to a patient may include the humidifier and the flow generator and/or the tube. A method of humidifying a flow of breathable gas comprises sealing a chamber containing a supplying a water and pressurizing the chamber by directing the flow of breathable gas into the chamber.