A humidification chamber (24, 50, 300, 400, 500, 600 or 700) for humidifying gases is provided comprising: a water tub that is configured to receive a volume of water; a gases inlet for receiving a flow of gases into an interior volume of the humidification chamber; and a gases outlet through which a humidified flow of gases may exit the interior volume of the humidification chamber. One or more fill apertures (120) are provided in fluid communication with the water tub such that the water tub can be filled with water through the one or more fill apertures. A sealing closure (850) is also provided and configured to be releaseably mounted on the humidification chamber (24, 50, 300, 400, 500, 600 or 700), the sealing closure (850) being configured to sealingly close each fill aperture (120) when the sealing closure (850) is mounted on the humidification chamber to resist gas and/or vapour escaping from the one or more fill apertures (120).