A humidifier includes a tub configured to hold a supply of water. The tub includes a tub lid and a tub base adapted to be coupled to the tub lid. The tub further includes a base plate formed of a heat conducting material and adapted to be coupled to the tub base and a seal positioned between the base plate and the tub base. The tub also includes a retaining mechanism with at least one projection that projects from the tub base and is configured to force the base plate against the seal so that the seal is secured against the tub base.