A humidifier for adding humidity to an air stream of an HVAC system includes a housing configured to be mounted to a surface of an HVAC duct. The housing may be configured to hold a humidifier pad in an operating position. The housing may direct air through the humidifier pad from a first major surface to a second major surface of the pad. In the operating position, the first and second major surfaces of the humidifier pad may be substantially non-parallel with the surface of the HVAC duct when the housing is mounted to the surface of the HVAC duct. In some cases, this may provide a more compact design. In some cases, a centrifugal fan, rather than an axial fan, may be used to draw air into the humidifier from the HVAC duct. The use of a centrifugal fan may reduce the noise associated with the operation of the humidifier.