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. A back side of the housing may be positioned adjacent to an opening in the surface of the HVAC duct, and the housing may be configured to hold a humidifier pad in an operating position. The housing may define a first air flow path from a first side of the humidifier pad to the opening in the surface of the HVAC duct when the housing is mounted to the surface of the HVAC duct. The housing may further define a second air flow path from a second side of the humidifier pad to a bypass aperture. At least part of the second air flow path may be defined by a bypass duct member. The bypass duct member may be field-reconfigurable between at least a first position resulting in the bypass aperture being located at a first location, and a second position resulting in the bypass aperture being located at a second location. In some cases, the first and second locations are on the left and right sides, respectively, of the humidifier housing.