Embodiments of the invention provide a protective cap assembly for a CIC hearing aid. The assembly comprises a perforated cap configured to be mounted over the lateral end of the hearing aid to protect the hearing aid from contaminants. At a least a portion of the cap includes a protective coating and a plurality of perforations. The placement and size of the perforations can be configured to provide sufficient aeration and drainage to reduce a relative humidity of the cap interior when the hearing aid is positioned in the ear canal. The perforations also operate as sound conduction channels for conducting sound to the cap interior. The perforations have a minimum size wherein a single perforation provides sufficient acoustic transmittance to a hearing aid component such that a hearing aid performance parameter is not substantially adversely affected. They can also be configured to provide splash protection for the cap interior.