Outdoor humidity levels are high in the author’s region, and it’s not uncommon for moisture diffusing through the exterior to condense on the back of an interior wall, causing paint to blister in larger areas.
Air leaks carrying humid air that condensed near an outlet damaged wall surfaces.
Similarly, humid air leaking into the wall condensed near a switch has led to elvated moisture and wall damage.
Dark stains on the inside of these cabinet doors indicate microbial growth most likely caused by a poor air seal between the conditioned space and the attic above. The inside surfaces of the cabinet door are cooler than the other surfaces inside the cabinet and most often below the dew point.
Dark stains above a door signal poor ventilation in a bathroom. The growth is concentrated over the door because the HVAC return in the hallway pulls air though the closed door when occupants are showering.
Cold air from a floor register is causing moisture to condense on a wood door, causing it to deteriorate. The moisture is likely coming from air leaks around the door. This moisture might not have been a problem if the surface of the door wasn’t frequently so cold.
There are two problems with this bath fan: The ceiling stains are from air leaks that are depositing high humidity and condensation on the outside of the fan housing. In addition, condensation from inside the ducts is dripping though the grille.