A bathroom exhaust fan reduces humidity by drawing humid air out of the room and replacing it with drier air from other areas. In a well-designed vent system, the air the fan removes exits the house through a duct.
Makeup air replaces what the fan pulls from the bathroom. Makeup air comes from many places. If the bathroom is located on an outside wall, the vacuum the fan creates pulls fresh air from around the cracks at the baseboard and from around window frames. If the vacuum is unable to replace the air from an outside wall, it draws air through the crack between the door and floor. The air the fan pulls from other parts of the home also requires replacement, which occurs through small openings in outside walls. If the installer does not seal the fan properly, the air enters the room from the attic through openings around the fan, creating a small circulation pattern and reducing its effectiveness at removing humidity.
Reducing humidity in a bathroom helps to prevent mold growth. Small bathrooms are more susceptible to humidity than larger ones, especially if they include a shower or the ceiling is uninsulated. In these small spaces, installing and using a vent fan regularly, along with providing adequate insulation helps to eliminate humidity problems.