Bathrooms are particularly prone to moisture problems caused by high humidity. Heat and moisture combine to produce very humid, moisture-laden air that fills the room and, upon contacting cooler tile, walls, and ceilings, condenses, turning back into water. This feeds mold, mildew, and even structural rot. To expel humid air before it has a chance to condense, every bathroom needs ventilation. In most cases, the best ventilation for a bathroom is an exhaust fan.
Though the bathrooms in most new homes are equipped with adequate bath fans, an older home is often either entirely without a fan or has one that is underpowered. When buying a new ventilation fan, be sure to check its rating for air movement, which is measured by cubic feet per minute (cfm). For an average-size bathroom, the minimal requirement is 80 cfm.
Replacing an existing fan with a new one is a relatively easy do-it-yourself project as long as the new unit will fit in the existing opening and can be connected to the existing exhaust duct.
Installing a new fan where there wasn't a fan before is more involved--but often easiest if you place it where there is an existing ceiling light fixture. Before you actually install the fan housing, this job will require cutting a hole in the ceiling at the light fixture's location, snaking a vent pipe pipe (or duct) from there to an exterior wall, penetrating the exterior wall for the vent, and adding a vent cap. For this job, purchase a fan that has a built-in light to replace the light you remove. If the ventwork is beyond your skill level, call in a contractor.