Bathroom Fan Venting

Bathroom fans are a common issue regarding moisture damage found in attics. Very often I find that the vent piping is not even installed. Lets discuss some common issues regarding this very important ventilation system. Typically bathroom ventilation systems consist of a ceiling fan unit that vents trough the roof or wall assembly. one important requirement is that the vent piping be vented directly to the exterior and is not using the attics ventilation system as part of it’s venting.


The fan unit can be controlled in several different ways.

  • A wall Switch
  • A timer switch
  • or a humidistat that automatically activates when the relative humidity reaches a preset level.

The fan unit should be kept free from debris. Dirt, dust and lint buildup can reduce airflow and lead to an inoperable fan. Moisture laden pet dander and lint are attracted to the fan unit due to its static charge. Bathroom ventilation fans should be installed in all bathrooms, even ones that have windows.

Common Issues

The following may indicte defects or other issues with your bathroom fan ventilation system:

  • Moisture staining or mold on walls and ceilings.
  • Rust or corrosion on metal. Most commonly this can be found on HVAC supply registers.
  • Visible mold or mildew on walls and ceilings
  • Peeling Paint
  • Frosty windows
  • High humidity

The most common defect in bathroom fan installations is the improper duct installation. The duct must be vented directly to the exterior. It is also common to see these ducts vented into the soffit of the roof overhang. While most municipal codes will allow this type of installation generally leads to deteriorated roof sheathing.

The most common improper installations are as follows:

  • Vents that terminate in the attic but above the insulation, these are easy to identify.
  • Vent ducts not installed or buried under the insulation, These can be harder to find, moving the insulation in the area of the fan is necessary.
  • Under attic/roof vents. The bathroom ventilation system is not allowed to use the attic venting as the system was not designed for this purpose.

Improperly installed vent ducts may appear to work fine in the bathroom but when the attic is examined after a period of time one would likely find extensive condensation damage and mold growth. During winter months just the warm air escaping into the attic space could be the cause of condensation damage. This can also cause extensive damage to the insulation. Wet or moisture damaged insulation would no longer be effective.

The most serious product of poor bathroom ventilation would probably be mold growth. Mold can appear in many different forms. Health problems caused by mold are related to high concentrations of mold spores in the air. Spores are like microscopic seeds in the air that are given off by mold fungi. Organic materials that have a moisture content of 20% or higher are conditions conducive to mold growth. The presence of mold is especially dangerous for persons suffering from allergies and asthma, and can cause serious or fatal fungal infections in those with lung disease or compromised immune systems. It should be stated that in order for a substance to be identified as a mold it must be lab-tested.

Decay or rot is also caused by fungi. Early decay cannot be seen, by the time decay becomes visible affected materials may have lost up to 50% of their strength. Mold spores are common in attic spaces due to condensation that forms during winter months, especially if indoor are leaks into the attic space. Even though mold in the attic is outside of the building envelope, mold spores can be drawn into the living space during times of low or negative pressure, usually created by the expulsion of household air from exhaust fans in bathrooms, dryers, kitchens and heating equipment. 

Improper Ventilation 

Ventilation ducts must be made from appropriate materials and oriented effectively in order to ensure that stale air is properly exhausted. Ventilation ducts must:

  • terminate outdoors. Ducts should never terminate within the building envelope;
  • contain a screen or louvered (angled) slats at its termination to prevent bird, rodent and insect entry;
  • be as short and straight as possible and avoid turns. Longer ducts allow more time for vapor to condense and also force the exhaust fan to work harder;
  • be insulated, especially in cooler climates. Cold ducts encourage condensation;
  • protrude at least several inches from the roof;
  • be equipped with a roof termination cap that protects the duct from the elements; and 
  • be installed according to the manufacturer’s recommendations.

The following tips are helpful, although not currently required. Ventilation ducts should:

  • be made from inflexible metal, plastic or other rigid material. Unlike dryer exhaust vents, they should not droop; and 
  • have smooth interiors. Ridges will encourage vapor to condense, allowing water to back-flow into the exhaust fan or leak through joints onto vulnerable surfaces.

Above all else, a bathroom ventilation fan should be connected to a duct capable of venting water vapor and odors into the outdoors. Mold growth within the bathroom or attic is a clear indication of improper ventilation that must be corrected in order to avoid structural decay and possible health issues.

As always, I hope you have taken some value from this post. If you have any home related questions please send them my way at the email below.



Aaron M. Zuehlke is the owner and inspector at Zuehlke Inspection Service, LLC, a full-service home inspection company serving Southern Wisconsin. Specializing in Home inspection, Radon Testing, Mold Testing/inspection, Residential Thermal Imaging, and Manufactured Home Foundation Certifications. He also manages several rental properties through Zuehlke Properties, LLC. He can be reached by email at or by calling the office at 608-931-7485.

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