Proper Venting of Bathroom Exhaust Fans

A new continuously operating bathroom exhaust fan can make a big difference in indoor air quality and household safety.  By ensuring the home has at least the minimum amount of fresh air flowing in and stale air flowing out, the always-on fan is a great addition to any home.

No matter how great the fan, it doesn’t make sense to pull air out of the bathroom and then exhaust it into your attic.  If your bathroom fan, new or old, is exhausting air into your attic; then it is depositing moisture into a place where it can cause problems.

Addressing Attic Air in Different Seasons 

In the winter time, air from your bathroom exhaust fan is warmer and more humid than the cold, dry air in your attic.  The warm, humid air condenses on the cold surfaces and creates conditions that can include wood rot and mold.  That’s not to mention the damage pooled water can cause to your drywall ceiling.

In the St. Louis summer time, attic air is already humid.  Adding more moisture only worsens the risk of mold, wood rot and moisture damage.

The bathroom exhaust fan should be vented in such a way as to push the bathroom air outside, not inside the attic.  A vent should be located either in the roof or at a gable end to allow the exhausted air to flow completely out of the attic.  A backdraft damper should be included near the fan to keep outside air from flowing back into the bathroom.

The duct carrying air across the attic from the fan to the vent should be insulated to at least R-4 in order to prevent condensation in the winter time.  The duct should be properly sized for the airflow.  That often means 4” diameter duct and could mean 6” or even greater.  The duct should provide the straightest, shortest path with the fewest bends and curves possible in order to support the required air flow.

 Last Things to Consider

Mistakes to avoid include running the exhaust to the soffits or terminating the duct near an existing vent.  In both cases the bathroom air is often blown back into the attic rather than being reliably exhausted out.  The last step is to cover the fan in the attic with an airtight enclosure in order to prevent air transfer between the attic and the house and to keep attic insulation from contact with the fan.

Enjoy your new continuously operating bath fun.  When vented properly, you and your attic will both breathe easier. For other ways to best insulate your home and improve air quality contact us today to schedule a home energy audit or an estimate for our other services!