Do Bathroom Exhaust Fans Need To Be Vented Outside?

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Your bathroom exhaust fan is a small but important part of your home’s HVAC (heating, ventilation, and air conditioning) system. 

The main thing to keep in mind is that you must always vent your bathroom exhaust fan outside your home. This is important to comply with most residential building codes, protect your family from health hazards, and protect your property from moisture damage.

Why is it Important to Vent Outside?

The primary reason for drawing moist air from your bathroom and pushing it outside with an exhaust fan is to prevent humidity buildup. 

Stagnant moist air can contribute to the growth of mold spores, which are dangerous health hazards. Too much moisture in your bathroom can also cause paint and wallpaper to peel and, in extreme situations, warp cabinet doors.

A secondary reason for venting outside is to improve the air quality inside your home. An exhaust fan will help move unpleasant odors and potentially irritating cleaning chemical fragrances to the outdoors, where they won’t bother you or your guests.

Bathroom Exhaust Fan Venting Options

You have several options when considering bathroom ventilation. In most cases, it’s a matter of personal preference.

Vent Through the Ceiling

This type of installation you’ll see most often because it is inexpensive and easy to do.

As the name suggests, the fan is mounted in the bathroom ceiling. Above the ceiling, a flexible or sheet metal duct runs from the fan, across the ceiling, out to an outside wall. The choice of duct type will usually be driven by whether you have a straight shot to the outside wall or require several twists and turns.

The choice of duct type will usually be driven by whether you have a straight shot to the outside wall or require several twists and turns. The duct is attached to a vent cover that is mounted on the exterior wall. Vent covers are usually plastic and have flaps that allow moist air to exit while keeping pests and debris outside.

Vent Through the Roof

Another popular form of venting is to go up through the roof instead of to an exterior wall. From a venting perspective, all that changes here is that the exhaust is the termination point of the duct.

When you vent through the roof, you should use a sturdy stainless steel vent cover that will withstand the elements.

Because you have to cut into your roof, it’s best to use a licensed roofer for this kind of installation to ensure that the job is done correctly and that all leaks are sealed.

Vent Directly Through the Wall

If your bathroom is on an exterior wall, you can use a vertical wall-mounted fan that will carry air right from your bathroom to the outdoors. In these cases, the distance to the home exterior is usually no more than 2-3 feet.

You can use an inexpensive plastic vent cover attached to the exterior of your home for this type of installation.

Vent Through the Soffit

The soffit is that part of your roof that hangs over the side of your house. This isn’t the best place to vent because the soffit has its vents, and there’s a risk of drawing the moist air that you’re expelling from your bathroom back into your attic.

If you don’t have any other options, a soffit venting installation can still be effective. Building codes vary and commonly prohibit this type of installation, so be sure to check beforehand.

If you vent through your soffit, you’ll want to install a spring-loaded vent cap. These are specially designed to snap back into position when the airflow stops to prevent critters from entering your home.

Can you join the Bathroom Vent and the Dryer or Kitchen Vent?

This might sound like a convenient, money-saving technique, but you should never do it because it presents a dangerous situation that probably violates your local building codes.

Your kitchen exhaust carries grease. Your dryer vent carries lint. You don’t want these materials to collect on their respective motors or the warm motor driving your bathroom exhaust fan because they may accumulate, ignite, and create a dangerous fire. Joining these vents can also lead to less sanitary conditions.

Why not Vent into the Attic?

This may seem like a good shortcut, but it’s a terrible idea. If you vent into your attic, you not only probably violate code but create a situation where the moisture that you’re removing from your bathroom will collect on the underside of your roof. This will accelerate deterioration and – worst of all – encourage the growth of health-threatening mold spores.

Terminating your vent duct at the ridge vent or soffit vent is not a workaround because doing so may cause the air to stay in the attic.

What about Venting into the Garage?

This is a bad idea for a lot of reasons. 

  1. Your local building code probably says that you can’t vent into any room in the house, including a garage.
  2. Venting into the garage is similar to venting into your attic – you will encourage the growth of mold and accelerate the deterioration of materials.
  3. The excess moisture you will create in the garage will contribute to rusting any equipment you have in your garage.

Is there a Limit to How Far you can Run a Vent?

You can usually run your bathroom exhaust duct a maximum of 25 feet if you’re using a 4-inch flexible duct.

What kind of duct should you use for bathroom exhaust?

You have two choices: foil or sheet metal. Here’s some information about each of these:

Flexible Foil Ducts

Aluminum foil ducts are the most commonly used ones because they are inexpensive and easy to work with. With these ducts, you never need to buy elbows to wind around obstacles between the exhaust fan and the duct termination on the outside of your home.

The only minor downside to foil ducts is that they can tear, but this shouldn’t be a concern if you handle them carefully.

Sheet Metal Ducts

Sheet metal ducts are superior performers because they are stronger than flexible foil and because they are smooth, meaning that they have no ridges inside that can impede airflow.

Major downside considerations are cost and the difficulty installing them because they don’t bend around obstacles.

What Size should my Ducts be?

Most bathroom exhaust fans today use a 4″ or 6″ duct. 

Can I Install a Bathroom Exhaust Fan Right Over the Tub or Shower?

Yes, you can. Just make sure that the fan is UL (Underwriters Laboratory) certified for such placement and on a GFCI (ground-fault circuit interrupter) circuit. 

You cannot install your exhaust fan over the tub or shower if the device has any heating function.

Conclusion

You must vent your bathroom exhaust fan directly outside for the safest and most sanitary complement to your HVAC system.