An improperly vented bathroom fan can cause a multitude of problems for homeowners.
I have seen bathroom ducts where the airflow was completely stopped due to a bird's nest at the exterior cover. Sometimes the duct has a sag that collects water and makes the bath fan useless.
And if you install the wrong duct size, the advertised CFM or airflow capacity of the bath fan can be reduced by half.
In this guide, I will go over the 6 steps to properly vent a bathroom fan as well as a few other installation best practices. Let's get started!
Step 1 - Pick The Exit Location
Homeowners need to first choose an exit location for the vent on the exterior.
The most common locations are the roof, exterior wall, or gable wall. Regardless of the place you choose, it is essential that the bathroom vent terminates to the outside and not inside the attic or wall cavity.
Sometimes the bathroom fan vents go to the soffit but it isn't recommended. Since warm air rises, venting to the soffit can cause mold issues on the soffit underside or in the attic (if sucked into nearby soffit venting).
Homeowners should pick an exit location that is as short as possible from the bathroom fan with the least number of bends.
The exit location should also be in the same direction as the exhaust port on the bath fan.
Step 2 - Pick The Vent Type
Homeowners should check with the manufacturer for the duct recommendations.
The two main duct options are between rigid metal and flex vents.
The best vent type for bathroom fans will always be rigid smooth metal. Metal vents have the least amount of air resistance and they are also the most durable. However, metal vents will also be the most difficult to install due to their inflexibility.
The second option is to use corrugated flex vents. If this vent type is chosen, it is important to stretch it so that the ridges don't impede airflow.
Regardless of the vent type, we recommend insulating the duct if it goes through an unconditioned area such as an attic or garage. Uninsulated ducts that go through unconditioned spaces is a common reason for condensation or moisture problems.
Step 3 - Choose The Duct Size (Diameter)
Choose the correct size bath fan duct for your home.
Homeowners should check with the bath fan manufacturer for the recommended bath fan duct size.
Most modern bathroom fans will require a duct diameter of between 4-inches and 6-inches. Many older bath fans use 3-inch diameter ducts.
If you install a 3-inch duct in a new bathroom fan that specifies a 4-inch duct, it will reduce airflow and performance. Ideally, the duct diameter should be the same size as the exhaust port on the bathroom fan.
Step 4 - Decide If Insulation Is Needed
If the bathroom fan duct goes through an unconditioned space such as an attic, crawlspace, or garage — homeowners should insulate the duct.
When a duct goes through an unconditioned area that is subject to exterior air, condensation (water) can form on the inside and outside of the duct.
Sometimes so much water can form inside the duct that it will stop airflow.
And if water forms on the skin of the duct, it can lead to mold growth or wood damage. Homeowners can buy bath fan ducts that come in 25-foot sections and include fiberglass insulation.
Step 5 - Connect Duct To Exhaust Port
Connect the vent to the exhaust port of the bathroom fan.
Simply slide the vent over the exhaust port and secure it with a zip tie, mastic, or aluminum foil tape (or combination). Make sure it is air tight and that it won’t come loose.
Step 6 - Connect Duct To Exterior Hood
Connect the duct either through the wall, ceiling, or attic and to the exterior cover.
The cover should have a flapper that keeps out exterior air and there should be a screen to prevent pests from entering.
Like connecting to the exhaust port, the duct can be secured with a combination of mastic, metal tape, or a zip tie.
Additional Best Practices
Here are a few essential best practices when installing a bath fan vent:
Duct Should Go 3-Feet Out Before First Turn
It is important that homeowners don't immediately make any turns when the vent comes out of the bath fan.
Homeowners should keep it straight at least a couple of feet before the first turn. This will reduce any performance loss if you have to make a bend.
Secure The Duct With Straps
If the bath fan duct goes through an attic, it should be secured with straps so that it doesn't move and to get rid of sagging.
If the bath fan duct isn't straight, it will greatly reduce performance, and the sags may capture condensation and stop airflow.
Ideally the bath fan should go in a straight line to the outside. This is usually just not possible unless the exterior hood is very close to the bath fan.
Nevertheless, homeowners shouldn't create any unnecessary bends or turns.
10-Feet Away From Air Intakes
On the outside of the home, the bath fan duct should terminate at least 10-feet away from any air intakes such as from a high efficiency furnace or a power vent water heater.
The exception to this building code is if the bath fan duct terminates at least 3-feet above the powered air intake.
3-Feet Away From Windows And Doors
The bath fan duct needs to terminate at least 3-feet away from any doors or windows.
This makes sense if you think about since you don't want bath fan moisture going back into the home if you open a window.
I hope you enjoyed my guide on venting a bathroom fan. Here are a few of the essential tips covered in this guide...
- The duct should always go the exterior
- Connect duct to bath fan and hood with a combination of mastic, zip ties, and metal (foil) tape
- Choose the correct size duct type and diameter
- Reduce the number of bends
- Keep the duct as straight as possible
- Don't allow any sags in the duct
- Insulate the duct if it goes through an attic, crawlspace, or garage.
And here are some additional resources related to bathroom fan venting:
- Bathroom Fan Venting Options (Wall, Roof, Soffit)
- Building Code For Bath Fan Vents
- How To Vent Bath Fan Through The Wall
- How To Insulate An Exhaust Fan Duct
- Is It Allowed To Vent A Bath Fan Into The Attic?
- How To Install A Bath Fan Roof Vent?