How Bathroom Fans Should Be Vented: A Complete Guide


Proper household ventilation is a vital component of keeping your house free of mold and moisture damage. Bathroom fans are located in the greatest moisture-producing section of every home, yet some residents wonder if they are really necessary.

Bathroom fans need to be vented. Vents should always feed to the outside of the residence. The most effective options are through the roof, through an exterior wall, or out of a soffit vent.

In addition to mold problems, failing to vent your bathroom fan properly can lead to peeling paint or wallpaper, soaked drywall, and water stains. If you’re interested in how to vent your bathroom fan properly, keep reading as we guide you through the available options.

Bathroom Exhaust Fan Venting Options

Since newer homes tend to be far more airtight than their predecessors, homeowners have to pay very close attention to their ventilation systems to ensure the removal of stale, moist, and dirty air.

Many homes have different forms of static ventilation that require no power like gable vents, soffit vents, box vents, or even turbines (source).

Some of these areas might seem like an excellent option to run fan ductwork through, but this requires caution since you could disrupt the airflow design of your home.

Related article: 6 Easy Steps To Choose The Right Bathroom Fan

Where you decide to vent will be dependant on several factors such as the frequency of wind changes in your area and the level of humidity you experience in your local climate.

Whether or not you have an attic will also affect how you decide to vent your bathroom fan, but you cannot vent into your attic without risking damage to your home.

Another key consideration will be if your bathroom has at least one exterior wall or if you have a more centrally-located bathroom. It’s always best to minimize the amount of ductwork, but bathroom placement can make this a challenge.

Your four most common options for ventilating your bathroom fan would be:

  • Directly through the roof
  • Through the wall
  • Through a gable vent
  • Through the soffit

In the next section, we will discuss the pros and cons of these options and some of their variations.

Ventilating through the Roof

Ventilating through the roof is often the best option when your bathroom is situated below an attic., and this is the most common solution for contractors.

Roof ventilation uses ducting to direct the air from a ceiling-mounted exhaust fan straight out through the roof.

Standard Roof Vents

Builder's Best 084635 Galvanized Steel Roof Vent Cap with Removable Screen & Damper, 4" Diameter Collar, Black

Roof vents typically have flashing, roof boots, or roof hoods installed at the end of the ductwork. Each has flanges that fit underneath your roof shingles to prevent water from seeping in from above.

You have to install these in such a way that the water will flow around them quickly, and you must be especially careful to ensure that they are properly caulked and sealed.

Flashing and roof boots are very rudimentary, providing a very basic seal around your vent pipe. 

Roof hoods or caps add a little extra protection to keep out water and unwanted pests. Roof hoods often have lightweight doors on the inside that allow air to escape out and that close due to gravity when the fan is off to drafts.

Pros of Venting through the Roof

One of the chief reasons that homeowners and contractors use standard roof vents is that they’re comparatively easy to install. Also, heat and moisture naturally rise, and roof vents direct heat and moisture away from your walls.

Venting through the roof is usually the best option if you have an attic since air intakes like soffit vents might tend to draw moist air into the attic. 

Cons of Venting through the Roof

The most obvious disadvantage of venting through a roof would be the risk of leaks if you didn’t install it right because you had to cut a hole in your roof.

Some do-it-yourselfers may shy away from this option since there is a lot that can go wrong while cutting the hole.

As long as you properly install and seal your flashing or roof hood, this shouldn’t be a major issue. However, because there is an increased risk of leakage, standard roof vents are more likely to require frequent maintenance.

Also, some home builders will advise you against a standard roof vent unless you have no alternative because heavy snow, ice, leaves, or dead animals can easily block these vents.

How to Vent through a Shingled Roof

First, locate the area of your roof you want to place your vent. Next, mark the spot of the vent hole from the inside using a nail. Drive the nail in between the rafters of the rafter bay and out through the other side.

The next step is to go outside and up on the roof to locate the spot where you just drove the nail.

To install your duct and vent hood or flashing, you’ll need to cut out a shingle. Measure your vent hood or flashing so that it’s centered on the nail, and cut the shingle so you can slide the hood in place.  

You’ll also need to loosen adjacent shingles and break the seal so you can fit the flange underneath them. Once your fitting is correct, you’re ready to cut your hole for the ductwork.

While it’s best to use a hole saw of the proper diameter, you can also use a reciprocating saw. With the hole cut, have someone pass one end of the duct up to you from inside through the roof.

If you’re installing metal duct pipe, cut two tabs on each side of the end going through the roof, and then bend these over so you can drive a roofing nail through each.

With your duct nailed in place, you’re ready to install the hood or flashing. You’re going to need to apply a bead of tripolymeric locking seal along three sides of the outer edge underneath the hood flange. 

Only apply this to the top, the left side, and the right side of the flange. You want any condensation that forms inside the hood to run out and down the side of the roof.

The hood or flange will go underneath the top layer of shingles, while your sealant will stick to the roof and the bottom layer of shingles.

Make sure you press it down into place and then lift the top layers of shingles to nail the flange to the roof on the same three sides. Finally, make sure you reseal all of your shingles.

How to Vent through a Metal Roof

One advantage of a metal roof is that it will tend to reflect a lot of the heat that would otherwise buildup in an attic. While there are many steps to venting a metal roof that follow the previous example, the roof boot for a metal roof is very different.

If you have a metal roof, you will use a flexible roof boot that can form fit to the sheet metal ribs. Before using a hole saw or reciprocating saw to cut a hole in the roof, you might use a hammer and chisel with metal sheers to cut a hole for the duct through the metal.

The rubber boot will have guide rings for you to cut to fit the diameter of your pipe. You need to cut it so that you have a tight seal but not too tight. If you happen to cut it too loose, you can go around it with sealant afterward.

When attaching the boot to the roof, you will need to apply a bead of sealant all along the bottom of the roof boot before applying it to the metal. Press the boot firmly so that it fits properly around the metal rib.

Next, you will have to use screws all along the outside edge of the roof boot to fasten it to the roof and make sure you have a proper seal. Once you have fastened the boot to the roof, apply another bead of sealant to the outer edge of the roof boot.

Venting Bathroom fan Through a Soffit

Soffit vents usually are a form of static ventilation used in tandem with some other type of static ventilation like ridge vents or gable vents. Roof soffits are the underside of the portion of the roof that hangs over the walls, commonly called an “eave.”

If you want to learn more I have an entire article dedicated to venting bathroom fan through the soffit.

The point of the soffit vent is to provide fresh air to the attic. This is why it is important to leave at least 10 ft. between your bathroom exhaust vent and the soffit fresh air intake.

Pros of Venting through Soffit Vents

Soffit vents are very low-profile, so they will not draw a lot of attention if you decide to vent your bathroom fan through a soffit vent. You also don’t have to cut a hole in your roof. You can simply route your ductwork to them.

Cons of Venting through a Soffit Vent

Directing moist, hot air through a soffit too close to the attic intake vent can cause problems. The soffits vent might just suck the moist air right back in the attic.

This is especially true during the winter as the warm and moist air drawn into the attic will condense on the roof sheathing. Ultimately, this situation leaves you no better off than if you had simply vented directly into the attic.

If you live in a particularly windy region, soffit vents, whether used to vent a fan or not, might not be the best option for you. Insurance companies will often advise homeowners against the use of soffit vents in areas frequently hit by hurricanes.

Directing bathroom air to the soffit also requires more ductwork to connect the exhaust fan to the soffit with more bends than for a roof vent or wall vent. For these reasons, many contractors will advise you never to ventilate your bathroom through a soffit vent.

Ventilating through a Wall

Venting your bathroom exhaust fan through a wall can be a good option if an exterior wall faces your bathroom. Under most circumstances, you wouldn’t want to run ductwork to a wall from a bathroom surrounded by rooms on all sides.

Related article: How to use a bathroom fan to ventilate any room of the house that is being painted

Ventilating your bathroom through a wall might also be your best option if you have to vent a bathroom located in a basement.

Ductwork to Wall Vent

For those who have a bathroom on the first floor of a multi-story building, they may choose to ventilate through the side of the building (source).

This option uses a ceiling-mounted fan connected to ductwork run through the ceiling joists to an exterior wall vent.

Pros of Venting through a Wall

Holes cut for wall vents are less likely to leak than for roof vents because the roof tends to shield them from rain falling from directly above. Even if the rain comes in from the side, it will travel down the wall to the ground and not into the vent.

Cons of Venting through a Wall

If the bathroom is more centrally located in the home and not near an exterior wall, it will require much more ductwork. As with venting through a soffit, the added ductwork and necessary bends in the ductwork can create problems. 

Venting through a wall tends to direct that moist air up against the wall, creating potential problems with water stains and mold. Not only is this unattractive, but it can cause serious damage down the road.

Venting through the exterior wall can also be particularly challenging if you have to cut through brick to install your vent.

How to Vent through a Wall

Installing a wall vent will obviously require you to cut a hole through the wall, and it’s especially important in this case to know where your studs, wiring, and plumbing lines are located. You may need to consult an electrician or plumber.

You will also need to remove any insulation that might be in the way between the ceiling joists and the wall where your vent and ductwork will have to go.

Once you identify where you want to place your exhaust vent, mark the outline of your hole with a pencil on the inside wall. You will need to find a way to transfer the same size hole to the exterior wall as well.

Cutting a hole is relatively easy with a properly-sized hole saw with wood or stucco siding, but it can be a little more challenging with a reciprocating saw or another cutting method. You also can’t use a hole saw to cut through a brick exterior.

One way you can do this is to use a long drill bit to drill locator holes through the exterior wall after you have cut your hole on the inside wall. From the inside, simply use the long drill bit to drill holes about every inch along the vent outline. 

If your exterior wall is composed of masonry, make sure you use a masonry bit to drill the locator holes.

Go outside and find the locator holes and connect them using a pencil or marker. Cut the vent hole and remove any debris that might get in the way of the duct. If cutting through masonry, it’s best to use a hammer and cold chisel.

With your hole cut, install your outside wall vent or cap using screws. Be careful since some vents can direct moisture to your siding and cause water stains or mold, especially with wood siding or stucco.

Your best option will likely by a flush-mounted vent that directs the air straight out. 

With your vent and ductwork installed, reinstall insulation and make sure it’s vapor sealed. Use vapor barrier tape to ensure a better seal. Reinstall your paneling, and you’re done.

Direct-Discharge Exhaust Fan

If you don’t have space to run ductwork through the ceiling joists of your bathroom, you can instead purchase a direct-discharge wall-mounted vent that vents air directly through the wall. 

Broan, for example, produces two through-the-wall exhaust fans, one rated at 180 CFM and the other at 270 CFM. These are capable of ventilating a space of 160 square feet and 240 square feet, respectively (source).

Pros of Direct-Discharge Fans

These carry all of the benefits of wall vents without the ductwork if your bathroom has an exterior wall. Some of these have moisture sensors that turn the fan on only when the moisture level is too high. (I have one of those and it is amazing!)

A proper direct-discharge fan will have a built-in damper to prevent backdrafts. Subtracting the ductwork from the equation also reduces the risks of condensation and requires no additional insulation.

Cons of Direct-Discharge Fans

Wall-mounted exhaust fans usually require additional wiring, and you may have to install a new electrical line. This may discourage some do-it-yourselfers who are not comfortable with electrical work.

It’s always best to have a professional install these to prevent injury and ensure that the installation is up to code.

Like a ducted wall vent, wall-mounted vents can also be harder to install when you have to go through brick.

Installing a Direct-Discharge Exhaust Fan

Installing a through-the-wall or direct-discharge fan is similar in many ways, just without all the ductwork.

When you purchase one of these units, they normally come with templates that you can use to mark your outlines on the inside and outside walls.

These units usually consist of a small, paintable grill for the inside wall, the fan and motor that go inside the wall, and a much larger vent that goes on the outside wall.

Do not attempt to wire these yourself without the proper knowledge and experience, leave that part to an electrician. You may also need them to install entirely new lines through your wall.

Behind a Gable Vent

Another option will be to run your ductwork through a gable vent in your attic if you have one. This is also assuming that your bathroom is toward one of the gable ends of the house. 

Like ridge vents, gable vents are often used in conjunction with soffit vents to circulate air in the attic, although ridge vents do tend to be more effective.

Pros of Venting through a Gable Vent

Venting your bathroom exhaust through a gable vent is another inexpensive and convenient option. Like ridge vents, gable vents are very aesthetically pleasing compared to other options.

Venting through a gable vent doesn’t create as many problems as venting through a soffit vent if properly sealed.

Cons of Venting through a Gable Vent

Again, like all cases involving extra ductwork, you increase the risks of condensation. You may also need a more powerful fan to force the air through the added ductwork.

Since gable vents are often used in tandem with soffit intake vents, adding ductwork to a gable vent you already have could create unintended ventilation problems by blocking the flow of air. So make sure you don’t use the existing gable vent.

Also, like soffit vents, changes in wind direction could blow moist air back into the vent if it doesn’t have a damper.

Venting under the Floor

Venting under the floor might be a good option for a home where the bathroom is far from an exterior wall.

In this case, you would have a wall-mounted fan with ductwork that ran on the other side of the wall and down through the floor. From there, it would travel through the floor joists out to the exterior wall.

Venting through the floor might be a good option for you if you have a small half-bath. Otherwise, half-baths usually rely only on fans that recirculate the air back into the bathroom after running it through a filter.

Pros of Venting through the Floor

The main benefit of venting through the floor would be in cases where you couldn’t vent through the roof. This could be because the ceiling is too high or the bathroom is underneath a stairway.

Cons of Venting through the Floor

This would require a more powerful fan motor to overcome the additional ductwork and natural convection. Natural convection would also tend to lead the expelled moist and hot air to rise up the side of the wall.

Use Proper Ductwork

The standard guideline is to keep the amount of ductwork to a minimum, making it as straight as possible. The longer and more convoluted the ductwork is, the harder that the fan motor has to work to pump air through it.

Quality ductwork is made either of smooth metal or PVC pipe, typically three to four inches in diameter. While flexible ductwork is comparatively inexpensive, this is an area that you don’t want to skimp on.

While you can get flexible ductwork into areas where you simply cannot fit metal pipe or PVC, you are greatly reducing your airflow by bending it at sharp angles.

With metal or PVC pipe, you should avoid 90-degree angles as much as possible. If you can, combine two 45-degree angles instead.

More ductwork also increases the risks of condensation, although you can offset this will proper insulation.

Since modern homes tend to be so airtight, it’s better to have some form of fresh air intake. Otherwise, your home will draw in air from the dirty cracks and crevices of your home.

Learn more about what kind of duct to use for venting a bathroom fan

In addition to exhaust fans and vents, you may want to consider some sort of fresh air intake system.

Bathroom Exhaust Fan Placement

You should install a bathroom ceiling fan somewhere between the shower and toilet. If the fan has a built-in heater, you may not want it too close to the shower.

Learn more about optimal bathroom fan placement

You also don’t want to install it too close to other vents or air conditioning units that could alter the air currents and slow the outflow of humid air.

Larger bathrooms will need more powerful fans with a higher CFM rating, and some might require more than one exhaust fan. Many of these now come with lights, motion detectors, and humidity detectors to increase their efficiency.

Final Thoughts

Proper bathroom ventilation is critical to avoid mold growth and damage to your home, so make sure you choose the ventilation option that best fits your circumstances. 

Each option has its advantages and disadvantages, but most contractors still favor venting through the roof where possible, and you should never vent directly into your attic.

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Joonas Pütsepp

I like it when I'm able to fix everything that needs fixing around the house. In order to do that, I have to do a lot of research. This site will cover everything I learn and maybe help others do the same.

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