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Venting a Bathroom Fan Into an Attic: How-To Guide

Venting a bathroom fan into an attic removes moisture.

Bathrooms need the correct ventilation. Poorly ventilated bathrooms can lead to all manner of issues. So, should you consider venting a bathroom fan into the attic?

We look at your options and give guidance so you can avoid crucial errors.

Key Takeaways

  • Don’t vent bathroom fan into attic, as it causes mold and rot. Vent through a gable wall or roof instead.
  • Building codes require proper ventilation, so follow guidelines and vent bathroom fan to the outside.
  • Consider fan size, vent material, and duct length when installing a bathroom fan for optimal performance.
  • Costs vary depending on installation type, but expect to pay around $200 for labor if venting through a gable wall.

Is It Okay To Vent a Bathroom Fan Into the Attic?

In short, no. You can vent a bathroom exhaust fan through the attic to the outside, but allowing steam and moisture to collect in the attic space leads to a whole heap of issues.

But why, we hear you ask? It was common practice in the 80s and 90s. Moisture and mold are very bad for wooden joists and rafters, insulation, cables, electrics, and any number of other things.

As moisture traps, wetness increases, allowing mold spores to form. This rots the wooden infrastructure of your roof, especially in the winter during colder spells, where the wood gets little opportunity to dry out.

Plus, as the air cools, moisture creates frost, which takes its toll on wooden beams. As the ice melts, the water drips onto the sheathing and drywall ceiling.

What Is the Building Code for Venting a Bathroom Fan?

The Environment Protection Agency (EPA) states that each bathroom must have a mechanical means of extracting moisture if there is no window that opens to provide ventilation.

The exhaust fan must have a CFM (Cubic Feet Per Minute) of 50 for intermittent operation and 20 CFM for continuous use. While model codes do not cover the configuration and direction of the vent system, they do publish guidelines that require the vents to exhaust to the outside.

  • Residential code: R303 Ventilation air from the space shall be exhausted directly to the outside.
  • Mechanical code: M1501.1 Outdoor discharge. The air removed by every mechanical exhaust system shall be discharged to the outside.
  • M1507.2 Recirculation of air. Exhaust air from bathrooms and toilet rooms shall not be recirculated in a residence or to another dwelling unit and shall be exhausted directly to the outdoors.

How Do You Vent a Bathroom Fan Into the Attic?

We have established that venting a bathroom fan directly to the attic is against all building codes and a terrible idea, but where do you vent it to?

We’ve also learned that all expelled air must vent to the outdoors.

Where Should Bathroom Exhaust Fans Be Vented To?

The proper way to vent your bathroom fan should be through the roof or a cable end. The vent has to go directly outside. It has a small flap that opens when air is expelled but closes when it is dormant.

It prevents outside air from entering the home when the vent is off. It also prevents nesting birds from taking up residence in your ducting. It’s a good idea to install a vent roof cover to avoid this.

Can You Vent a Bathroom Fan Through a Gable Vent?

The correct way to vent a bathroom fan is through a gable vent with a hood cover to prevent outside air from escaping into the house when the fan is off. It is cheaper to do it this way than to put the vent through the roof as this requires a specialist roofer, which ramps up the costs.

How Far Can I Vent a Bathroom Fan?

This is an interesting question because there is no right or wrong answer. It depends on several factors.

  • The fan size and CFM rating, or the design flow rate according to your local codes.
  • The vent material (smooth or corrugated).
  • The vent diameter.
  • The number of elbows required along the duct length.
  • The distance between the exhaust fan and the building’s exterior.

You should also consult the manufacturer’s manual to see what their recommendations are. As a rule of thumb, using a 4-inch diameter duct to carry the fan exhaust, the accepted maximum length is 25 feet.

How Much Does It Cost To Vent a Bathroom Fan Outside?

The smallest outlay is for the equipment and supplies. You should expect to pay somewhere around $70 for a 25 ft insulated bathroom vent. Next comes the labor costs. If you opt for a DIY approach, this reduces the price considerably.

If you vent to the roof, you will need a roofer, adding to the overall budget. Plus, roof vents are more complicated to install. Expect to pay anywhere between $500 and $750 in labor costs alone.

Going through a gable wall is considerably cheaper, reducing the price to about $200 for labor. Your best course of action is to get a couple of quotes and then decide.

How Do You Vent a Bathroom Fan With No Outside Access?

Venting a bathroom fan without outside access requires some imagination. First, you could locate the fan on the floor and run the ducting between the floor joints to an exterior wall. This would require a fan strong enough to get the moisture to the outside.

Second, you could install a recirculating fan, especially if you have low levels of bathroom moisture. The fan pushes air through a filter, making it better at removing odors.

Third, if you live in an apartment block with high ceilings, commercial ducting could be the solution. It could be fed to the nearest outside wall and disguised using a soffit.

To help increase the airflow, install a small grill under the bathroom door. This helps to draw air from the rest of the house to dissipate moisture.

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Where Do Most Bathroom Fans Vent To?

Most bathroom fans vent to the exterior of the house through the wall. Some systems use ducts and pipes, but others are installed to vent through the roof.

Does the Bathroom Vent in the Attic Need to Be Insulated?

You should insulate bathroom vents in the attic because the moisture they eliminate will cause the humidity levels to rise in your attic. This can cause problems, with mold being the number one enemy.

Insulating the vent can prevent these problems, but ensure you use the proper insulating material. Check the R-value of the materials. This will show how resistant they are and how well they can handle temperature fluctuations and moisture.

Does a Bathroom Need a Vent if It Has a Window?

Bathrooms with windows don’t always need vents, but municipal laws and building codes in your area might require a vent, nevertheless.

A bathroom window’s primary role is to allow natural light into the bathroom, but it can also be a good means of ventilation. However, when you have to ventilate the bathroom by opening the window in winter, you’ll wish you had a vent installed too.

Plus, bathroom windows are small and might not ventilate the space properly. Since the role of a vent is to get rid of excess moisture and unpleasant odors, you should consider installing one whether you have a bathroom window or not.

Should a Bathroom Exhaust Fan Be Over the Shower?

The ideal location for the fan would be immediately above the bathtub, but any location close to the water would do. Place the fan in the middle of the bathroom if there is more than one fixture.

Can a Shower and a Toilet Share a Vent?

A shower and a toilet can share a vent, but it might not be the best idea. If both have waste trap arms, their drainage lines can be connected. Be aware that mixing them may cause drains to become clogged, which in turn may create unpleasant odors to emanate from those drains.

What Is the Problem with Venting an Exhaust Fan into the Attic?

The biggest problem with venting an exhaust fan into the attic is that it will raise humidity and potentially cause structural damage.

Since your bathroom vent is designed to eliminate moisture and unpleasant odors, all those will go to your attic. All that air likely has harmful particles you don’t want in any area of your house.

As moisture in the attic builds up, it can cause problems like mold. Not only is this a breathing hazard, but it also damages the house’s structure. If your attic is not insulated, this condensation can freeze in the winter, thus leading to more problems.

Vent Like a Pro

Moisture inside your home is inevitable. It happens when we shower, wash our hands, cook, and even breathe. Removing the wet air is necessary to avoid dampness, mold, mildew, and harmful pollutants circulating around our homes.

Bathroom fans are crucial in this battle, so how and where you fit it matters. Get it right the first time, and you save a whole heap of agony later.

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About the Author

Mark Weir

Mark spent 24 years working in real estate, so he knows his way around a home. He also worked with contractors and experts, advising them on issues of planning, investments, and renovations. Mark is no stranger to hands-on experience, having renovated his own home and many properties for resale. He likes nothing better than seeing a project through to completion.