You are asking a really common question. Attics tend to exist outside of the building’s envelope, with passive vents allowing air to flow between the interior space and outside. Because of this, people think that any air vented into the attic will be channeled outside with no problems.
The air will eventually make it outside in this way, but it will first cause some issues in the attic. These can impact the whole house.
Bathroom fans cannot vent into the attic. Doing so contravenes the building codes and poses a risk to the structural, thermal, and electrical integrity of the house as well as the health of the occupants.
Terminating Bathroom Fan in Attic Is Prohibited
Per Section M1501.1 of the International Residential Code (IRC), no mechanical exhaust system can release exhausted air into attics:
“The air removed by every mechanical exhaust system shall be discharged to the outdoors in accordance with M1594.3. Air shall not be exhausted into an attic, soffit, ridge vent or crawl space.”
A bathroom fan is such an exhaust system and must follow the above code. Bathrooms are specifically referenced by the IRC in Section M1505.2:
“Exhaust air from bathrooms and toilet rooms shall not be recirculated within a residence…and shall be exhausted directly to the outdoors. Exhaust air from bathrooms, toilet rooms and kitchens shall not discharge into an attic, crawl space or other areas inside the building.”
So, the first reason you shouldn’t vent a bathroom fan into the attic is because it is a direct violation of the building codes.
Where Is the IRC Applicable?
The IRC is applicable on an international scale. Although created in the United States, the IRC was designed with an international audience in mind.
Most states in the USA adopt the IRC as their residential building code standard.
That being said, local codes should be checked along with the IRC because state-specific changes can be made to the IRC to ensure nuanced applicability in that specific region.
In the case of the particular codes described in the previous section, local codes have almost certainly left such regulations unchanged because the issues associated with indoor venting are not going to change from state to state.
If there are any changes, they may have been adapted to be more strict or specific, rather than omitting the regulations or making them more lenient.
What if the Attic Is Ventilated?
As I said, the attic vents make people think that it’s okay to vent into an attic. However, even if an attic is ventilated, a bathroom fan cannot release the exhaust into the attic.
This is because the attic is still considered an indoor space, even when it is outside the envelope and has a ventilation system. Indoor spaces cannot have exhaust vented into them.
Why Bathroom Fans Cannot Vent Into the Attic
Bathroom fans cannot vent into attics because of the heat and moisture that is being exhausted.
Venting warm and moist air into a home can lead to serious problems. Such problems can lead to compromised health, safety, and the ability to make use of the attic or even sell the house.
The difficulty in selling the house can come from the home not being up to code and damages being done to the attic.
Various issues could potentially arise from a bathroom fan venting into an attic.
An attic with a bathroom vent releasing exhaust fumes into it could experience structural damage.
Attics are mostly made up of wood. Wood will readily absorb the water, which warps and weakens it.
If the wood absorbs and contains more than 30% water, its strength is compromised, which poses can be dangerous for anyone living in the home.
For example, when your roof trusses suddenly start rotting and buckling, you are in for some serious trouble!
Additionally, metal in the attic can be damaged by warm, damp air that is pumped in from the bathroom.
Water condensing on metal surfaces can lead to rusting and structural complications because rusting weakens the affected metal.
Insulation is also negatively impacted when dealing with warm and damp air.
Attics typically have a large amount of insulation in them so that heat isn’t lost as easily through the attic/roof.
Insulation that is damp or saturated doesn’t work as effectively as insulation kept in proper conditions.
Insulation that isn’t keeping heat in goes against its only real purpose. Damp insulation becomes more of a liability as it no longer keeps heat in, and the cost of maintaining it increases.
Your house loses its thermal envelope and you end up paying more to heat or cool it.
Insulation can also get moldy as it takes on more moisture. The paper backing of insulation is a great food source for mold. Later, I will explain a bit more about why you don’t want mold in your home.
Electrical damage is likely to occur in attics that have air from a bathroom vent being pushed into them.
Warm, damp air that enters into the attic has a possibility of condensing. This means that water droplets could start to form and cling to electrical wires that are present in the attic.
Water conducts electricity, so this can then lead to short circuits, which can cause fires.
Additionally, depending on how the water interacts with the wiring in the attic and where it goes to, people in the house may be shocked. Such electrical shocks could be life-threatening.
Mold could find a prime environment in an attic with a bathroom venting air into it. After all, you are trying to exhaust the air to prevent your bathroom from forming mold.
Mold spores are constantly present but only grow in certain settings. Mold thrives in conditions that are warm, dark, and moist. An attic with a bathroom vent exhausting into it is just that.
Not only could mold thrive on the insulation in the attic, but it can also grow on ceilings, cement, wood, and brick.
Once mold begins to grow on a surface, it uses the thing it is living on for food. This can then affect the structural integrity of whatever the mold started growing on.
Mold spores are then likely to spread even more. This is concerning because mold can cause health problems like skin irritation, rashes, and serious respiratory infections.
Infestations and Unwelcome Guests
Insects and smaller animals (rats, mice, squirrels, raccoons, etc.) are attracted to warm, damp, and dark areas. A bathroom vent leading to an attic creates a space just like this for them.
Attics are already one of the easiest spaces for animals and insects to access. Adding warmth and a source of water (condensation) would only make an attic that much more attractive for these unwelcome guests.
This is a problem because animals and insects living in a person’s home can cause damage to the structure when they make their home or by chewing on the wood and wiring. They can even chew through the ducting.
They also bring bacteria, which can grow and expand in a dark, warm, and damp environment, and spread throughout the house, leading to health problems for people living in the home.
House Gets Hotter
Anyone who has been in an attic knows just how hot they can get. Growing up, I always sulked when my Dad would ask me to get the Christmas decorations down from the attic because I knew I would come back a sweaty mess!
An attic that has a bathroom vent leading to it will be made hotter by the air that the bathroom vent is releasing.
In the winter months, if ice starts to build on a roof, water could become trapped by the ice and against the roof. This water can start to form because the roof is much warmer due to the bathroom vent.
This means that the ice touching the roof turns to water while the outer portion not in contact with the roof remains ice.
This can cause water to pool and remain in contact with the roof, which could cause the roof to leak into the attic.
In combination with the compromised insulation, this excess heat is also likely to be felt throughout the house.
My Bathroom Fan Vents into the Attic
How Did This Happen?
A bathroom fan venting into an attic could happen for different reasons.
The contractor may have decided not to go through the hassle of proper installation and relied on the fact that the owner did not know better.
If the bathroom fan was installed by the previous owners as a DIY job, they might not have been aware of the regulations.
It is not uncommon for vents to pass through the attic. If connections in the vent wear down/are not sealed correctly, and the air is inadvertently vented into the attic even though the termination point of the vent is outside the home.
Fixing the Problem
There are a few options if you find that your bathroom vent has somehow started releasing air into your attic.
One option would be to extend the vent and run it out of the gable end of the house. The gable is the portion of the wall that closes the end of a roof.
To do so, you would cut a hole through the wall and add another section of ducting to allow the pipe to terminate outside. Install the correct seals and flashing around the new hole in the gable.
There is also an option to vent out of the roof rather than the wall. You will cut a hole in the roof, extend the ductwork, and install the correct flashing, seals, and roof vent hood.
A third option is to vent it down through the soffit.
If the vent does lead outside, and the release of the air is coming from gaps in the venting itself, then other options are available.
In this instance, you can cover the leaks with metal tape. Alternatively, you can replace that section of ducting.