If your bathroom has an exhaust fan, then looking into the makeup air requirements is a good idea. Exhaust fans in bathrooms move air to the outside of the house, which affects the internal pressure balance.
While it’s good to ask, makeup air units are hardly ever required in domestic bathrooms. Let’s look at why this is, what signs would indicate that your setup may be the exception, and more.
In general, makeup air is not necessary for bathroom fans because they don’t often exhaust enough air to warrant it. If the fan is not functioning effectively, there are gas appliances in the bathroom, or the drains are gurgling and smelly, then makeup air should be supplied.
Makeup Air Is Not Commonly Required
Makeup air replaces air that is being exhausted so that the home (or room) is not at a negative pressure.
Although bathroom fans do exhaust air, it is quite rare for bathroom fans to require makeup air. This is because bathroom fans are almost never strong enough to create negative pressure in the bathroom.
The rule of thumb is normally that there should be one CFM (cubic feet per meter) per square foot. CFM is a measure of airflow volume.
However, since bathrooms are typically some of the smallest rooms in the house, the air that they require to balance the negative pressure created is easily obtainable from adjoining rooms, and is not likely to cause significant pressure changes in these other, larger rooms.
In addition, bathroom fans are also seldom in the same room as other appliances. So, there are not likely to be appliances that can compound the amount of exhausted air or can suffer from the lack of sufficient air supply (gas-fuelled appliances).
Determining if Makeup Air Is Necessary
There are some situations where makeup air is needed in a bathroom, and it is important to know whether or not your bathroom falls into the category of needing makeup air.
Does the Fan Interfere With Appliance Function?
Section G2407.4 of the International Residential Code (IRC) states that exhaust fans that interfere with the operation of appliances (referring specifically to gas-fuelled appliances) require a supply of makeup air.
Bathroom fans can interfere with their own function if they remove enough air that they reach the point of creating negative pressure in the bathroom. This could then cause the bathroom fan to experience backdraft.
A bathroom fan that is backdrafting is not going to be effectively removing steam from the bathroom. It lingers and causes all the damage you are trying to avoid through the fan’s installation.
In addition, if you have a gas dryer or water heater in your bathroom, then the negative pressure can be enough to cause backdrafting of harmful combustion gases and also possibly interfere with the supply of combustion air.
Is the House Very Airtight?
The degree of how airtight a home is can also play a role in how likely the negative pressure in the bathroom is to become significant.
The exhausted air has to be replaced. A less airtight house will allow air to come into through gaps and cracks, and, unless the fan is very powerful, you are unlikely to experience this air movement as actual drafts.
If, however, the house is very airtight, the replacement air will be pulled from undesirable places.
Air can be pulled up from the drains to try and rebalance the pressure system. This means that all the air in your sink, shower, bath, and—yup, you guessed it—your toilet can come flooding into your bathroom.
These gases can smell really bad, but they can also be bad for your health. Specifically, methane gas could be pulled into the bathroom. Methane gas is combustible and also dangerous to inhale.
In addition to pulling gases from drains, negative pressure in an airtight house will also cause problems with water drainage. This is because as the water is draining, air is pushing in the opposite direction. This slows the speed at which the water drains.
Oversized Bathroom Fans Can Cause Issues
A fan that is too large for the bathroom in which it is installed can cause problems for that bathroom, problems that resemble the signs indicative of the need for makeup air.
A fan that is too large can actually remove too much air, creating the same issues described above due to the negative pressure system created.
In such cases, it isn’t a matter of supplying makeup air. Really, it is a matter of swapping out the oversized fan with one that is appropriately sized. It will do the job without the detrimental side effects.
How to Provide Makeup Air for a Bathroom Fan
Makeup air can be provided in a bathroom in a few different ways, and the most effective strategy is to use a combination of these methods.
One way of providing makeup air for a bathroom is to undercut the door. This allows for air to enter the bathroom underneath the door.
A passive vent can also be added to one of the walls in the bathroom that connects the indoors with the outdoors and has louvers to close it when it is unnecessary.
There could also be a vent leading into the bathroom attached to the house’s HVAC system. This will allow for air to be pushed into the bathroom as well.
Actual makeup air units are typically unnecessary.
Sources
https://inspectapedia.com/odor_diagnosis/Backdrafting_Odors.php