Section M1507. This section notes that air exhaust from the bathroom must be sent outdoors, not indoors, to the same residence or indoors to any other dwelling unit. It cannot move air to a crawlspace or attic.
Code Requirements for Bathroom Exhaust Fans
Chapter 15 of the International Residential Code (IRC) requires bathrooms and toilet rooms to have a minimum local exhaust capacity of 50 cfm intermittent or 20 cfm continuous.
To fulfill the local exhaust airflow requirements of ASHRAE 62.2 (2010) and the International Residential Code (IRC 2021, Section M1507), bathroom fans should have a mechanical exhaust capacity of ≥ 50 cfm for intermittent operation or ≥ 20 cfm of ventilation when operated continuously.
For better performance, duct runs should be as straight as possible. If a bend is necessary, 2 to 3 feet of straight duct should be allowed to run from the fan exhaust port to the first elbow in the duct run.
How far can you run ducting for an exhaust fan in the bathroom? There is no guidance on the duct length; only a straight run is preferred to a duct with bends or elbows. On average, the vent duct can run from 2 to 30 feet.
The most common types ducts that can be used for a bathroom exhaust fan installation are rigid ducting, semi-rigid ducting, flexible ducting or a rigid duct and flexible duct combination.
The duct should vent to the outside.
For continuous indoor air quality ventilation, a heat or energy recovery ventilator (HRV or ERV) should provide 0.35 air changes per hour. This calculation must consider the complete occupied volume of the house. This rate can be more easily calculated by allowing 5 CFM per 100 square feet of floor area.
Yes, the toilet and sink can share the same vent, as can the shower. It's common to have one main vent pipe for multiple fixtures, although you might need a larger pipe. You should check with local building and plumbing codes to confirm you'll have adequate piping for your home.
A fan that is too big can cause a bathroom to be cold and noisy. This is why calculating the correct CFM is important.
If your new bathroom vent fan is the same size as your old one, you can install it right away. If it's bigger, you'll have to cut a larger hole with a drywall saw. If it's smaller, you can caulk around the edges of the housing unit to fill in the gaps once the fan is installed.
In general, the vent should be as close as possible to the fixtures it serves. This proximity helps ensure the vent can effectively equalize air pressure and prevent sewer gas entry. However, there are some common guidelines to consider: The vent should be within 5 to 6 feet of the fixture drain it serves.
The most common option for bathroom exhaust fan venting is the ceiling to wall venting option. The second most common is the roof vent installation. The third most common is probably the vertical wall installation.
One Foot In, One Foot Out – Ideally, you should place a bathroom fan in the ceiling about 1 foot from the shower and between the shower and the door.
A ventilation fan brings fresh air into a space, improving circulation, while an exhaust fan removes stale, humid, or polluted air, typically from kitchens or bathrooms. The key difference lies in airflow direction: ventilation fans pull outdoor air in, while exhaust fans push indoor air out.
Behind or within the Wall: In most cases, the vent pipe for a toilet should run vertically from the toilet's waste pipe (known as the closet bend) and extend upward to connect with the main vent stack or a branch vent line that ultimately exits through the roof of your home.
While building codes vary by location, most communities and states require an established form of bathroom ventilation. Exhaust fans are required in bathrooms that don't have a window or if the window is smaller than 3 square feet. The window must also open.
M1505.
Exhaust air from bathrooms and toilet rooms shall not be recirculated within a residence or circulated to another dwelling unit and shall be exhausted directly to the outdoors.
When drain lines aren't properly vented, low pressure inside the pipe can suck water out of the traps, letting sewer gas in. Air pressure imbalances inside the drain pipe can restrict water flow and cause sluggish draining, often mimicking a partial clog inside the pipe.
Other venting options includes running the duct up through the roof or down through the soffit. Note that the bathroom vent fan must always exhaust to the outdoors; never allow the duct to simply blow into an attic, crawlspace or other enclosed area.
One of the major disadvantages of flex duct is that it lacks any form of structural support. Because of its soft, bendable make-up, flex duct can easily sag or compress over time due to its own weight. This lack of structural support can lead to reduced air flow and ineffective HVAC performance.
This is a sure way to cause blockages that can lead to larger issues.
The installation of a recirculating exhaust fan is one of the easiest ways to vent a bathroom without an exterior exit. Recirculating fans filter the air via a set of charcoal or HEPA filters before releasing it back into the bathroom, in contrast to ordinary exhaust fans, which exhaust air outside.