Bathroom exhaust fans have a few key code requirements according to the International Residential Code (IRC) as well as the National Electrical Code (NEC).
It is essential to also keep in mind that many ‘code violations’ may be exempted for an existing home if it was constructed prior to the code taking effect.
The IRC is known as a model code that many U.S. states have adopted, but your local state, city, or county takes precedence over the IRC and may have exceptions.
Below are 7 bathroom fan venting codes from the IRC and NEC code handbooks:
- Bathroom Fans Are Usually Required For New Homes
- CFM & Diameter Determines The Maximum Length of Exhaust Fan Duct
- Duct Diameter Minimum Is 3-Inches
- 50-CFM Is The Minimum Fan Speed of Bath Fans
- Bathroom Vent Fans Can’t Terminate In An Attic
- Bath Fans Are Allowed Above Showers If Rated (And Has GFCI Circuit)
- Flex Or Metal Duct Can Be Used For Bathrooms
Code #1 – Bathroom Fans Are Usually Required For New Homes
Bathroom exhaust fans are generally required for full and half baths in every new home.
I can’t even remember inspecting a brand new home that didn’t have an exhaust fan in every bathroom.
However, most older homes will be ‘grandfathered in’ to this code requirement.
Your local county may also just require an openable window or a ductless bath fan in the place of a standard exterior vented bath fan. You can see our article on ventless (recirculating) bathroom fans here or your bathroom ventilation options here.
Check with your local building code department to determine whether a bathroom fan is required if you are renovating your bathroom and a permit is required.
Code #2 – CFM & Diameter Determines The Maximum Length of Exhaust Fan Duct
Sometimes my clients wonder, how long can a bathroom exhaust fan run? A 50-cfm bath fan that has a 3-inch duct can have a maximum 5-ft duct.
But in general, the maximum duct length for bathroom fans (and dryer ducts) will depend on:
- how strong your bath fan is (cfm rating)
- the diameter of the duct (usually 3″ or 4″)
- the length of the duct run
The IRC (section 1504.2) has a helpful table on allowable lengths as shown below. I invite you to read our reviews of the best ducts for bathroom fans here.
The CFM (fan speed) of bathroom fans are usually in the range of 50-cfm to 150-cfm.
Code #3 – Duct Diameter Minimum Is 3-Inches
The minimum exhaust fan duct size for a bathroom fan is a 3-inch diameter.
Older bathroom vents tend to have the 3-inch diameter duct hose, while newer and more powerful bathroom fans have 4-inch to 6-inch diameters.
The larger duct allows more air to be exhausted, but it also extends the maximum allowable duct length.
As an example, if you have a flex duct (non-smooth metal duct), and if your bathroom fan has a 100-cfm rating — then the longest duct according to the IRC is 42-feet for a 5-inch diameter duct.
Code #4 – 50-CFM Is The Minimum Fan Speed of Bath Fans
The minimum fan speed of a bathroom fan must be at least 50-cfm according to the IRC (section 1505.4.4).
I have never seen a bathroom fan with a CFM rating less than 50-cfm in a home store. You can read my full guide on picking the correct bathroom fan CFM right here.
The general rule is that you want at least 1-cfm for every square foot of your bathroom up to 100-sf.
If your bathroom is 75-sf, then ideally you want a bathroom fan with at least a 75-cfm speed rating.
Code #5 – Bathroom Vent Fans Can’t Terminate In An Attic
If you are installing a new bathroom fan, you may be tempted to just exhaust it into a ceiling cavity or maybe into an attic.
But this will definitely violate the bathroom exhaust fan termination requirements as stated in IRC section 1501.1. You can read my complete guide on whether you can vent a bathroom fan into an attic here.
The bathroom vent to attic code states that you can vent through an attic and exhaust to the outside, but you can’t exhaust into an attic.
No bathroom fan should ever be exhausted into an attic, crawlspace, or other interior area of a home.
Motivation of This Bathroom Vent Duct Code
Frankly, during my home inspections, I have seen many bathroom fans vented to attic spaces, and I always include it in my reports.
The reasoning behind these code requirements is that bathroom fans are exhausting a lot of moisture, and this moisture can lead to mold and wood decay if it isn’t vented to the outside.
And it can also negatively impact indoor air quality even without visible mold growth.
If you will be exhausting your bathroom fan through (not into) an unconditioned space like an attic or crawlspace, I highly recommend that you install an insulated flex duct.
There are pre-insulated flex ducts on the market that will help prevent condensation problems on the duct itself.
These ducts typically come in 25-foot long sections and include a layer of fiberglass insulation.
Read Also >> How To Insulate Your Bathroom Fan Duct?
Code #6 – Bath Fans Are Allowed Above Showers If Rated (And Has GFCI Circuit)
Bathroom fans are allowed to be installed above showers and tubs under certain circumstances.
The exhaust fan in shower code is touched upon in the National Electrical Code (NEC) chapter on “Damp or Wet Locations” in Section 314.15.
Basically, the electrical code states that the fan (or fan and light combo) will need to be listed by the manufacturer as being designed for such an installation.
The NEC also states that the box needs to prevent the intrusion of water or moisture (it can’t be installed in a weird location where water goes into it).
And the most important thing about bathroom fans in wet locations is that they need to be connected to a GFCI circuit.
This means that if there is some type of electrical short (possibly caused by water), then the power to the bathroom fan will shutoff.
All manufacturers require GFCI connected bathroom fans if installed above a shower or bathtub.
Read Also >> How To Vent Bath Fan Through The Wall?
Code #7 – Flex Or Metal Duct Can Be Used For Bathrooms
According to the IRC, you can use either flex or rigid metal ducting for your bathroom exhaust vent.
If you are planning on installing a new duct, rigid and smooth metal ducting is still the best choice.
These metal ducts are sturdier and maximize airflow with the smooth interior walls. The downside is that they are more expensive and difficult to install.
However, the IRC only states that metal ducting is required for dryer vents, and NOT bathroom vent fans.
As a home inspector, I have seen many improperly vented bathroom fans, but the biggest issue is not venting bath fans to the exterior.
And it isn’t hard to see why, it can be a real pain to install wall or roof vent covers. (I recently wrote a detailed guide on installing bathroom fan roof vent covers.)
But it is worth the effort to prevent any potential mold issues or even wood degradation.
The extra moisture spewed out by a bathroom fan can also attract termites, critters, and other insects like ants.