Do you want to learn about bathroom fan venting code?
Probably the most important bath fan vent code is that they need to exhaust to the outside.
In this guide, I will go over…
- Where it can vent to
- Maximum duct length
- Minimum CFM needed (fan speed)
- Windows replacing fans
- And installing bath fans above showers or tubs
Bath Fan Ducting Is Laid On In The IRC And NEC
Bathroom exhaust fans have a few key code requirements according to the IRC or International Residential Code as well as the National Electrical Code (NEC).
Most of the bathroom fan venting code is laid out in IRC Chapter 15 ‘Exhaust Systems and IRC Chapter 16 ‘Duct Systems’.
Your local state, city, or county takes precedence over the IRC and will probably have numerous exceptions. Always consult your local building code or local building department for the final verdict.
Bath Fan Vents Need To Be 10-Ft From Intakes
The IRC states in section 1504.3 that bathroom fans should terminate at least 10-ft away from vent intake openings. For example, if you have a high efficiency furnace, you will have a PVC pipe that extends to the outside which is a fresh air intake.
It makes sense that you want your bathroom fan vent to be several feet away from something that is sucking in air. You don’t want the moisture-heavy air getting sucked into a furnace causing problems.
The IRC also states that you can have the bathroom fan vent hood above a furnace but by at least 3-ft. You also need to have the exhaust to be 3-ft away from windows, doors, intake vents, and property lines.
Read Also: How To Clean Bathroom Fans?
CFM & Diameter Determines The Maximum Length of Exhaust Fan Duct
Sometimes my clients wonder about the maximum duct length for bathroom fans (and dryer ducts) and the IRC (section 1504.2) has a helpful table on allowable lengths.
Basically, you will need to know the fan speed or CFM of your bathroom fan which usually in the range of 50-cfm to 150-cfm. They are almost never greater than 200-cfm unless it is a commercial exhaust fan.
You will also need to know the duct diameter that you are using. Older bathroom vents frequently have a 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.
Read Also: What Are The Best Decorative Bathroom Fans?
50-CFM Is The Minimum Fan Speed of Bath Fans
And according to the IRC (section 1505.4.4), at least 50-cfm is the absolute minimum CFM required in a house. This is a bit a of no-brainer because I have literally never seen a bathroom fan with a CFM rating less than 50-cfm.
The general rule is that you want at least 1-cfm for every square foot of your bathroom up to 100-sf. So if your bathroom is 75-sf, then ideally you want a bathroom fan with at least 75-cfm speed rating.
You can read my full guide on picking the correct bathroom fan CFM right here.
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 code as stated in IRC section 1505.2 and 1501.1.
The IRC states that no bathroom fan should be exhausted into an attic, crawlspace, or other interior area of a home.
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 includes a layer of fiberglass insulation.
A Window Can Replace A Bathroom Fan
The short answer is that yes, under IRC code, a window is allowed to replace a bathroom fan.
The IRC says (section R303.3) that if you have a window that is at least 3 square feet in size, and half of it is openable, then a bathroom fan is not required. And this makes sense because you can always just crack open the window after taking a shower to release the moisture.
Of course, in my opinion, a bathroom fan is always best for a variety of reasons such as influencing the conditioned HVAC air, allowing pests to enter the home etc.
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 National Electrical Code touches on this topic in their 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 it needs 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.
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.