Are you installing a new dryer vent and want to know about the code requirements?
Dryer vent code helps prevent fires, moisture damage, and other venting problems during installation.
The most important dryer venting code requirement is that the dryer duct should always go to the outside and never vent into an attic, soffit, or crawlspace.
In this HomeInspectorSecrets.com guide, you will find some of the most essential dryer vent code requirements stated by the IRC (international residential code), such as:
- Maximum Length: The longest allowable length of dryer ducts is 35-feet.
- Duct Material: The duct should be made out of smooth metal (no ridges).
- Distance From Openings: The duct termination (exterior cover) should be at least 3-ft from windows and doors
- Flex Vent Length: The flex vent or transition duct (goes from the back of dryer to wall) cannot be more than 8-feet.
- Independent Vent: The dryer duct cannot be connected to a plumbing vent, HVAC vent, or chimney.
Below, we detail all of the 11 key dryer vent code requirements. Keep scrolling to learn the nitty-gritty of the IRC dryer vent code to keep your home safe.
11 Key Dryer Vent Code Requirements
1. An Independent Vent
Sometimes people ask us if the dryer vent can be connected to another vent and according to the IRC — it cannot.
The dryer vent needs to be independent and it can't be connected to a range hood vent, plumbing vent, HVAC vent, or chimney.
Anytime a dryer vent is improperly connected to another vent, there is a greater risk of lint buildup which may lead to a fire, dryer damage, or moisture damage.
2. Must Go Outdoors
The dryer vent location code states that the duct must terminate outside and should not be vented into the house.
It cannot be terminated inside an attic, soffit, wall cavity, or another enclosed area.
Dryers send out gallons of moisture during the day, and all of this moisture can damage wood sheathing, wood framing, and drywall if it is improperly ducted to an interior space.
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3. Distance From Windows & Doors
The dryer vent location code states that the exterior hood needs be at least 3-feet away from any openings such as windows, doors, or HVAC intakes.
Since the dryer is sending out a lot of moisture and lint particles, you don't want the dryer exhaust to be sucked into the home through a window, HVAC intake, or door.
Inhaling dryer exhaust is hazardous to your health and may damage HVAC equipment (or your high-efficiency water heater).
4. The Duct Material
The dryer vent system (the duct inside the wall) must be made out of metal and have a smooth interior finish.
Don't confuse this dryer venting code with the transition duct which is the short & flexible duct that goes from the dryer to the wall. The dryer duct that goes into the wall and then terminates to the outside needs to have at least a 0.0157-inch metal thickness (0.3950-mm) and #28-gauge metal.
The dryer duct also needs to be at least 4-inches in diameter but you should follow the manufacturer's recommendations because a lot of newer bathroom fans require 6-inch ducting.
5. Dryer Vent Installation Code
When you install a new dryer vent and the walls are still open (pre-drywall), you need to support the dryer duct at 12-foot intervals with straps or some type of fastener.
The male or insert end of the dryer duct needs to be inserted into the female duct end in the direction of airflow in order to avoid lint blockages.
Each joint of the dryer duct needs to be sealed with metal tape or mastic in order to prevent air leaks. Also, each duct joint needs to be fastened together with screws (or another type of fastener) but the screws cannot go into the duct more than 1/8-inch so it doesn't obstruct the airflow.
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6. The Flex Vent Length (Transition Duct)
The dryer vent length code for the short vent that goes from the back of the dryer to the wall (also called the transition duct) cannot be more than 8-feet.
This transition duct, normally made out of flexible material, cannot be concealed behind anything—it must be out in the open behind the dryer.
7. Wall Duct Maximum Length
The dryer vent length code for the in-wall duct system (as measured from the wall to the exterior hood) cannot be more than 35-feet in length. This maximum length does not include the transition duct that is immediately behind the dryer.
The length of the main duct will also need to be reduced based on the number (and degree) of bends in the duct.
Below is a table that includes the duct diameter, elbow degree, and effective duct length. You will need to subtract the effective duct length from the total duct length. This makes sense because each bend in the duct reduces the dryer vent airflow.
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Effective Duct Length
8. Provide Copy Of Manufacturer Instructions (New Construction)
If you installed a new construction dryer vent duct, you will need to leave a copy of the manufacturer instructions that includes the maximum duct length and duct diameter.
A copy of the instructions needs to be provided at the duct concealment inspection (prior to drywall finishing work).
9. Length Identification (New Construction)
For new construction, the dryer vent length code requires that if the total duct length is greater than 35-feet (and it is allowable by the manufacturer) then the duct length will be on display through a permanent tag (or label) at the duct location.
10. Dryer Duct Cap
If a duct has been installed, but the dryer hasn't been installed yet, then the duct needs to be capped and labeled for "future use".
Also, if the laundry room has specified space for a dryer, then the wall duct system needs to be installed.
11. Protection Plates
This dryer vent installation code requirement states that protection plating must be installed on the framing if it is possible that nails or screws may go into the duct.
Protection plates need to be installed on the home's framing if there is less than a 1/4-inch between the dryer duct and the wood framing.
The protection plate must be made out of steel and be at least 0.062-inch thick.
Protection plating installed above sole plates and below top plates needs to extend at least 2-inches.
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What Do Home Inspectors Look For?
It is always important for home inspectors to thoroughly examine the dryer duct because they can be fire hazards if they aren't properly installed. Here are a few things we look for:
When I inspect a home, I always take a peek behind the dryer with my flashlight to look for problems.
If the dryer vent is crushed or has a kink in it, it will always be cited in my home inspection report. A kinked dryer vent can reduce airflow by 50% or more, leading to a lint clog, and possibly a lint fire.
Turn On Dryer And Check Airflow
There is only one way to know if the dryer vent is working properly and that is to turn on the dryer and check for strong airflow at the exterior hood.
If there is little or no airflow, then I know there is an obstruction or disconnection somewhere in the dryer vent and it should be professionally cleaned or repaired.
Exterior Hood Cover
The exterior hood cover needs to have a single flap or louvers that open when the dryer is on and closes when the dryer is off.
This is also called a backdraft damper and it prevents exterior air (and pests) from infiltrating the dryer vent.
If the damper is stuck, missing, or not working properly, it can obstruct airflow and allow birds to make nests inside the duct (a very common problem).
No Metal Screens
I also check for metal screens that cause lint buildup and subsequent airflow blockage.
There should never be a metal screen and qualified dryer vent covers have larger holes that allows stronger airflow.
Dryer Vent Cleaning
When was the last time the dryer vent was cleaned? Dryer vents need to be cleaned at least every two years as a safety precaution.
Cleaning your dryer vent also keeps your dryer running properly.
Professional dryer vent cleaners use professional air compression tools and power brushes that can clean wall ducts up to 30+ feet that homeowners cannot accomplish.
The Bottom Line
To sum it up, having a properly installed dryer vent is important to prevent a fire hazard, maintain indoor air quality, and quickly dry your clothes.
Even though this guide addresses the most important IRC dryer vent installation code requirements for dryer vents, I always recommend checking with your local city/county or state because their code will almost certainly supersede the IRC (though most of it will probably be the same).
The International Residential Code is a so-called model code that is updated every 3-years and many U.S. states and countries base their building code upon it. Even though in many cases states copy word-for-word the IRC, your local dryer venting code will probably have exceptions and small modifications.
There are also numerous code exceptions for dryer vents that may be allowed by the duct manufacturer and which can take priority over local code or the IRC as well.
I hope you enjoyed this dryer vent code guide. Please share your dryer vent experiences or ask us a question below.