New Jersey builds to the International Residential Code, and the dryer exhaust requirements in it are short, specific and broken constantly. Here is what they actually ask for, why each rule exists, and which ones we find violated most often.
Your local code official is the authority, and this is a plain-language summary rather than a substitute for them.
The duct has to be rigid metal, and smooth on the inside
The concealed run is smooth-wall rigid metal duct, typically four inches in diameter for a residential dryer. Not plastic. Not the flexible foil accordion sold next to the dryers.
The reason is mechanical, not bureaucratic. Every ridge in a corrugated duct is a place lint stops. Foil sags between supports, and a sag becomes a low point where moisture collects and lint mats. It also tears, and a torn duct inside a wall discharges warm wet air into the cavity.
A short listed transition connection between the machine and the wall is normal and allowed. The concealed run is not the place for it.
Joints go in the direction of airflow, and no screws inside
Sections are joined so the downstream piece fits inside the upstream one, in the direction the air moves. That way lint travels past a joint instead of catching on a lip.
Fasteners must not protrude into the duct. A screw sticking into the airstream is a lint collector, and over a couple of years it will build its own plug. Joints get taped or sealed rather than screwed through.
The length limit counts elbows, not just tape measure
There is a maximum developed length for a dryer exhaust, and elbows count against it heavily. A ninety-degree turn is treated as the equivalent of several feet of straight duct, and a tighter radius counts for more than a gentle one.
The practical consequence: a run that measures twenty feet on a tape can already be over the limit once four turns are added. Manufacturers also publish their own maximum for the specific machine, and where the two disagree the shorter one governs.
This is why we survey the route before quoting rather than measuring a straight line on a floor plan. It is also why “it is only twenty feet” is not the reassurance people think it is.
It has to terminate outdoors
Outside. Through a wall or a roof, with a hood.
Not into an attic, a crawlspace, a garage, a soffit or a chimney flue. A dryer puts several litres of water into the air every week along with the lint, and discharging that into an enclosed part of the building is how attics grow mould and insulation gets ruined. We are called to fix this often enough that it is worth checking yours.
The termination needs a backdraft damper and no screen
The hood has a flap that opens under airflow and closes when the machine stops, which keeps weather and wildlife out.
It must not have a screen or mesh. Screens catch lint by design, and a screened dryer termination blocks within months. If somebody added one to keep birds out, the correct fix is a proper hood with a working damper, not a mesh.
There are also clearance requirements from openings, and the hood needs to be positioned where the discharge is not blowing straight into a window or an air intake.
Booster fans, where a run is genuinely too long
Where the layout makes a compliant length impossible, a listed dryer exhaust booster fan can be used. It is not a device to bolt onto a badly planned run to make the numbers work: it is for the case where the building geometry leaves no shorter path. Booster fans need power, they need access for service, and they need cleaning like everything else in the line.
If somebody proposes one on a run that could have gone to a nearer exterior wall, ask why.
The violations we find most
- Flexible foil accordion duct used for the concealed run.
- Termination into an attic or a crawlspace.
- A screen over the exterior hood.
- Screws through joints, projecting into the airstream.
- A run with far more elbows than the length budget allows, usually because it took a convenient path rather than a short one.
- A crushed transition hose behind a machine pushed too far back.
Every one of those is fixable, and most of them are visible without opening a wall.
What to ask for when you have an install quoted
Ask what material the concealed run is, where it terminates, how many elbows are in the proposed route, and whether the exterior hood has a damper and no screen. Ask whether the installer will run the machine and check the flap from outside before leaving.
Those five answers tell you whether you are getting an install that will still be working in ten years, or one that will need cleaning twice a year for the rest of its life.
