Most confusion about dryer duct comes from treating it as one thing. It is two, and the rules for each are different.

The exhaust duct is the permanent run, the part inside walls, ceilings or the attic that carries air to the outside of the building. The transition duct is the short visible connector between the back of the dryer and the wall opening. Material that is perfectly acceptable for one is prohibited in the other, which is why advice that does not draw the distinction ends up either wrong or uselessly vague.

The exhaust duct: rigid metal, no exceptions

Every concealed run has to be rigid metal, aluminum or galvanized steel, with a smooth interior. The commonly referenced specifications are four inches in diameter and a minimum wall thickness around 28 gauge. Nothing flexible belongs inside construction, regardless of what it is made of or what it is listed for.

The reason is simple enough. A concealed duct cannot be inspected, cannot be reshaped once it sags, and cannot be replaced without opening a wall. It has to be the material that holds up unattended for decades, and that is rigid metal.

The transition duct: what code actually requires

This is where the common advice gets sloppy, including our own earlier version of this page.

Building code is largely silent on what a transition duct is made of. What it requires is that the duct be listed to UL 2158A, the standard covering clothes dryer transition ducts. That listing is not a formality. To carry it, a duct has to pass flame resistance, surface burning, crush and bending, puncture, corrosion and tension testing.

The other transition duct requirements matter just as much and get ignored constantly:

• Eight feet maximum length
• A single continuous piece, not sections joined together
• It must remain visible. A transition duct concealed inside construction is not permitted, ever
• Joined with clamps or foil tape, never screws

Short and straight is the goal. Every extra foot of slack coiled behind the machine is airflow you are giving away.

Flexible foil, honestly

Our previous version of this page said avoid foil outright. That is good practical advice and slightly stronger than the truth, so here is the accurate version.

Foil transition duct that carries a UL 2158A listing is permitted by code, and most of what hardware stores sell as transition duct does carry the listing. What makes it a bad choice is performance rather than legality.

The corrugated wall creates drag before lint is involved at all, and the effective opening narrows in practice, with a nominal four-inch duct behaving closer to three and a quarter. Lint then catches in every ridge. It sags between supports, it kinks, and it flattens the moment a dryer gets pushed back against the wall. LG documents the consequence directly: foil corrugation creates back pressure the airflow sensor reads as a blockage, so the machine reports a restricted duct that is not actually clogged yet.

Several dryer manufacturers go further and prohibit foil in their own installation instructions. That matters more than it sounds, because code also requires the dryer to be installed per the manufacturer manual. When the manual is stricter than code, the manual wins.

Plastic and vinyl: never

White vinyl duct puts a combustible material directly in a line carrying hot air and lint, it deforms with heat, and it will not contain flame if lint ignites. It is prohibited by most codes and by essentially every dryer manufacturer, and using it is likely to void the appliance warranty as well.

If you find plastic duct on a dryer, it is a replacement, not a cleaning.

The transition ducts actually worth using

Semi-rigid aluminum. The long-standing preferred choice. Smooth interior, holds a true four-inch diameter, bends enough to make the connection, and is widely UL 2158A listed. Keep it short and keep the bends gentle.

Aluminum ribbon transition duct. A newer category built from wound aluminum ribbon over a wire frame. It is as flexible as foil to install but crush resistant, and it holds its diameter under compression and stretching. If clearance behind the machine is awkward, this solves it without giving up airflow.

Periscope or slim duct. A rigid telescoping aluminum piece designed for shallow closets, where the dryer sits within a few inches of the wall. Worth knowing about here, because condo laundry closets routinely have less clearance than the transition duct wants, and the usual improvisation is to crush a foil hose flat behind the machine and forget about it.

Length and elbows matter as much as material

The exhaust duct has a ceiling of thirty-five feet of developed length, and bends count against it. The standard math deducts five feet for each ninety-degree elbow and two and a half feet for each forty-five. A run with four ninety-degree turns has spent twenty feet before covering any actual distance.

Where a run exceeds the equivalent length limit, some code editions require a label near the connection stating the length, so the next person installing a dryer knows what they are connecting to. Booster fans are not a permitted workaround.

This is the part that catches older coastal condo buildings, where interior laundry closets vent horizontally to an exterior wall. Long runs are not automatically illegal, but they are worth measuring rather than assuming. See how to trace your run.

Installation details that matter as much as the material

• Joints face the direction of airflow. Assembled backwards, every joint becomes a ledge for lint.
• Foil tape, not screws. A screw projects into the duct interior and collects lint at the exact point nobody can see.
• Support at least every twelve feet and at joints. Unsupported duct sags, and lint settles at every low point.
• Terminate outdoors with a damper. A gravity or automatic damper, and a louvered one performs better than a single hinged flap.
• No screen or bird guard on the termination. They clog with lint quickly, which is why they are not permitted there.
• Keep the termination clear of other openings. Commonly at least three feet from windows and doors, and around ten feet from an air conditioning or heat pump condenser. That second one gets ignored constantly in tight South Florida side yards, and lint on a condenser coil becomes an air conditioning problem on top of a dryer problem.
• Ducts passing through a garage have heavier sheet steel requirements and no openings into the garage.

The rule that overrides all of it

Install the dryer the way its manufacturer says to. Code requires that too, and where the manual is stricter than the code, the manual governs. If your manual prohibits foil, foil is not an option in your installation even though it is legal in general.

What this means practically

If your concealed run is foil or plastic, that is the actual problem, and no cleaning schedule fixes it. You are cleaning more often than you should need to and getting worse performance in between. Replacing it with rigid metal is a one-time job that changes both.

If the concealed run is already rigid metal and only the transition duct is wrong, that is a much smaller fix and often gets handled during a cleaning visit. Either way, the first step is finding out what is actually back there, which is not something you can tell from the front of the machine.