Dryer vent materials and size: what the code allows
A dryer vent must be a 4-inch metal duct with a smooth interior, at least 0.0157 inch thick (No. 28 gage), under section M1502.4.1 of the International Residential Code, so PVC and plastic are out. Behind the dryer, the code allows a transition duct: one length, listed and labeled to UL 2158A, no longer than 8 feet, and not concealed within construction. The 2021 and 2024 editions of the code print the same words for these rules.
Dryer vent material code, material by material
| Material | As the exhaust duct (the run to the outside) | As the transition duct (dryer to the exhaust duct) | Source |
|---|---|---|---|
| Metal duct | Yes, if the interior is smooth, the metal is at least 0.0157 inch thick and the duct is 4 inches | Only a single length listed and labeled to UL 2158A | IRC M1502.4.1, M1502.4.3 |
| Flexible foil duct | Same code test; the USFA says to replace coiled-wire foil with rigid, non-ribbed metal duct | Only if listed and labeled to UL 2158A; Whirlpool says a flexible metal duct must be a type the appliance maker identifies as suitable | IRC; USFA; Whirlpool |
| Plastic or vinyl flexible duct | No: the code requires metal | Only a single length listed and labeled to UL 2158A; the USFA's 2012 report says all manufacturers then stated in their manuals not to use plastic, flexible dryer ducts between the vent and the dryer | IRC M1502.4.1, M1502.4.3; USFA |
| PVC pipe | No: the code requires metal | Only a single length listed and labeled to UL 2158A | IRC M1502.4.1, M1502.4.3 |
The exhaust duct runs from its connection with the transition duct to the outlet terminal (M1502.4.6.1). The code wants it supported at intervals of no more than 12 feet, with the insert end of each section pointing in the direction of airflow (M1502.4.2), and it must end outside at a termination with a backdraft damper and no screen (M1502.3). The rules for joints, screws and tape are further down this page.
Are foil dryer vents safe?
Not in the U.S. Fire Administration's view. Its dryer fire report says even flexible foil vents are not a good choice for venting clothes dryers, because flexible vents can twist and let lint build up and catch fire if it meets enough heat. Its advice is to replace coiled-wire foil or plastic venting with rigid, non-ribbed metal duct. Whirlpool's venting specification says not to install a dryer with flexible metal (foil type) duct, and that any flexible metal duct used must be a specific type the appliance maker identifies as suitable for dryers; it says flexible venting materials are known to collapse, be easily crushed and trap lint, which obstructs airflow and raises the risk of fire.
The NFPA's clothes dryer safety tip sheet says rigid or flexible metal venting material should be used to sustain proper air flow and drying time. If a foil hose connects your dryer today, look for a UL 2158A listing label on it; the code accepts a transition duct only when it is listed and labeled to that standard (M1502.4.3).
Flexible vs rigid dryer vent
Smooth metal for the exhaust run; a listed transition duct for the short connection behind the dryer. The exhaust duct must have a smooth interior finish (M1502.4.1), and where it runs through wall or ceiling cavities, the cavity must let the duct go in without deformation (M1502.4.2). The transition duct has the opposite rule: it may not be concealed within construction, and it may not be longer than 8 feet (M1502.4.3). Those 8 feet do not count toward the exhaust duct's maximum length, which the code sets at 35 feet from the transition duct connection to the outlet, reduced for fittings (M1502.4.6.1). The full length rules are on how long a dryer vent can be.
- Rigid metal: the USFA's recommended replacement for foil and plastic venting, and the duct Whirlpool uses to define a single dryer venting system. The code's own test is metal with a smooth interior, at least 0.0157 inch thick.
- Foil: the same listing test applies behind the dryer, but the USFA calls flexible foil vents not a good choice and Whirlpool says not to use foil-type duct.
UL listed dryer vent hose: the transition duct rule
The hose behind the dryer has four requirements, all in M1502.4.3: a single length, listed and labeled in accordance with UL 2158A, not greater than 8 feet, and not concealed within construction. The USFA's report restates the rule and adds that only flexible transition ducts listed by UL or another approved product safety testing agency should be used. Look for the listing mark on the duct itself before you buy it or accept an installation; the code asks for a duct that is both listed and labeled. The USFA also says to inspect the venting system behind the dryer to make sure it is not damaged, crushed or restricted.
What size is a dryer vent?
4 inches. The code says the exhaust duct "shall be 4 inches (102 mm) nominal in diameter" (M1502.4.1). As an alternative method, it also lets the dryer maker's installation instructions determine the size and maximum length of the exhaust duct (M1502.4.6.2), so check the manual for your model. Whirlpool defines a single dryer venting system as one dryer attached to a residential-type 4-inch rigid metal vent system, and says it does not approve anything other than rigid metal or flexible metal duct in a 4-inch diameter.
Bigger is not better, in Whirlpool's explanation: with an identical blower system, a duct larger than 4 inches lowers the air speed inside it, which reduces the air stream's ability to keep lint suspended and moving through the pipe. For a tight wall, Whirlpool accepts oval duct, meaning a 4-inch smooth round duct compressed to fit inside a 2x4 wall, with a smallest outside dimension of 3 1/8 inches and a length reduction once the oval sections total more than 20 feet. At the outside end, the code requires the termination's passageway to stay undiminished in size, with an open area of at least 12.5 square inches (M1502.3.1).
Can you use PVC for dryer vent?
No. The code requires the dryer exhaust duct to be metal (M1502.4.1). The USFA's dryer fire report says problems can occur when plastic or vinyl exhaust materials are used to vent dryers, and, writing about plastic, flexible dryer ducts, that the plastic itself can provide additional fuel for a fire. Whirlpool gives a second reason: a PVC pipe wall does not retain heat as well as metal, so moisture from the clothes condenses more easily inside, lint sticks to the walls and collects, and the vent can end up severely restricted or blocked. Whirlpool notes that some local codes may allow alternate vent designs, and says it still does not approve anything other than rigid or flexible metal duct in 4 inches.
The one place chapter 15 of the code permits schedule 40 PVC is a kitchen down-draft exhaust duct installed under a concrete slab (M1503.4). That exception sits in the section on cooking exhaust ducts and does not cover dryers.
Screws in dryer vent joints
Short ones only. The code says ducts shall not be joined with screws or similar fasteners that protrude more than 1/8 inch into the inside of the duct (M1502.4.2). The same section requires every joint to be sealed and mechanically fastened. Where screws or nails from finish work could pierce a duct inside a wall, the code calls for steel shield plates at least 0.062 inch thick on the framing wherever the duct sits less than 1 1/4 inches from the finished face (M1502.5).
Can you use duct tape on dryer vent?
Only tape that meets a named standard, and never as the only thing holding a joint. The code says dryer exhaust duct joints shall be sealed in accordance with section M1601.4.1 and shall be mechanically fastened (M1502.4.2). M1601.4.1 requires tapes used to seal metallic and flexible air ducts to comply with UL 181B and to be marked "181 B-FX" for pressure-sensitive tape. The code does not name duct tape or foil tape; it names the standard, so check the roll for the 181 B-FX mark before any dryer vent tape goes on. The 2021 and 2024 editions use the same wording.
If your vent is foil, plastic or crushed, replacing it is a repair job: see dryer vent repair, or dryer vent installation for a new run and its cost. To clear lint from a vent that is already to code, see how to clean a dryer vent or book professional dryer vent cleaning with the form below.
Dryer vent materials: common questions
How long can a dryer vent hose be?
Are all dryer vents the same size?
Can you use plastic pipe for dryer vent?
Best material for dryer vent
Are flexible dryer vents safe?
What tape to use for dryer vent?
Can you use galvanized pipe for dryer vent?
Sources
- International Code Council, 2021 International Residential Code, chapter 15, sections M1502 and M1503: codes.iccsafe.org
- International Code Council, 2024 International Residential Code, chapter 15, section M1502: codes.iccsafe.org
- International Code Council, 2021 International Residential Code, chapter 16, section M1601.4.1: codes.iccsafe.org
- International Code Council, 2024 International Residential Code, chapter 16, section M1601.4.1: codes.iccsafe.org
- U.S. Fire Administration, "Clothes Dryer Fires in Residential Buildings (2008 to 2010)", August 2012: usfa.fema.gov
- NFPA, "Clothes dryer safety" tip sheet: nfpa.org
- Whirlpool, "Dryer venting specifications" (W11688180A): whirlpool.com