Dryer Wall Vent: Complete Guide

A dryer wall vent is the exterior hood and duct run that carries hot, moist, lint-laden air out of your house. It looks like a minor part, but a bad one adds 20 minutes to every cycle, drives up your energy bill, and is the leading cause of the roughly 14,000 dryer fires reported in the U.S. each year. Getting the wall vent right is equal parts efficiency and safety.

This guide covers the hood you mount outside, the duct sizing behind it, the code rules that govern the whole path, and how to keep it clean. Whether you are installing a new run or fixing a lazy dryer, the fundamentals are the same.

What a Dryer Wall Vent Actually Is

The system has three parts. First, the exterior hood or wall cap that penetrates your siding. Second, the rigid duct inside the wall and through the floor or attic that carries the air. Third, the flexible transition duct that connects the dryer to the wall duct. People use “dryer wall vent” to mean the exterior hood specifically, but a good install treats all three as one path, because the weakest link sets your drying time and your fire risk.

Standard duct is 4 inches in diameter. That size is set by the appliance and by code, and shrinking it chokes airflow. The exterior hood should match at 4 inches and mount so the flapper opens freely and closes when the dryer is off.

Choosing the Right Wall Cap

You have three main hood styles, and the choice matters more than most people think.

• Louvered hood: Two or three flapper slats. Cheap, around $8 to $15, but the small slats catch lint and pests love to nest behind them.
• Single-flap hood: One large damper that swings open with airflow. Around $12 to $25. Far better lint clearance and my default recommendation.
• Damper with a wide mouth (“dryer wall vent” hoods): A large 4-inch opening with a swinging or floating damper, often with a removable bird guard. $15 to $30.

Avoid any hood with a fine screen or fixed mesh. Lint packs into the mesh within weeks and turns your efficient dryer into a slow, hot fire hazard. If your existing cap has a screen, cut it out. A pest guard belongs on a magnetic or removable design you can clean, not permanently blocking the airflow.

Duct Sizing and Maximum Length

The International Residential Code limits a dryer exhaust duct to 35 feet of developed length. Every 90-degree elbow subtracts 5 feet from that budget, and a 45-degree elbow subtracts about 2.5 feet. So a run with two 90s can be no longer than 25 feet of straight duct. Many newer dryers ship a manufacturer chart that allows longer runs with a booster fan, but 35 feet is the baseline.

Use smooth-wall rigid metal duct, 4-inch, for everything inside the wall, floor and attic. The smooth interior sheds lint instead of catching it. Never use the white plastic or thin foil accordion hose anywhere in the concealed run; it is a fire accelerant and a lint trap, and it is banned by code inside walls.

Installing a Dryer Wall Vent Step by Step

For a through-the-wall install, plan the exit point first. It should be at least 3 feet from any window, door, or soffit intake, and it must not terminate under a deck or in a crawlspace.

1. Drill a locator hole from inside, then use a 4-1/4-inch hole saw from outside to cut through the sheathing and siding.
2. Run 4-inch rigid duct from the interior wall to the exit, keeping the total length within your elbow budget.
3. Seal duct joints with UL-listed foil tape, not sheet-metal screws. Screws protrude into the airflow and snag lint.
4. Slide the hood’s collar into the wall duct, bed the flange in exterior sealant, and fasten to the siding with corrosion-resistant screws.
5. Connect the dryer with a rigid or semi-rigid metal transition duct no longer than 8 feet, and clamp both ends.

The no-screws rule inside the duct is not a suggestion. Every screw point becomes a lint anchor, and over a year those anchors build a felt-like mat that restricts airflow and feeds a fire.

Code and Safety Rules You Cannot Skip

Beyond the 35-foot limit and the smooth-metal rule, code requires that the duct terminate outdoors, never into an attic, crawlspace, or garage. Venting into an attic dumps gallons of water vapor into your insulation and framing every week, and it will grow mold and rot the sheathing. The termination must have a backdraft damper and must not have a screen.

Gas dryers add a second concern. Their exhaust contains combustion byproducts, so a blocked or leaking vent can push carbon monoxide back into the house. If you run a gas dryer, keep a working CO detector nearby and treat any backdraft or hard-to-close damper as urgent.

Signs Your Dryer Wall Vent Needs Attention

Watch for these symptoms, which almost always trace back to a restricted or failed vent:

• Clothes still damp after a normal cycle, or drying time creeping past 45 minutes.
• The top of the dryer is hot to the touch, or the laundry room feels humid.
• A burning or musty smell during a cycle.
• The exterior damper does not open with airflow, or stays open when the dryer is off.
• Lint visible around the hood or a stalled flapper.

Any one of these means air is not moving. A dryer that has to run two cycles is not just costing you money; the trapped heat is exactly the condition that trips thermal fuses and starts fires.

Cleaning and Maintenance

Clean the full path once a year, more often if you dry heavy loads or have a long run. A dryer vent brush kit runs $15 to $30 and reaches through the wall duct. Disconnect the transition hose, brush from both the dryer end and the exterior hood, and vacuum the debris. While you are outside, wipe the damper and confirm it swings freely.

If your run is long or has multiple elbows, a professional cleaning costs $100 to $180 and includes a camera check. That is cheap insurance against a lint fire. A clean dryer wall vent pays for itself in shorter cycles and a lower power bill, and it is the single most effective thing you can do to keep your laundry room safe. Treat the vent as a system, keep it metal and smooth, and inspect the exterior hood every time you clean the lint screen.

Wall Vent vs. Roof Vent vs. Soffit Vent

Where the exhaust exits changes performance and maintenance. A side-wall termination is the best of the three: it’s the shortest path in most homes, it stays accessible from a ladder for cleaning, and gravity helps drain any condensation back outside. That’s why the wall vent is the default recommendation whenever the dryer sits on an exterior wall.

A roof termination is sometimes unavoidable on interior laundry rooms, but it forces the moist air to climb vertically, invites more lint buildup, and puts the cleanout on the roof where few people go. Soffit venting is the worst option and is banned in many areas, because the rising warm, damp air gets pulled straight back into the attic through nearby intake vents, feeding mold and rot. If you have any choice in a new build, run the dryer to a side wall.

Booster Fans for Long Runs

When architecture forces a run past the 35-foot budget, a dryer duct booster fan can make it work. These inline fans sense airflow or dryer operation and give the exhaust an extra push, effectively extending the allowable length. Expect $150 to $400 for the fan plus installation. They’re a real solution for a laundry room stuck in the center of a large house, but they add a moving part that needs its own periodic cleaning and eventual replacement. Try to shorten and straighten the duct before adding a booster; fewer elbows beats more fan every time.

Common Wall Vent Mistakes to Avoid

• Using foil or plastic flex duct in the concealed run. It sags, traps lint, and is a fire accelerant.
• Fastening joints with screws that protrude into the airflow and snag lint. Use foil tape instead.
• Crushing the transition hose by shoving the dryer tight to the wall. Leave a few inches so the duct isn’t kinked.
• Installing a screened cap, which clogs within weeks. Use a damper-only hood.
• Venting into a garage, attic or crawlspace, which dumps moisture and, on gas dryers, carbon monoxide indoors.

Avoid those five and keep the whole path smooth metal, and your dryer wall vent will move air freely, dry loads in one cycle, and stay off the list of fire hazards in your home.