Venting a dryer through an interior wall requires a duct that runs to an exterior wall or roof

You cannot terminate a dryer vent inside your home — the moist, lint-filled air must exit to the outside. If your dryer sits on an interior wall with no direct path to an exterior wall, you will run rigid or flexible ductwork through your walls, ceiling, or basement to reach the outside. The duct must slope slightly downward toward the exit point (at least 1/4 inch per foot of run) to prevent moisture from pooling inside the pipe.

The most common setup is to run the duct horizontally through a wall cavity, then up or down to an exterior vent hood. You will need to drill or cut an opening through the exterior wall, install a vent hood on the outside, and seal around it to prevent air leaks and pest entry. The entire run from dryer to outside should be as short as possible — every foot of duct adds resistance that makes your dryer work harder and dry clothes slower.

Key Takeaways

  • Dryer vent ductwork must run from your dryer to an opening on an exterior wall or roof, sloping downward at least 1/4 inch per foot to prevent moisture backup.
  • Rigid metal duct is more durable and easier to clean than flexible duct, and building codes in many areas require it for interior runs.
  • The duct opening on the outside must have a vent hood with a damper that closes when the dryer is off, preventing cold air and pests from entering.
  • Ductwork longer than 25 feet, or with multiple bends, may require a booster fan to maintain proper airflow and drying speed.
  • Sealing the duct joints with mastic sealant and insulating the duct in unconditioned spaces reduces energy loss and condensation inside the pipe.

Choosing between rigid and flexible ductwork

Rigid metal duct is the stronger choice for interior wall runs. It resists crushing, does not sag, and stays cleaner inside because lint cannot stick to smooth metal as easily as it does to the ridged interior of flexible duct. Most building codes allow or prefer rigid duct for runs that pass through walls and attics. Rigid duct comes in 4-inch diameter (the standard for most household dryers) and connects with slip-fit joints or band clamps.

Flexible duct is easier to install around obstacles and costs less upfront, but it sags over time, traps lint more readily, and can be crushed by drywall or insulation if not protected. If you use flexible duct, keep the run as short as possible and support it every 3 feet with a strap or clamp. Never use vinyl or foil-backed flexible duct — use only the metal-reinforced type rated for dryer vents.

Check your local building code before you buy materials. Some jurisdictions require rigid duct for any run longer than 8 feet, or for any run that passes through a wall or attic. A call to your building department takes five minutes and saves you from tearing out work that does not meet code.

Routing the duct through walls and ceilings

Start by deciding where the duct will exit the exterior wall. The exit point should be as close to the dryer as possible — ideally on the same wall, or one wall over. Measure the horizontal distance and the vertical distance from the dryer to the exit point. If the total run is longer than 25 feet, or if you need more than two 90-degree bends, plan to install a booster fan inside the duct to maintain airflow.

Drill a pilot hole through the exterior wall from inside the house so you can see where it emerges outside. This hole tells you exactly where to cut the larger opening for the vent hood. Use a drywall saw or reciprocating saw to cut a 4.5-inch hole through the exterior wall (slightly larger than the 4-inch duct diameter, to allow room for the vent hood flange). If you hit a stud or joist, adjust your exit point slightly rather than cutting through framing.

Inside the wall cavity, support the duct with metal straps or clamps every 3 feet. Do not let the duct rest on the bottom of the cavity — this traps moisture and lint. Slope the duct downward toward the exit at a minimum of 1/4 inch per foot. If you need to make a vertical rise (for example, from a basement dryer up to a first-floor exterior wall), the duct must still slope downward overall toward the exit, not upward.

Installing the exterior vent hood

The vent hood is the fitting that sits on the outside of your house and allows air to escape while keeping rain, snow, and pests out. Choose a hood with a damper — a flapper that closes when the dryer is off. Without a damper, cold outside air flows backward into your home, and insects or rodents can enter the duct.

Slide the duct through the hole you cut in the exterior wall and connect it to the back of the vent hood. Most hoods have a collar that slides over the duct and is secured with a band clamp or set screws. Align the hood so it sits flat against the exterior wall, then drive the fasteners (usually screws or nails) through the hood flange into the wall sheathing. Use at least four fasteners, one near each corner of the flange.

Seal the gap between the hood and the wall with exterior-grade caulk or flashing tape. This prevents water from running down the wall and into the wall cavity. If your exterior wall is brick or stone, use a vent hood with a brick veneer ring, or install metal flashing behind the hood to direct water away from the opening.

Sealing and insulating the ductwork

Every joint in the ductwork is a place where warm, moist air can leak out and cool down, causing condensation inside the duct. Seal all joints with mastic sealant — a thick, sticky compound that stays flexible and does not crack as the duct expands and contracts. Squeeze mastic around the joint, then wrap it with a band clamp or duct tape (use real duct tape rated for high temperature, not cheap cloth tape). Do not rely on duct tape alone; it dries out and peels away within months.

If the ductwork runs through an unconditioned space — an attic, crawlspace, or exterior wall cavity — wrap it with 1-inch foam pipe insulation. This keeps the air inside the duct warm, reducing condensation and heat loss. Tape the insulation seams with foil tape so moisture cannot seep in from the sides.

After installation, run the dryer for 10 minutes and feel the duct at several points along its length. You should feel warm air moving through it. If the duct feels cool or cold, condensation is forming inside, which means the duct is losing too much heat or the slope is wrong. Check that the duct slopes downward and that all joints are sealed.

When to add a booster fan

A booster fan is a small electric fan installed inside the ductwork that pushes air toward the exit. You need one if your duct run is longer than 25 feet, or if you have more than two 90-degree bends, or if your dryer is slow to dry clothes even though the vent hood damper opens freely. Booster fans are usually installed in the attic or basement, near the midpoint of the run, and are wired to turn on when the dryer runs.

A booster fan costs between $100 and $300 installed, and it uses a small amount of electricity — typically less than $10 per year. It is cheaper than running your dryer 20 minutes longer on every load. If you are unsure whether you need one, run the dryer and hold your hand near the exterior vent hood. If you feel only a weak breeze, a booster fan will help.

Common mistakes to avoid

Forgetting to slope the duct downward. Even a slight upward slope allows moisture to pool inside the duct, where it condenses and drips back toward the dryer. This water can damage the dryer's heating element or create mold inside the duct. Measure carefully and use a level to confirm the slope.

Using vinyl or foil duct. These materials are not rated for the heat and moisture of a dryer vent. Vinyl can melt, and foil can tear. Use only metal-reinforced flexible duct, or better yet, rigid metal duct.

Terminating the vent inside the house. Some older homes have dryers vented into the basement or attic. This dumps all the moisture into your home, raising humidity and promoting mold growth. The vent must go outside, period.

Leaving the duct unsupported. Flexible duct that sags or rests on the bottom of a wall cavity traps lint and moisture. Support it every 3 feet and keep it off the bottom.

Skipping the damper. A vent hood without a damper allows cold air to flow backward into your home and pests to enter the duct. Always use a hood with a working damper.

Frequently Asked Questions

Can I vent my dryer into the attic instead of outside?

No. Venting into the attic dumps moisture into an enclosed space where it cannot escape, promoting mold growth and rotting the roof framing. Building codes prohibit this. The vent must terminate on an exterior wall or roof.

How often should I clean the dryer vent duct?

Clean the duct at least once a year, or more often if you notice the dryer taking longer to dry clothes. Disconnect the duct from the dryer, use a vent brush or shop vacuum to remove lint, then reconnect it. If the duct is longer than 20 feet, you may need a professional vent cleaning service with a powered brush.

What size hole do I need to cut in the exterior wall?

Cut a 4.5-inch hole for a standard 4-inch dryer vent duct. The extra 0.5 inch allows room for the vent hood flange and gives you a little wiggle room when positioning the hood. Use a drywall saw or hole saw to make a clean opening.

Do I need a permit to vent a dryer through an interior wall?

Most jurisdictions require a permit for any work that involves cutting through exterior walls or running ductwork through framing. A permit ensures the work meets code and protects you if there is ever a problem. Call your local building department before you start.

What if my dryer is in the basement and the exterior wall is upstairs?

Run the duct vertically up through a wall cavity or chase (a framed space built specifically for ducts and pipes), then horizontally to the exterior wall. The duct must still slope downward overall toward the exit. A vertical rise of 10 feet followed by a horizontal run to the outside is fine, as long as the total slope is downward.