Installing Dryer Exhaust — Duct, Cap & Code Requirements (2026)

A dryer exhaust system consists of three parts that work together: (1) the transition duct, which is the short flexible connector between the back of the dryer and the wall duct opening — this should be semi-rigid aluminum, 4 to 6 feet maximum; (2) the main exhaust duct run, which travels through walls, floors, or ceilings to the exterior — this must be rigid smooth-wall metal (aluminum or galvanized steel); and (3) the exterior exhaust cap, a weatherproof fitting with a backdraft damper flap that lets exhaust air exit while keeping rain, animals, and outside air from entering. All three components must be 4 inches in diameter and made of metal — plastic and vinyl are code-prohibited.

The exhaust route should be as short and straight as possible. Start by identifying the nearest exterior wall to the dryer location — typically a direct path of 5 to 15 feet for most laundry rooms on an exterior wall. If the laundry room is in the center of the house (common in apartments and condos with shared duct systems), the route may need to travel through interior walls and up to the roof. For each planned bend, note whether it is a 90-degree or 45-degree turn. Calculate equivalent length: physical length + (number of 90° elbows × 5 ft) + (number of 45° elbows × 2.5 ft). This sum must not exceed 35 feet per the IRC.

The correct exhaust duct materials depend on where in the system they are used. For the in-wall (concealed) main duct run, use rigid 4-inch aluminum or galvanized steel duct with a minimum wall thickness of 0.016 inches. For the visible transition behind the dryer, use semi-rigid aluminum flexible duct (not plastic, not thin foil). All joints must be secured with sheet metal screws and sealed with aluminum foil HVAC tape. The exterior cap must have a backdraft damper that opens fully when air flows and closes when the dryer is off.

Cut the 4-inch exterior penetration using a hole saw from outside for best control. With the hole cut, insert the exhaust cap from outside — the cap's round pipe stub pushes through the hole, leaving the rectangular or round flange flush against the exterior surface. Apply a bead of weatherproof exterior caulk around the cap flange before seating it. Secure the cap to the wall with galvanized or stainless screws through the flange. After installation, the cap should sit flush against the siding with no gaps, and the damper flap should hang freely in the closed position. Apply a second bead of caulk around the perimeter of the installed cap flange for a weatherproof seal.

Start at the exterior cap and work inward toward the dryer, adding rigid duct sections end-to-end. For each section, crimp one end slightly to create a slip connection — the crimped end is the upstream end (toward the dryer), and it inserts into the downstream fitting (toward the exterior cap). This joint orientation means any air that escapes leaks toward the outside, not into the wall cavity. Secure each joint with two sheet metal screws and seal completely with foil tape. For elbows, secure both slip connections with screws and tape. Support long horizontal duct runs with hanger straps at 4-foot intervals to prevent sag. Once the duct reaches the laundry room, attach the semi-rigid aluminum transition duct from the wall opening to the dryer's exhaust port using hose clamps at each end.

After completing the installation, check all joints visually — foil tape should cover every seam with no gaps. Run the dryer on high heat for 10 minutes. Stand at the exterior cap and verify: the damper flap opens fully (not partially obstructed by the cap housing), you can feel strong warm airflow, and no air is leaking from any connection inside the laundry room. A piece of tissue paper held near any joint that shows no movement confirms an airtight seal. If airflow at the exterior is weak, check for a kinked transition duct, disconnected joint inside the wall, or an obstructed exterior cap damper. Professional cleaning service costs $80–$180 if the duct later needs cleaning after installation.