Proper Clothes Dryer Venting

Clothes dryers vent about a gallon of water for each typical load of laundry. If this vent empties into a crawlspace, attic, or other building cavity, the potential for moisture-related damage is significant. Clothes dryers must vent directly to the outside using approved exhaust duct. Vinyl, nylon, and foil ducts are not recommended and if used will void the warranty of most dryers (Cranor 2005). These spiral duct designs often trap lint, which can clog the duct, requiring the dryer to work harder and longer to dry clothes and also causing it to increase dryer temperature. Not only is a poorly exhausting dryer less efficient, it can also be a fire hazard due to the flammability of the collected lint (NFPA 2013). The use of PVC pipe for dryer exhaust ducts is also not allowed per all standards and specifications.

Per the requirements of the 2009 International Residential Code (IRC) M1502, the clothes dryer exhaust duct should be made of metal with a smooth interior finish and a duct diameter of 4 inches. Short, straight, horizontal duct runs are the most effective. The duct length from the wall to the outlet terminal should be no more than 25 feet, including equivalent length for any fittings. Or, follow the duct length recommendations specified by the manufacturer (2009 IRC M1502 - See Compliance Tab). The duct should be secured in place and supported at 4-foot intervals. Avoid sags in the duct. The transition duct (the visible piece of duct connecting the dryer to the wall) can be up to 8 feet and should be listed in accordance with UL 2158A; there are some semi-rigid metal transition duct products that are UL listed. The exhaust duct should terminate outside the building at least 3 feet from any building opening and at least 10 feet from an AC or heat pump condenser. The duct termination should be fitted with a closeable gravity or automatic damper. A louvered damper is more effective than a single hinged door. No screen should be installed in the duct terminus (2009 IRC M1502.3 - See Compliance Tab).

How to Install Dryer Exhaust Duct

1. Install a rigid metal (aluminum or galvanized steel) exhaust duct connecting the transition duct to the exhaust duct outlet terminal. The exhaust duct should be rigid metal 4 inches in diameter and no more than 25 feet in length or as specified by the duct manufacturer. This length should be decreased by 2.5 feet for every 45-degree bend the duct makes and 5 feet for every 90-degree bend the duct makes. Any joints should be installed in the direction of air flow.
2. Support the duct at least every 4 feet, and at all joints. Avoid sags. Air seal any joints with foil tape.
3. At the interior wall, connect the exhaust duct to a transition duct that connects directly to the dryer. The transition duct should be UL-2158A listed and should be no longer than 8 feet. Short, direct, semi-rigid metal transition duct is preferred. The ducts should be joined with clamps or foil tape, not screws or similar fasteners that can protrude into the inside of the duct and trap lint.
4. Locate the exhaust terminal at least 3 feet from any door or window and away from the fresh air intake of a high-efficiency furnace, water heater, or any other HVAC intake. It should also be located away from air conditioning or heat pump condensing units installed outside. Avoid terminating the duct at the roof. If possible, locate the exhaust terminal so that the exhaust duct will run horizontally with a slightly downward slope (1/4 inch per foot) to reduce the possibility that condensation will accumulate (Cranor 2005).
5. At the terminating end of the exhaust duct (at the exterior wall), install a backdraft damper. The hood opening should point down and should be at least 12 inches above the ground (Cranor 2005). A louvered damper is more effective than a single hinged door. No screen should be installed in the duct terminus. Flash and caulk the exterior wall penetration.
6. Insulate ducts in unconditioned spaces, especially in cold climates, to avoid condensation.

Figure 1 - The dryer exhaust duct should vent directly to the exterior in a short, straight line, via a smooth, rigid metal duct. 

Figure 2 - At the outlet of the dryer exhaust duct, install a hooded vent with a louvered damper.