How To Vent A Clothes Dryer

Q: Previous owners moved the laundry room to the second floor and terminated the dryer vent in the attic space above, since it’s not near an exterior wall. What’s the best way to vent it to the outside? A: It’s become popular to put laundry rooms on the second floor for the convenience, but venting a clothes dryer into an attic is a big mistake. Warm, moist air from the vent creates a perfect climate for mold to grow on the roof framing, sheathing, and anything you’ve stored in the space. In cooler weather, that damp air will condense on cold surfaces, soaking insulation and possibly damaging the ceiling below. And in snowy climates, all that extra warmth contributes to the formation of ice dams. To avoid these problems, consider the most direct route to the outside—through the roof. In that case, you’ll need to connect the ductwork to a hooded roof vent that offers minimal resistance to airflow from the dryer. Installing such a vent, even on an asphalt-shingled roof with a low slope, is best left to a roofing pro with proper fall-protection equipment and who knows how to properly flash around roof penetrations. Once the vent is installed in the roof, you can connect it to the dryer duct from inside the attic. Seal the connection to the vent, and any seams in the duct, with foil tape. Make sure to seal around the duct where it comes into the attic using a fire-stop sealant such as Fire Barrier CP 25WB+ made by 3M. Then wrap the metal with R-8 duct insulation to minimize internal condensation. You also might be able to run the duct to an exhaust vent on an outside wall at least 3 feet from a window or door. Dryers can operate with straight duct runs as long as 35 feet, but you have to subtract 5 feet from the run for every 90-degree turn the duct takes. If the dryer is still within reach of an outside wall, then have a handyman cut a hole for the duct and connect it to the vent. Whichever direction your duct goes, only use the kind made with 4-inch galvanized steel. Its smooth walls ensure good airflow, keep lint buildup to a minimum, and are fire resistant. The lint-trapping corrugations in flexible plastic or foil-faced flexible ducts are a fire hazard, and slow down airflow, which adds to drying time, and forces the dryer motor to work harder.

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