Mark Rosenbaum, P. E. of Energysmiths, Meriden, New Hampshire, responds: First of all, wherever possible, do not vent the exhaust from either a dryer or a bathroom into or through the attic or any other unheated space in the home. These exhaust streams are full of moisture and hence have a high potential for condensation. Also, the dryer and the bathroom fans should be vented separately and should never be vented out the soffit vent. Soffit vents are intake vents, so any air exhausted at that point will likely be sucked back up into the attic.
I assume that your laundry is in an interior space and venting directly through an exterior wall is impossible. Vent the dryer with 4-in. dia. metal duct, going vertically from the dryer through the second-floor ceiling. Seal the ceiling penetration so that the duct can expand in length as it heats up during use (a standard roof boot, such as you’d use to flash a vent pipe, would work). Continue the ductwork horizontally through the attic to the closest exterior wall.
Pitch the attic ductwork back toward the dryer so that any condensation will run back toward the heated space. Support the ductwork adequately to maintain this pitch—don’t allow the ductwork to sag in the middle. If you come off of the dryer with a tee connector instead of an elbow, condensate will run back to the lower portion of the tee instead of back into the dryer. This will give you a place to inspect whether condensate was indeed forming and to drain the water away if necessary.
Penetrate the wall and use an Energy Saving Dryer Vent (Heartland Products, P. O. Box 777, South Kathryn Road, Valley City, N. D. 58072; 800- 437-4780), which is designed to provide a tight seal when the dryer is not in use. Follow the dryer manufacturer’s recommendation for maximum allowable duct length. My Sears dryer, for instance, allows a maximum of 44 ft. of 4-in. duct with two elbows, and 35 ft. of 4-in. duct with three elbows.
The ductwork should be taped with a high-quality duct tape. Tape both longitudinal and transverse seams. Do not use sheet-metal screws to connect the ductwork because the screw tips protrude into the duct and will collect lint. Insulating the duct with at least 1 in. of fiberglass insulation will keep the duct warm during dryer operation and will minimize condensation on the duct walls. The more insulation you install, the less likely you are to have condensation in the ductwork. Insulation that’s carefully wrapped around the duct will work better than fiberglass batts just piled on top.
Because the dryer vent’s air seal to the outdoors cannot be made between the heated space and the unheated space (the second-floor ceiling), natural convection in the ductwork when the dryer is not running may still convey enough water vapor to the attic ductwork to cause condensation. Keeping wintertime relative humidity levels in the house to 40% or below will minimize this problem.