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Ask the Carpenter: How to vent a gas dryer properly

Ask the Expert Home Improvement

Q. I had a problem with the exhaust for my gas dryer and found that the builder had laid 4-inch flexible ducting from an interior wall up through the attic and across to an exterior wall. From there it ran another 16 feet across the rafters of an unheated porch roof. The ducting sagged between the porch roof rafters, and the accumulated moisture froze, almost completely blocking the flow. The flex duct in the attic was laid between two joists and covered with the attic floor insulation. At some point the moist exhaust backed up into the attic, and there is frost on the roofing nails and the underside of the sheathing.

Is there a rigid duct that has “clean-outs’’ that I can run in place of the flex duct? Do you have any suggestions on how to remove the moisture in the attic when the frost starts to melt? I am thinking of laying plastic sheeting over the insulation and letting the water evaporate when it gets warm.

AUDREY MARSH

A. I’m not sure about clean-outs in gas dryer vents, but it looks like you need a shorter run and insulated pipes. Plastic sheeting may work, as well as running a fan.

Ensure that any dryer ducts traveling through an unheated attic or eaves are insulated. Un-insulated pipe will sweat or create frost and lint buildup, which can cause a fire. While you are at it, ensure that all of the pipe seams are facing up and sealed with foil tape to prevent seepage.

The ducts should not exceed the dryer manufacturer’s recommendation. The duct must not exhaust into attics, crawl spaces, basements, chimneys, or any wall cavity. Always vent to the exterior with an approved exterior dryer vent hood and backdraft damper flap. The damper is important to prevent outside air intrusion and the entry of small animals.

Dryer duct best practices

Use rigid aluminum or rigid galvanized steel ducts (especially if concealed). If flexible metal duct must be used, install the semi-rigid type.

The dryer duct should be as straight and short as possible. Minimize 90-degree turns; sharp turns cause back pressure and create resistance to airflow. Two 45-degree bends are more efficient than one 90-degree. For best performance, separate all turns by 4 feet of straight duct.

Ideally, you should install a rigid, seamed exhaust duct. When possible, horizontal runs of dryer ducts should slope slightly downward (¼ inch per foot) toward the exterior termination to reduce the possibility of condensation accumulating and then collecting lint.

The hood should point down and have at least 12 inches of clearance between the bottom of the hood and the ground, and ducts should always have adequate support, especially at each joint.

A dryer duct should never exhaust near the fresh-air intake of any heating, ventilation, or air conditioning intake or water heater.

In cold climates, insulating the dryer exhaust duct in unheated spaces may help limit the condensation from forming inside the duct and collecting lint.

Furnace-Vent-Exterior-Home
Moisture from a furnace vent has been damaging the siding of this reader’s home. —Handout

Q. I had a high-efficiency furnace installed. Because of the outflow of steam/moisture, I need to repaint the house wall and roof overhang in that area every year. I asked whether making the vents shorter and more flush with the house would help, but was told the vents were properly installed. Do you have any suggestions? Is there a paint/primer/sealer product out there that will stop or slow the yearly peeling?

MEPOTOCKI

A. I, too, am at a loss as to why you have to repaint every year. Do you know whether those clapboards were back primed? You could try installing cement-board clapboards in that area. At a minimum, you could replace a course or two as an experiment. I’d love to know the results.

Rob Robillard is a general contractor, carpenter, editor of AConcordCarpenter.com, and principal of a carpentry and renovation business. Send your questions to homerepair@globe.com or tweet them to @robertrobillard. Subscribe to our free real estate newsletter at pages.email.bostonglobe.com/AddressSignUp