Made of flexible aluminum foil with an embedded spiral wire for reinforcement, this type of duct is appropriate for making the transitions from the appliance to the main rigid duct that runs to the outdoor vent hood outlet. It generally should not be used for the entire duct run. Foil duct can accordion from about 1 to 8 feet, which is usually enough for most installations. Double-check to make sure that this type is approved for your dryer model and allowed by local code. Transition ducts must not be concealed inside walls or other building cavities and usually have a maximum allowed length of eight feet. Foil duct is held in place with dryer clamps or large hose clamps. Metal tape can also be used to secure the flexible aluminum duct, too, but clamps make it easier to remove the tubing for cleaning. Aluminum foil duct is inexpensive and relatively easy to work with, but it has ribbed walls that can trap lint, leading to potential fire hazard. It is best used for the relatively short sections to join the dryer outlet to the main vent. Semi-rigid metal (usually aluminum) duct tubing is flexible and similar to foil tubing but quite a bit more rigid. Like foil duct tubing, semi-rigid duct tubing should be used only for the transition ducting running from the dryer outlet to the main duct, and it cannot be concealed in walls or floors. It is typically installed with clamps, which make it easier to remove for cleaning. Semi-rigid metal duct is often stronger and has a smoother interior (catches less lint) than foil duct. Semi-rigid metal duct is also relatively inexpensive. While it is sometimes used for an entire exposed duct run, such use is questionable. This type of duct tubing has some inner corrugation and thus tends to trap some lint. Semi-rigid tubing should be used for the entire duct run—dryer outlet to exhaust vent hood— only if the duct run is quite short and direct (less than 8 feet), only if it is exposed rather than hidden, and only if your local code allows such use. Slim duct, sometimes called a periscope duct, is a telescoping rigid aluminum tubing used to make transition ducting in tight spaces. It has a wide rectangular shape except for the two ends where it connects to the dryer and the vent outlet, which have round collars. It allows you to push the dryer very close to the wall. This can be a great space-saver as long as the dryer and the exhaust vent hood are close together. Most slim ducts can be adjusted from about 27 inches to about 48 inches in length. It is also adaptable to vertical, horizontal, and diagonal installations. Slim ducts are a bit more expensive, but they offer a good option where wall clearance is a problem. They can be somewhat tricky to install and sometimes are used in conjunction with short lengths of semi-rigid flexible tubing for transitions. Rigid metal duct is the only suitable duct material for concealed duct installations— that is, ductwork that runs inside walls, floors, or other enclosed areas. Available in aluminum and galvanized steel, rigid metal duct is very smooth on its interior, so air flows through with little friction, and it catches the least amount of lint of any duct material. Rigid metal duct is more expensive than other types, but it is extremely durable and efficient. And building codes generally allow for runs as long as 35 feet, compared to the 8-foot maximum for flexible transition tubing. Adjustable metal elbows, also with smooth walls, can be used to make the transitions between the rigid vent and the dryer outlet on one end and the vent hood on the other end. This kind of configuration—with smooth-walled vent running from dryer all the way to vent hood outlet—makes for the very best installation. Working with rigid metal duct and adjustable collars can be a bit tricky, however, so some homeowners will opt to have the installation done by a professional. The flexible plastic (usually vinyl) duct is similar to aluminum foil duct but has a thin plastic wall rather than metal foil. Despite its popular use for many years, the plastic duct is no longer allowed by most building codes. The ribbed interior of the flexible duct easily catches lint.
In general, it is best to choose tubular rigid metal duct tubing for as much of the duct run as possible, as this type provides the best air volume and few inner obstructions to catch flammable lint.Slim metal duct is the next best choice where wall clearance is an issue. It, too, has smooth walls that won’t trap lint, though the air volume may be slightly poorer than with tubular metal duct.Semi-flexible metal duct is a good choice for the transition ducting that runs from the dryer outlet to the rigid metal duct, as it can be bent to form fairly smooth elbow and curve shapes. But even here, a better choice would be adjustable rigid metal elbows that have perfectly smooth walls.The worst choices (and usually an illegal choice) are flexible vinyl or plastic duct and foil duct. Both tend to trap lint, and plastic duct tubing can easily melt if heat buildup becomes too great.
