Flexible ducts can be constructed from a variety of materials; for example, paper spread over a wire frame is one of the cheapest available and is often supplied with vented clothes driers.
In more permanent installations, ranging from kitchen extractors to domestic ventilation systems, it is common to find flexible ducts made from synthetics such as PVC (polyvinyl chloride), polyurethane, and high-density polyethylene. They are also made from steel wire or bands, sealed within layers of other flexible materials. Aluminium foil often features in their construction.
Layers of other materials are often applied to improve their thermal or acoustic insulation or deliver a smoother inner surface that will provide less resistance to the airflow or prevent static electricity.
The advantages of flexible ducting
As you might expect, the main advantage of flexible ducting is its flexibility.You can also use spiral duct products but flexible makes it easier to haul into loft spaces, down cellar steps, or between joists and rafters. The standard for most ductwork installations is spiral wound steel, which is available from suppliers such as https://www.dustspares.co.uk/ductwork-parts/galvanised-ducting/galvanised-steel-spiral-duct.html however, navigating a difficult route through walls and floors is sometimes almost impossible without some flexible duct.
Most types are also stretchable, to varying degrees, which means it does not have to be cut to precise lengths when connecting other ductwork parts. This can shorten the installation time.
Flexible ducts are also useful as the collecting ends of workshop dust and fume extractors.
The disadvantages of flexible ducting
Ironically, most of the advantages are also disadvantages. Flexible ducts are prone to sag, bow, buckle and bend, none of which are good for the airflow or for their seals with other ductwork parts. As a result, they waste energy and sometimes struggle to deliver sufficient airflow to distant rooms. As their inner surface is usually ridged and uneven, this further impedes the airflow and collects dust and grime that is very hard to remove; consequently, most products will fall short of the health and safety regulations for commercial kitchens and food preparing premises.
On a long run, more brackets may be needed; however, due to their poor energy efficiency and hygiene problems, the length and number of turns allowed in a flexible duct are sometimes limited by building codes.
In short, flexible ducting is useful in tight corners but is neither as efficient nor as easy to maintain as rigid ducting.