Flexible and sheet metal ducts are the typical models used when installing a central air system. Duct board is used in some homes and offices, whereas others don't have any ductwork whatsoever in their central air conditioning system.
Flexible ducts are often used with sheet metal or duct board systems. Contractors typically use flexible ducts as bridges between the main duct and individual heating or cooling registers. Flexible ductwork is good for areas in which the rigidity of sheet metal would make installation difficult or impossible. Contractors usually use durable plastic material surrounded by insulation to create flexible ducts.
Using sheet metal for duct construction creates a more durable system. However, sheet metal is also more expensive than other materials, as of 2015. Sheet metal ducts are not insulated, so they can lose heat into the crevices of the home, making them less energy efficient. For this reason, galvanized steel ducts are sometimes surrounded by insulation.
Duct board is made out of fiberglass coated in aluminum foil. This material is typically less expensive to install, since a contractor can simply use a utility knife to cut it to size.
Some homes do not use ductwork for their centralized air, relying instead on the space between walls and ceilings. Contractors sometimes cap off a section between studs with aluminum flashing in order to transform these spaces into improvised ducts.