Sheetmetal Ductwork

There are three kinds of ductwork for heating and air conditioning systems:

Vinyl Flex: Made of a vinyl liner with wire reinforcement, it includes insulation and a vapor barrier. It's the most commonly used (95% of time). It is the easiest to install and least expensive for both materials and labor because it bends and requires fewer separate fittings than traditional galvanized ductwork. It is available round in 4-inch to 20-inch diameters.
Aluminum Flex: More expensive than vinyl, also includes insulation and a vapor barrier, but because rodents could chew through vinyl, this is used primarily under houses because it acts as a rodent barrier. It is available round in 4-inch to 20-inch diameters.
Galvanized Steel (Rigid Pipe): It's the most expensive for both materials and labor; it needs more fittings to make turns, and needs to be sealed and insulated on the outside. It is primarily used in garages to be fire resistant. It is available round or rectangular.