Ducts are conduits or passages used in heating, ventilation, and air conditioning (HVAC) to deliver and remove air. The needed airflows include, for example, supply air, return air, and exhaust air. Ducts commonly also deliver ventilation air as part of the supply air. As such, air ducts are one method of ensuring acceptable indoor air quality as well as thermal comfort.

In air conditioning, ducting provides uniform air conditioning for large open environments such as libraries, public facilities, and large retail stores.

Material: Galvanized Mild Steel
Galvanized mild steel is the standard and most common material used in fabricating ductwork because the zinc coating of this metal prevents rusting and avoids cost of painting. For insulation purposes, metal ducts are typically lined with faced fiberglass blankets (duct liner) or wrapped externally with fiberglass blankets (duct wrap). When necessary, a double walled duct is used. This will usually have an inner perforated liner, then a 1–2' layer of fiberglass insulation contained inside an outer solid pipe.

Bare Galvanized Steel Duct:


Fiberglass Insulated Galvanized Steel Duct:


Material: Flexible ducting
Flexible ducts (also known as flex) are typically made of flexible plastic over a metal wire coil to shape a tube. They have a variety of configurations. The insulation is usually glass wool. Flexible duct is very convenient for attaching supply air outlets to the rigid ductwork. It is commonly attached with long zip ties or metal band claps. However, the pressure loss is higher than for most other types of ducts. As such, designers and installers attempt to keep their installed lengths (runs) short, e.g. less than 15 feet or so, and try to minimize turns.