Insulation is applied to ductwork to enhance thermal performance and prevent condensation and dripping. Duct thermal performance needs enhancement since air transported through a supply duct is at a temperature different than that of the surroundings. Insulation reduces the rate of thermal loss to those surroundings. Without insulation, the air would need extra heating or cooling in order to arrive at the design supply air temperature. Return air ducts only need to be insulated if they pass through environments that adversely affect the return air temperature. Exhaust air ducts normally do not need insulation. Supply air ducts may be left un-insulated if they run exposed through the space being conditioned; this arrangement also reduces system first cost.
Insulation prevents condensation and dripping from ducts. Un-insulated cold air ducts very often have surface temperatures below the local dew point. At this temperature, condensate will form and eventually drip off, causing an uncontrolled accumulation of moisture on the outside surface of the duct. Duct insulation eliminates the formation of condensate and consequently prevents rusting and staining.
Extra heating (or cooling) energy required to compensate for reduced thermal performance of un-insulated duct has a negative effect on the HVAC system's life-cycle cost. Therefore, duct insulation always presents an optimization problem. Since insulated duct costs much more than un-insulated, the recommended air velocity becomes a key factor in optimization. For instance, a higher air velocity reduces duct surface area and thus insulation cost.
Because of the relatively small temperature differences between supply air ducts and the spaces through which they ductwork are routed, a one-inch-thick fiberglass blanket is almost always sufficient. Insulation should be wrapped around the duct's exterior. A protective cover with a vapor barrier such as an aluminum foil, referred to as FKS, should be included in insulation specifications. Care must be exercised to protect exterior insulation integrity where insulation comes in contact with hangers, supports, and other structural members. Interior duct insulation (lining) should not be used in laboratory or cleanroom applications because the insulation tends to entrain microscopic particles into the airflow.
Special consideration must be given to ducts exposed to weather. Lagging materials or heavy metal covers over the insulation are commonly used to protect ductwork. A life-cycle cost analysis may be necessary to determine optimum insulation thickness when ducts encounter temperature extremes.