Duct Insulation

Generally, you will not need insulation for heating and air-conditioning ducts that are enclosed in already well-insulated portions of your house. But, if the ducts for either your heating or your air-conditioning system run above the ceiling or below the floors, they should be insulated.

Whether ducts are in the attic or beneath the floor, insulation is installed in pretty much the same way using special duct insulation:

Seal seams with metal duct tape.
Cut the insulation so it's long enough to wrap around the duct with a two-inch overlap.
Wrap the insulation around the duct with the vapor barrier on the outside, faced away from the ducts.
If your ducts already have some insulation, check to see if any moisture has collected in it. If so, replace it with new insulation.
Note: some duct systems have insulation on the inside that cannot be seen. Check to make sure you do not have this type of duct system before adding additional insulation

Once the ducts have been installed, and the trunks and runs established, it's time to finish off the heating system by insulating the ducts. The degree to which you insulate your ducts depends on local building codes for energy efficiency, and whether the ducts are in conditioned or non-conditioned areas. If a duct runs through an unheated crawl space, an unheated basement, or garage, it must typically be insulated to R-4 or greater. If ducts are long or poorly insulated, they often result in cool blows, or gusts of air that have lost the desired temperature. In such a case, running the heat can actually cool a room rather than heat it. The same can be said for cool conditioned air, which must maintain its temperature or fail to cool the room at the other end.

Sealing and Insulating the Ducts
The first step toward fully insulated ducts is to seal all of the points in the duct run. This is done with a mastic that is brushed on then left to dry for at least 24 hours. The mastic seals the joints and protects against air loss and drop in velocity. How quickly the air moves through the ducts determines how well it will hold its temperature. Well-sealed joints allow the ducts to carry air at maximum capacity. Mastic is used because it will not degrade or shrink away from the joint. Duct tape should not be used for this purpose, since the tape degrades and decomposes, leaving the joint leaky and unprotected.

Once the ducts have been sealed, an insulating jacket is wrapped around the ducts and plenum. Like wall insulation, duct insulation is typically made of fiberglass. Insulators may use rigid insulation or faced fiberglass. The fiberglass surface faces in toward the duct, while the foil face protects the outside of the wrap, preventing conductive heat loss and protecting the fiberglass within. The insulation is applied in sheets that are cut to fit and seamed, usually along the sides. While some installers use foil tape for this purpose, most insist on attaching the pieces to one another with a staple gun. Staples are less likely to degrade or lose grip over time.
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.