HVAC insulation can take the form of duct liner, duct wrap and duct board. Some forms of HVAC insulation muffle sound in addition to improving efficiency of the HVAC system.
Duct liner is used inside of air conditioning and heater vents. Duct liners typically dampen the noise HVAC equipment makes as well as preventing heat loss. Some duct liners contain protective coatings that inhibit the growth of bacteria and mold. Duct liners are simpler to cut to length and install than duct board and are lighter weight than other forms of HVAC insulation. In some cases, duct liners lower the resistance air encounters as it is pushed through the ductwork, reducing the energy required to run the HVAC system.
Duct wrap is made from a blanket like insulating material similar to the insulation used in attics and walls. It wraps around the duct, insulating it. Insulating with duct wrap allows you to move ducts and add extra insulation where needed. However, if the duct wrap gets wet, the mold growing in it can extend into the duct itself. Adding extra layers of duct wrap reduces the risk of leaks but increases the cost. Duct wrap is easily fitted to round and square ducts and can be replaced in sections simply by cutting away any sections of the existing wrap. Duct wrap can even be wrapped around ducts with an existing duct liner for greater insulating effect. You can buy duct wrap with reflective foil that prevents the duct work from absorbing heat from the attic as it distributes cooled air throughout the house.
Duct board must be cut to fit the intended path of the duct work. It often costs more to install than sheet metal lined ducts, but it is durable and prevents the heat loss associated with sheet metal ducts. And it is not subject to the same risk of tears over time or being chewed through by animals in the attic. Duct board is easier to clean than flexible ductwork. It often absorbs sounds that may otherwise travel room to room through the air ducts like air rush noise, noise generated by the HVAC system and conversations from other rooms.