To prevent condensation from forming on the outside of a duct, it must be:

Insulating the outside of a duct is essential for preventing condensation from forming on its exterior. When warm, humid air comes into contact with a cooler surface, such as an uninsulated or poorly insulated duct, condensation can occur. By insulating the outside of the duct, you create a thermal barrier that keeps the outside surface of the duct warmer, thereby reducing the likelihood that it will reach the dew point of the surrounding air.

This approach effectively minimizes the temperature differential between the duct's surface and the air outside, which is critical in controlling condensation. Proper insulation on the outside ensures that the duct maintains a more stable temperature that does not allow for condensation to form, thus protecting against moisture-related issues like mold and corrosion.

Other options may address different concerns related to duct work, such as the integrity of the connection or reducing vibrations, but they do not specifically target the prevention of exterior condensation as effectively as outside insulation does.