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 a duct carries cold air, the temperature of its surface can drop below the dew point of the surrounding air. If the outer surface of the duct is not insulated, the cooler temperatures can cause moisture in the air to condense, leading to water droplets forming on the duct. This can result in several issues, including corrosion of the duct material, damage to nearby structures, and potential mold growth due to extended moisture exposure.

By insulating the outside, the temperature of the duct's surface is maintained above the dew point, reducing the risk of condensation. Insulation effectively creates a barrier that minimizes heat transfer, keeping the outer surface warmer than the ambient air. This practice is crucial in maintaining energy efficiency within HVAC systems and prolonging the lifespan of ductwork systems.

Other options may not specifically address the condensation issue. For example, inside insulation would insulate the air within the duct but wouldn't prevent the surface from cooling. Flex connectors aid in reducing vibration and movement but don’t impact condensation. A moisture barrier is useful in protecting against moisture intrusion but does not directly prevent the cooling of the duct surface responsible for condensation.