How does duct condensation or “sweating” occur?
Condensation or “sweating” is a complex problem that is primarily seen in high humidity areas. There are a large number of factors that can lead to condensation. Condensation occurs on any surface that is colder than the wet bulb temperature of the air surrounding it. In air ducts it is most common to see this on the outer vapor barrier of the duct. However, it can also be on the inner core in flexible ducts, or the sheet metal of a sheet metal system. We also see condensation at the fittings or plenums. Any place where the temperature of a surface is colder than the wet bulb temperature of the surrounding air you will have condensation.
