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 dew point 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 dew point temperature of the surrounding air you will have condensation.

To prevent sweating it is necessary to either raise the temperature of the sweating surface, or lower the dew point temperature of the air. It is only possible to lower the dew point temperature by decreasing the amount of moisture in the air (dehumidify) or by bringing in (mixing) drier air from the outside or inside. It is usually easiest to increase the temperature of the sweating surface by adding insulation.

When it is necessary to route an insulated duct through a truss/joist support, care should be taken to prevent tearing of the vapor barrier and insulation as this will allow moisture into the duct wall and reduce the thermal performance..

Localized compression of the fiberglass should have minimal, negative effect on the total, thermal losses of the duct. However, when the installation is in a critical condensation zone, localized compression could result in localized sweating.

The A/C contractor or designer should consider this information in view of total design performance.

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