Condensation occurs when air is cooled below its dew point temperature. The study of air containing moisture (or plain old air as we know it) is called psychrometrics (pronounced si-crow-met-ricks), and deals with the relationships between temperature, relative humidity, absolute humidity, dew point and several other properties of the air/moisture mixture. A basic psychrometric relationship is that air can only hold so much moisture at a certain temperature. When the air is full of moisture, the relative humidity is 100%. When the air contains half as much moisture as is can at a temperature, the air is at 50% relative humidity. The next relationship is that if you cool the air, the relative humidity increases. (Cool air can’t hold as much moisture as warm air) At some point, the air becomes saturated. Cooling it any further causes condensation. This is the dew point.
So how does this relate to sweating ducts? Air conditioners make air cold. The cold air is forced through ducts. As a result, the outside surface of the ducts is cooled. If the air outside the ducts is humid enough, condensation will form on the ducts. The colder the air in the ducts and the more humid the air around the ducts, the more chance of forming condensation. Note that sweating ducts has nothing to do with moisture in the air inside the ducts. Solutions to sweating ducts involve 1) warming the surface, and 2) drying the air around the ducts. Insulation is added to the exterior of ducts to help warm the duct surface. The insulation should be enclosed in a vapor barrier to keep moisture from moving through the insulation itself. Joints in the ducts, insulation and vapor barrier should be sealed. The insulation and vapor barrier should extend completely to the registers, or condensation can form on the exposed ends.
If the ducts are in a crawlspace, a complete vapor barrier on the soil is an essential first step. Increasing crawlspace ventilation may help in some parts of the country, but be careful because increasing ventilation in other areas can actually increase the condensation. In basements and crawlspaces, sometimes adding a dehumidifier is necessary. Most duct condensation issues I have seen are the result of problems with duct insulation. In some cases, fixing the insulation solves the problem. Adding insulation typically does not solve the problem. In cases where the insulation is in good shape, crawlspaces and basements have been wet, or ducts have been pressed together.
When is the air conditioner at fault? Some newer air conditioning systems and controls actually make the air inside the ducts colder. This is an attempt by the manufacturer to help make the air in the house dryer, but often causes more condensation on the outside of ducts. Dirty filters can restrict air flow through the system, resulting in colder air. This is the easiest one to deal with: keep your filters clean. Otherwise, make sure the duct insulation and vapor barrier are continuous, contiguous and complete. And keep the air around ducts dry by covering exposed soil in crawlspaces, keeping ducts apart, and reducing other moisture sources in the air as much as possible.