Why Are My Windows And Air Conditioning Ducts Sweating?

Solutions to the Case of the Sweating Duct

Reprint of an article dated Mar 1, 2007 12:00 PM, By Kevin O'Neill

It's that time of year when windows are sweating and people are wondering if their air conditioning systems are working properly.  Usually they are, but to be better informed we are republishing an article we published last year.  Enjoy.
 

With the ever-increasing popularity of putting the air conditioning ducts outside the conditioned space; many contractors and homeowners in humid climates are now experiencing sweating ducts. Most of these sweating ducts are carrying cool supply air, although sometimes return air ducts can sweat, especially if the house is kept very cold and the return ducts run through cool vented crawlspaces.

Ducts sweat because the outside surfaces of the ducts are below the dew point of the air in their immediate surroundings. The primary ways to stop ducts from sweating is to raise the surface temperature of the ducts or to lower the dew point of the air surrounding the duct by some form of dehumidification. The dew point of air is the temperature at which the amount of water vapor in the air is the maximum amount the air can hold. The dew point is the same as air at 100% relative humidity (see Table 1).

Make The Ducts Warmer

There are several ways to raise the outside surface temperature of the ducts. The first is to raise the temperature of the air inside the ducts.