Duct Sweating

Ducts in your attic sweating is not the same as your body sweating. Ducts that are sweating are more like your car having those beads of condensation in the morning. This map of the US shows Florida fully contained in the area with the highest seasonal dew point. When an object reaches the dew point temperature, condensation starts to form (we call it sweating).

dew point


Duct Sweating

There is a complex formula for the calculation of a homes ductwork meeting the dew point. Some important things a homeowner needs to know:

  • the air inside your ducts is about 20 degrees below your room temperature when your AC is cooling.
  • if you lower your thermostat to 65 degrees at night, the 45 degree air temp inside your ductwork will cause the temp of outer skin of your ducts to fall below the dewpoint on every summer night.
  • People who live near the water (higher humidity areas) or have a tile roof (cooler attic temperatures) have a greater possibility of duct sweating problems.
Hayward HeatPro Pool Heat Pump

Sweating Ductwork is a major and complex problem and has no quick fix. The good news is that the best fixes start with giving your AC system a tighter seal and thus making it more efficient. Difficult fixes involve replacing ductwork with better insulation or even replacing home insulation with spray on foam. After the repair investment these fixes pay for themselves over time with energy savings.

In Depth

How It Works


Dry Bulb across the bottom is the temperature of the air that a thermometer would sense. The lines relating to dry bulb temperature run vertically up and down. Wet Bulb temperaure along the upper curve is taken with a thermometer with the sensing bulb end wrapped in a wetted cloth and moved through the air (twirled in a circle or placed in return airflow). Wet Bulb represents the lowest temperature that can be reached by the evaporation of water in the current air. Wet Bulb are the about 45 degree slashing lines through the graph. At 100% humidity dry bulb = wet bulb. The many curved lines in the graph represent relative humidity which can be calculated with a wet bulb and a dry bulb temperature. Find where the dry bulb vertical temp line crosses the wet bulb downward to the right slashing line and read the curved humidity line. (example 80 dry bulb and 63 wet bulb crosses just below the 40% humidity line, so 39% humidity)

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