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  • R. Alex Giordano, PE

Don't Sweat It! Understanding Duct Sweating

During the sweltering heat of summer, an air-conditioning (AC) system becomes our greatest ally in maintaining indoor comfort. However, despite its efficient cooling capabilities, AC systems may encounter an issue called "duct sweating." This phenomenon occurs when condensation forms on the surface of the air ducts, leading to potential problems like mold growth and reduced system efficiency. In this article, we will explore what duct sweating is, its causes, and practical ways to mitigate it to ensure optimal performance and indoor air quality.

What is Duct Sweating?

Duct sweating is a common problem that occurs in air conditioning systems, especially in humid climates. When warm, humid air passes through the cold surfaces of the ductwork, it loses its moisture through condensation, forming water droplets on the duct's exterior. This moisture accumulation can lead to various issues over time, including mold growth, water damage to the ducts and surrounding areas, and reduced efficiency of the AC system.

Causes of Duct Sweating

On a fundamental level, duct sweating is an example of condensation when water vapor becomes liquid. Condensation occurs when the surface temperature of an object is lower than the dew-point temperature of the air in which the object is immersed or subjected to. See appendix for definition of dew-point temperature. Several factors contribute to the occurrence of duct sweating. Understanding these causes is vital in implementing effective mitigation strategies. The primary culprits include:

  1. High Humidity Levels: Excess moisture in the air increases the likelihood of condensation forming on the ductwork. Regions with high humidity levels, especially during the summer months, are more susceptible to this issue.

  2. Inadequate Insulation: Poorly insulated or uninsulated ducts are more prone to temperature fluctuations. When cold air travels through such ducts, the temperature difference between the air inside and outside the duct increases, leading to condensation.

  3. Air Leaks: Ductwork with gaps, cracks, or loose connections allows warm, humid air to infiltrate the system. This warm air comes into contact with the cold duct surfaces, causing condensation.

  4. Improper Duct Design: Incorrectly designed duct systems can create areas of stagnant air or low airflow, promoting moisture accumulation.

Mitigation Strategies

Thankfully, there are several effective strategies to mitigate duct sweating and maintain a comfortable, healthy indoor environment:

Duct Insulation: Properly insulating the ductwork is one of the most effective ways to prevent duct sweating. Insulation helps maintain a consistent temperature, reducing the likelihood of condensation.

Dehumidification: Installing a whole-house dehumidifier or using standalone dehumidifiers can significantly lower indoor humidity levels, reducing the chances of condensation on the ducts.

Air Sealing: Regularly inspect and seal any gaps or leaks in the ductwork to prevent warm, humid air from entering the system.

Proper Duct Sizing and Design: Ensuring the duct system is correctly sized and designed helps maintain adequate airflow, reducing the potential for stagnant air and moisture buildup.

Regular Maintenance: Schedule routine maintenance for your AC system, including duct inspection and cleaning. Keeping the ducts clean and free of debris reduces the chance of mold growth.

Increase Ventilation: Improve overall ventilation in your home by using exhaust fans in kitchens and bathrooms. Proper ventilation helps maintain balanced indoor humidity levels.


Duct sweating is a common issue that can compromise the efficiency and air quality of your AC system. Understanding the causes of duct sweating and implementing effective mitigation strategies can help prevent this problem, ensuring that your AC system operates optimally and keeps your indoor environment comfortable and healthy. Regular maintenance and proactive measures are essential to ensure a trouble-free cooling experience during the hot summer months. By addressing duct sweating, you'll not only enhance your AC system's performance but also create a more pleasant living or working space.


Air Psychrometrics: Dew-Point temperature and Condensation.

  • “Air” is a mixture of small amounts of water vapor droplets, molecules of Oxygen, Nitrogen, and other gases.

  • Sponge Analogy: Think of a fixed volume of air as a sponge at a given pressure and temperature. This sponge can hold a certain (x) amount of water as water vapor (gaseous form of the water molecule H2O). Because of the heat energy this sponge has, (corresponding to the air temperature at which it is at) the sponge vaporizes and absorbs any water droplets it encounters. This vaporization of water droplets continues at this temperature until the spopnge can no longer vaporize and absorb water droplets. At this point the air (sponge) is holding all the water it absorbed from the air as gaseous vapor. But could this volume of air (sponge) continue vaporizing and hold more water as vapor? That depends on how much water was in the air to begin with and the maximum amount of water that sponge could absorb at the given temperature and pressure; the sponge's capacity.

  • Relative Humidity: Relative humidity (RH) is expressed as a percentage of the amount of water vapor the sponge is holding over the maximum amount of water that sponge could hold at the given temperature and pressure. That is to say if the sponge hasn’t reached its max capacity yet but there is no more water to vaporize, then the air (sponge) is at a percentage of what it could hold (Relative Humidity).

  • Saturation: If on the other hand there is a lot of water i.e., high humidity, more than what the sponge could hod, this vaporization continues until the air (sponge) can hold no more water, the sponge has thus reached the maximum amount of water it can hold as vapor at the given temperature and pressure. At this point the air is said to be saturated.

  • Generally speaking, keeping the pressure steady and raising the temperature increases the amount of water that volume of air (sponge) can hold (raising temperature of air is analogous to increasing the size of the sponge). The opposite is also true; lowering the temperature of air decreases the amount of water the air can hold (decreasing size of sponge). So as temperature decreases, the RH increases. Very cold air is analogous to a very small sponge that is on the verge of being completely soaked or saturated; it can’t hold much more water.

  • Dew Point Temperature: The temperature to which a volume of air must be cooled (sponge shrunk in size) so that it absolutely cannot hold any more water vapor is called the dew-point temperature; this temperature corresponds to 100% RH. The air (sponge) is saturated with water vapor.

  • Condensation: As the air temperature of this volume of air (sponge) dips below its corresponding dew-point temperature, water will start to condense back into droplets out of the air. This is analogous to the wringing or squeezing of the sponge.


  1. ASHRAE. (2021). Chapter 9: Psychrometrics. In ASHRAE Handbook - Fundamentals. Atlanta: American Society of Heating, Refrigerating, and Air-Conditioning Engineers.

  2. ASHRAE. (2021). Chapter 22: Moisture Management in Buildings. In ASHRAE Handbook - Fundamentals. Atlanta: American Society of Heating, Refrigerating, and Air-Conditioning Engineers.

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