By: Don Hardy

Occasionally we see containers that arrive and the cargo inside is all wet with unsightly blotches of mold.

You find that there are no holes in the container, the container is a completely sealed, undamaged and clean unit. Yet the interior of the container and your cargo has wetness, mold and mildew on the boxes. Water / condensation rains down from the ceiling of the container.

What is to blame?

If your containers are loaded in a warm tropical environment (ex China, India, Vietnam) where the air was humid and warm, then shipped to the United States, where temperatures were far cooler, your cargo may have experienced a problem called “cargo sweat” or “container rain.”

 Inside container rain drips down on boxes 

A container is a closed system with its own unique climatic system inside. It differs from a warehouse in that the variations in temperature are usually much greater. It is not unusual to have containers where temperatures range from freezing to over 50° Celsius (122° Fahrenheit) during the course of a single voyage.

The central fact is that warm air can hold more moisture than cold air. That means that if warm air is cooled, it becomes more humid. If it is cooled enough, some of the moisture may rain out and condense. This is exactly the same phenomenon that causes dew in the grass or fog on a cool day.

The resulting damage may not be covered by filing a cargo claim with the carrier.

 An example of cargo “sweat” appearing on the inside of a container 

Possible Solutions:

1)     If possible, load containers inside warehouse when cooler or in air conditioned low humidity environment.

2)     Never load containers in rain or drizzle.

3)     Use Container Desiccant to wick moisture from the air


Most desiccant manufacturers have guidelines for their products that will help you determine the type and number of units to hang inside of the container.

4)   Consider / request ventilated containers. These are typically used for coffee beans etc. but are based upon availability.

Why does this happen?

The Relative Humidity (RH) is a percentage measure of how much moisture the air holds, as compared to the maximum amount of moisture air at that temperature can hold. That means that completely dry air has a RH of 0%. The RH can never be more than 100%, or any excess moisture will rain out. There is little risk of damage to any cargo if the RH is below 50%.

The humidity changes when the temperature does.

 Relationship between Temperature and Humidity

In a container, a fast temperature change of 5°to 10° Celsius is often enough to cause problems. Water will condense on the coolest available surface, which usually is on the container ceiling or walls. From there it may drip down onto the cargo and cause damage, also known as container rain. At other times it condenses on the cargo, say on the inside of the pallet wrap, also known as cargo sweat, which is usually even more damaging.




Even without any condensation, elevated humidity over a period of time is sufficient to cause damage. Many metals will corrode or discolor at a rather modest level of humidity of 60% to 70%. At higher levels of humidity, above 80% molds can grow, labels peel and corrugated boxes start to soften.

It is important to realize that the humidity of the air changes only as a result of the change in temperature. When air cools it becomes more humid, even though the moisture content in the air remains the same.

The humidity in a container will go up and down throughout the voyage, as a result of changing temperatures. If the temperature changes rapidly enough there are sure to be moisture risks, even if the container may be fairly dry.

In a container, moisture evaporates into the air during periods when the container is warm. The warm dry air can accept a lot of moisture. Warm moist air from the outside can also enter into the container through container breathing. When the container cools down, that air becomes very humid. This is when the risk for moisture damage increases. 

But the temperature does not only have to change over time to make a difference. It is also risky when different parts of a container are at different temperatures. When warm air moves into a colder part it becomes humid and perhaps even condenses moisture. Tons of moisture can be redistributed within a container during a voyage through such processes. Strange patterns of damage may arise, such as mold only in certain parts of the cargo.

Please feel free to share this information with your factories and overseas partners. If you have any questions please feel free to contact Don Hardy (

Sources: 1,2,3,4