Inside a container
All containers contain moisture from the time of loading. No container is completely airtight. Moisture will move in and out of the container during the course of the voyage known as "container breathing".
By reducing the amount of moisture entering the container and by using desiccants to remove moisture from the air you can prevent the buildup of moisture to levels where it may cause damage. Therefore, you will avoid the effect known as container rain.
Is the container airtight?
A minimum requirement is that the container is in good condition, without holes or damaged doors. This should be checked for every container before loading. Especially the doors are vulnerable to damage that may not easily be noticed. Check the ventilation holes. Certainly, no container is airtight, but a container in good condition only allows air and moisture to move in and out of the container slowly. This significantly reduces the amount of moisture moving into the container under common circumstances. Tape the ventilation holes if you are shipping a dry cargo.
Is the container dry?
A container that has been washed before loading may contain a lot of water. Pay special attention to the container floor. The humidity of the wood should not be above 15%. All pallets and wooden dunnage must be dry, certainly not above 20%. It is easy to check the moisture content of the wood with a handheld moisture reading device.
Warm air can hold more moisture than cold air. When the temperature goes down, the air becomes more humid. If it is cooled enough, some of the moisture may condense. A fast temperature drop of 5° 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.