If you are storing items in a standard shipping container that is placed outside and exposed to the elements your goods are susceptible to condensation occurring inside the container, usually collecting on the ceiling. The root cause of moisture damage in storage containers is the simple fact that warm air can hold more moisture than cold air just like the dewy grass in the morning after a cool summer night.
A shipping container is a metal box that is quickly affected by ambient temperature changes. When the container is warm, the atmosphere inside the container can become humid. Moisture gets into the air in the container from the outside or by evaporation from the cargo. You may also check http://shippingcontainers.net/ online to gather the more information about the storage container.
When cooled during the evening, water droplets will form, usually on the coolest place like the ceiling, when then the droplets are large they drip onto your stored items. If this is not addressed, you will get a micro rain cycle, causing water damage on your stored items.
No container is air tight it will "breathe" as a result of temperature cycles. When the air inside the container cools, the pressure drops. Air and moisture moves in from the outside to equalize the pressure.
The opposite happen when the air inside the container heats up, but it is easy to show how a repeating cycle of breathing can cause a buildup of moisture inside he container, especially if there is absorbing packing materials.