September 14, 2026

Container Rain: Why Goods Arrive Wet When Nothing Leaked

Close-up of water droplets beaded on a grey steel surface, representing the condensation that forms inside a steel shipping container

You open the container and the cartons along the top row are soft. Some have brown tide marks. Bare metal parts have a fine orange bloom of rust, and there is a smell. But the container has no holes, the seal was intact, and nothing leaked at the port.

That is usually condensation, and the shipping trade has a name for it: container rain. It is one of the most common kinds of sea freight damage and one of the most misunderstood, because it seems impossible that a sealed steel box could let water in.

It did not let water in. The water was inside from the start.

Where the water comes from

A loaded container is not a dry box. Moisture goes in with the cargo from three places.

The air. Air always carries water vapour. A container stuffed on a humid day in southern China seals that humid air in with your goods.

The packaging. Corrugated cartons, paperboard and other wood-based materials are hygroscopic: they absorb moisture from the surrounding air, and release it again when conditions change.

The pallets and dunnage. This is usually the biggest source. Virginia Tech's Center for Packaging and Unit Load Design puts freshly sawn ("green") pallet wood at 50–100% moisture content, against roughly 19% for kiln-dried. A single green pallet can hold more than ten pounds of water, and it gives that water up during the voyage.

Add the goods themselves if they are organic (cotton, paper, leather, wooden products, food) and a surprising amount of water ends up sealed inside a steel box.

How it turns into rain

No leak is needed. The mechanism is ordinary physics.

A container spends its voyage on a ship's deck or in a stack in the sun. By day the steel roof and walls heat up and warm the air inside. Warm air holds more moisture, so the pallets and cartons give water up into it. At night, or when conditions turn cooler, the steel cools much faster than the air it contains. When that warm, moist air meets a surface colder than its dew point, the vapour condenses into droplets on the underside of the roof.

The droplets grow and drip onto the top layer of cargo. Repeat that cycle every day of a voyage and the top tier of cartons can arrive soaked while the bottom of the container looks dry.

A second version, usually called cargo sweat, happens to the goods directly. If cargo is cooler than the air around it — goods loaded in a cold northern Chinese winter, for instance — and the doors are then opened into warm, humid air, moisture condenses straight onto the goods, the way a cold bottle beads with water on a hot day.

Virginia Tech's research also notes that the most significant changes often happen around loading and unloading rather than mid-voyage. That matters, because those are the stages you and your supplier can actually influence.

What it does to goods

Cartons lose strength, so stacks collapse and the contents get crushed
Labels and print smear or peel, a real problem if you sell the goods in their retail packaging
Bare metal rusts: tools, fittings, fasteners, machinery parts
Mould and odour appear on textiles, leather, paper and anything organic
Electronics can corrode in ways that only show up later

A lot of this is cosmetic to the product itself but fatal to the packaging. For retail goods, damaged packaging often means unsellable stock.

The insurance catch most importers miss

This is the part that turns a bad day into a real loss.

Our cargo insurance guide explains why cover is worth having. What it does not go into is how moisture claims tend to be argued.

The 2009 Institute Cargo Clauses (A), the broadest standard cargo cover, exclude loss caused by packing or preparation that is insufficient or unsuitable to withstand the ordinary incidents of the transit. Two details in that exclusion matter a great deal to anyone buying from China:

For that exclusion, "packing" is defined to include stowage in a container
It applies where the packing was done by you or your employees, or before the insurance attached

Now think about how most China shipments work. Your supplier loads the container at their own factory. Depending on your policy and exactly when cover begins, that stuffing may well have happened before your insurance attached. If wet pallets or unsuitable packing turn out to be the cause, the insurer has grounds to argue the exclusion applies, even though you never touched the cargo.

The same clauses separately exclude loss caused by the inherent vice or nature of the goods themselves.

None of this means every moisture claim fails. The insurer generally has to show that the packing actually caused the loss, and a surveyor will typically check for holes, damaged door seals and signs of outside water getting in before concluding it was condensation. But you should not assume that "insured" means "covered" for this particular kind of damage.

What you can actually do

Most prevention happens at origin, so it has to be in your instructions to the supplier before loading, not a complaint afterwards.

Specify dry pallets in writing. Ask for kiln-dried timber or plastic pallets in your purchase order. An ISPM 15 heat-treatment stamp shows the wood was treated against pests; it does not by itself mean the wood is dry
Ask for container desiccants for moisture-sensitive goods. These are large absorbent bags, usually hung along the upper side walls of the container. How many you need depends on the route, the goods and the packaging, so ask your supplier or forwarder rather than guessing
Use vapour barriers for anything moisture ruins: shrink-wrap, sealed polybags, or corrosion-inhibiting (VCI) wrap for bare metal
Keep cartons dry before loading. Goods stored in a damp warehouse carry that moisture into the container with them
Check the empty container. Holes, rust patches that show daylight and damaged door gaskets let outside water in and turn a condensation problem into a leak
Get the loading checked. A pre-shipment inspection can include a loading check (pallet condition, desiccant placement, container condition) as well as the quality of the goods
Do not leave the container sitting. Every extra day at a hot port is another heat-and-cool cycle. Clearing promptly after arrival at Chattogram reduces your moisture risk as well as your storage bill

If you ship LCL, you control less. Your cartons share a container with other shippers' cargo and pallets, and the consolidator does the stuffing. That is a sensible trade-off for smaller shipments, but moisture-sensitive goods need better individual packing to make up for it.

If it has already happened

Treat it as transit damage, not a supplier quality problem. The response is different from dealing with wrong or defective goods.

Photograph before you unload further: the container ceiling, the door seals, the top tier of cartons, any water marks
Keep the damaged cartons and the pallets. They are evidence of what caused the damage
Notify your insurer promptly and ask whether they want a surveyor to attend before goods are moved or thrown away
Record the damage on the delivery paperwork rather than signing for the goods as received in good condition

Whether you also have a claim against your supplier depends on what your purchase order said about packing. That is the best reason to put packing requirements in writing before your next order, rather than after your first wet container.

Insurance terms differ between policies and insurers, and how an exclusion applies to a particular loss depends on the facts and the exact wording. Read your own policy, and ask your insurer or broker how they treat condensation claims before you ever need to make one.

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