We opened the container and the cartons were soaked
Preventing container condensation (moisture) damage - why it "rains" inside a container, which products and seasons are most at risk, the causes that start at the factory, packing, pallet, loading and desiccant standards, why cargo insurance claims get rejected, and a checklist

Hello, this is GreenFrog Seoul.

A company importing bamboo kitchenware shipped a 40-foot container from a factory in Guangdong at the end of June. When the container was opened at a warehouse after arriving in Busan, water was dripping from the roof. The top layer of cartons had gone soft and collapsed, and inside them the cutting boards and cutlery sets were covered in white mold. About 30% of the shipment could not be sold.

The importer had cargo insurance and assumed the loss would be covered. The insurer's loss adjuster, however, concluded that "the products had a high moisture content before shipment and no desiccant was used," and cut the payout sharply on the grounds of insufficient packing and inherent vice. The factory kept repeating that "everything was fine when it left."

This kind of damage is usually called container rain or cargo sweat. A container is a sealed steel box, so when the moisture trapped inside meets a change in temperature, it condenses on the walls and roof and drips onto the cargo. There was no water ingress or accident at sea, yet the goods arrive wet, and first-time importers often have no idea why. This post explains how condensation forms, which products and seasons are most at risk, and what can be done at the factory and during loading to prevent it.

In the moisture damage cases we handle, goods that were already damp when loaded at the factory far outnumber problems that genuinely started at sea. Under-dried wooden products, rain-soaked pallets and outdoor loading in the rainy season are the usual suspects.

1. Why it rains inside a container

Warm air can hold more water vapor than cold air. When a container door is closed in a hot, humid place, the air inside is full of moisture. Once the ship is at sea and night falls, or the container moves into a colder region, the steel walls cool first. Air touching the steel drops below its dew point, and the vapor turns into droplets. When those droplets run down the roof and walls and fall onto the cargo, that is container condensation.

Most of the moisture comes from things loaded into the container. Under-dried wood and bamboo, paper cartons, wooden pallets and the humid air let in during loading all release water. A single wooden pallet can hold several liters of water depending on its condition. That is why water can "appear" inside a container even when nothing leaked in transit.

2. Products vulnerable to moisture and typical damage

Some cargo is far more sensitive to condensation than others. If you import any of the products below, plan your moisture protection before shipment.

Wet cartons alone can cause heavy losses. Corrugated board loses much of its compression strength when it absorbs moisture, so the bottom cartons buckle and the stack above collapses. We covered how to specify export cartons in Episode 15.

3. When and where the risk rises

Condensation is most likely when both humidity and temperature swings are high. For cargo moving from China to Korea, be especially careful in the following situations.

It is easy to relax because the voyage to Busan is short, but if you add up the time from factory dispatch to port delivery, waiting for loading and moving to the warehouse after arrival, the goods sit in the container longer than you think. For the costs of long port stays, see Episode 96.

4. Most causes start at the factory

Condensation happens at sea, but the moisture that feeds it is usually loaded at the factory. These are the three causes we see most often.

Under-dried products and packaging

Wood and bamboo products that have not been dried properly release moisture during transit. Factories sometimes cut drying time to meet deadlines, so it is worth writing a numeric moisture content standard into the purchase order. Freshly printed cartons and cartons stacked outside the warehouse on rainy days also hold a lot of moisture.

Wet pallets

Wooden export pallets must be heat treated (HT) or fumigated under ISPM 15, but an HT stamp does not mean the pallet is dry. Pallets stored outdoors can be soaked through by rain. For moisture-sensitive products, consider requiring dry pallets or switching to plastic pallets.

Loading conditions

Loading outdoors with the doors open on a rainy day lets humid air and rainwater straight into the container. Sometimes the plywood floor is still wet from the previous cargo, or there are small holes in the roof. Plenty of factories load without checking the container at all.

5. Prevention, step by step

You cannot eliminate condensation completely, but reducing the moisture that goes into the container and absorbing what remains cuts the damage dramatically.

How many container desiccants do you need?

Container desiccants are usually hanging or pole-type products based on calcium chloride. As they absorb moisture, water collects inside or turns into a gel. In practice, 4 to 8 units for a 20-foot container and 8 to 16 for a 40-foot are common, but absorption capacity differs by product and the amount needed changes with the goods and the season. If you are shipping moisture-sensitive products in the rainy season, use more than the baseline.

Hang desiccants high on the container walls or in gaps between the cargo, and keep them from touching the products directly. A full desiccant bag that bursts can soak the goods it was meant to protect. Desiccants cost only a small amount per container, which is trivial compared with a single damage incident.

Ventilated containers

High-moisture agricultural goods such as coffee beans or cocoa are sometimes shipped in ventilated containers. For general manufactured goods, though, the usual approach is a dry container with good packing and desiccants. Ask your forwarder whether a special container is necessary. For choosing a forwarder and shipping terms, see Episode 35.

6. Why cargo insurance may not pay out

Many importers assume that because they have cargo insurance, moisture damage will naturally be covered. The reality is different.

Whether you claim against the insurer or the factory, you will have to prove when and why the goods got wet. Take photos and video as soon as the container is opened, and record water marks on the roof, the state of the desiccants and where the wet cartons were. Notify the insurer before moving the cargo. We covered cargo insurance terms in Episode 57 and claims against factories in Episode 36.

7. Sample wording to send your supplier

Moisture measures need to be requested at the order stage so the factory can build them into cost and schedule. Raise them just before shipment and the answer will likely be "packing is already done." We recommend sending the request in Chinese as well.

Sample wording (moisture content)
Please dry all wood and bamboo parts to a moisture content of 12% or below before dispatch. Before packing, send us photos of moisture readings taken on randomly selected products.

Sample wording (packing and pallets)
Please put silica gel in every unit and seal each one in a PE bag. Use dry heat-treated pallets, and do not use pallets or cartons that have been exposed to rain.

Sample wording (loading)
Before loading, please send photos of the container floor and roof. On rainy days, load indoors or under cover, install at least 12 container desiccants for a 40-foot container, and send photos of the installation.

The 12% moisture figure is only an example. Agree on a value that suits the product and the destination climate with the factory and your inspection company. What matters is writing the standard down as a number and getting photo evidence.

8. Three real cases

These are anonymized versions of cases we see often. Amounts and timelines are illustrative.

Case 1 - Under-dried bamboo products

This is the bamboo kitchenware importer from the opening. It turned out the factory had halved the drying time to meet the deadline and loaded in the rainy season without desiccants. The loss came to about KRW 18 million, but the insurer paid only part of it, citing insufficient packing. From the next order, the importer made a moisture content standard, measurement photos and desiccant installation photos mandatory, and the problem has not recurred.

Case 2 - Rain-soaked pallets

An importer of furniture. The products were properly dried, but the factory used pallets that had been stacked outdoors. On arrival, mold and water stains appeared only on the bottom cartons touching the pallets. The importer switched to plastic pallets and now requires pallet condition to be included in loading photos. For furniture import practice, see Episode 72.

Case 3 - Metal tools arriving in winter

An importer that shipped tool sets from Guangzhou in December. Nothing went wrong in transit, but when the cold cargo was moved straight into a heated warehouse and the cartons were opened, moisture formed on the metal surfaces and rust appeared a few days later. It was cargo sweat. The importer now leaves winter arrivals unopened for about half a day so they can adjust to warehouse temperature. Simply opening the cartons a little later would have prevented the damage.

9. Common mistakes

• Asking the factory for moisture measures only just before shipment
• Assuming pallets are dry because they carry an HT stamp
• Relying on cargo insurance without documenting packing and desiccant measures
• Using too few container desiccants or letting them touch the products
• Moving cargo before taking photos on arrival and then being unable to prove when damage occurred
• Opening cold cargo in a warm warehouse right away in winter
• Not telling the forwarder that LCL cargo is moisture-sensitive

10. Condensation prevention checklist

Ordering and production

• Checked whether the product is moisture-sensitive and wrote moisture measures into the PO
• Agreed a numeric moisture content standard for wood and bamboo products
• Decided on silica gel, PE bags and VCI paper for unit packing

Loading

• Received photos of moisture readings and packing condition
• Confirmed that the pallets are dry
• Received photos of the container floor and roof
• Confirmed the number and placement of container desiccants
• Avoided outdoor loading on rainy days

Insurance and arrival

• Confirmed that the cargo insurance cover includes condensation damage
• Took photos and video immediately on opening the container
• Let winter arrivals adjust to warehouse temperature before opening

Final thoughts - moisture is loaded at the factory, not at sea

Container condensation looks like bad luck, but when you dig in, the cause is usually clear. Under-dried products, wet pallets and loading in the rain mean the moisture is already on board before the doors are shut.

Prevention does not cost much. Set a numeric moisture standard, get photos of packing and pallet condition, and put a modest amount of desiccant into each container, and most damage can be avoided. Those records also become your evidence for insurance and factory claims if something does go wrong.

There is one thing you can do right now. Open the purchase order for your next shipment and check whether it says anything at all about moisture. If it does not, start by adding a moisture content standard and a request for desiccant installation photos.

GreenFrog Seoul supports you on the ground in China, from checking drying and packing processes and measuring moisture content to supervising loading and confirming desiccant measures. If you already have moisture damage, we can start by sorting out the cause and the evidence. If you are preparing a new shipment, we can start with the moisture measures to put in your PO.