A container can leave the factory with every document in order — correct HS code, clean bill of lading, a valid certificate of origin — and still arrive at the port with stained cartons, rusted fittings or mould across half the load. Nobody dropped it. Nobody flooded it. The damage happened quietly, inside a sealed steel box, somewhere in the middle of a three- or four-week voyage. This is container sweat, and it catches out exporters who have done everything else correctly, because it has nothing to do with documentation and everything to do with what happens inside the box once it is shut.
What container sweat actually is
There are two related problems that get lumped together under the same name. The first is container rain: moist air inside the box condenses on the cold steel roof as the ship moves through different temperature zones, and the water drips straight down onto whatever is packed below. The second is cargo sweat: the goods themselves release moisture — grain, coffee and cocoa all do this — and that moisture condenses on the coolest surfaces it can find, which is usually the cargo next to it or the container walls. Both are driven by the same mechanism: a temperature swing large enough to push humid air past its dew point inside a sealed space with nowhere for the moisture to go.
A container loaded in a mild European autumn and discharged six weeks later in Lagos or Abidjan will pass through exactly that kind of swing more than once. Nights at sea, a hot deck under direct sun, a stop in a humid port before the final leg — each cycle adds another chance for condensation to form.
Which cargoes are most exposed
- Hygroscopic goods — rice, pulses, grain, coffee, cocoa — because they generate their own moisture as temperature changes
- Anything packed in plain cardboard, which wicks water, loses rigidity and can collapse under the weight stacked above it
- Machinery and metal parts, where condensation shows up as rust spots or pitting rather than staining
- Textiles, leather and similar goods, where the visible damage is mould and a smell that is hard to shift afterwards
The risk does not end when the container reaches port. During the rainy season at several West African terminals, containers sit in open yards awaiting customs clearance, and a box that survived the voyage dry can still take on damage during unloading if it is opened in the wrong weather.
Packaging that actually holds up
Generic export packaging is built for handling and stacking, not for a sealed, humid environment over several weeks. What tends to work in practice:
- Desiccant bags (silica gel or calcium chloride) sized to the container’s volume and the cargo’s moisture content, not a fixed quantity used on every shipment regardless of route or season
- Vapour corrosion inhibitor (VCI) paper or film around metal parts, which protects even where some condensation does form
- A plastic liner or shrink-wrap acting as a vapour barrier between the cargo and the container walls and floor
- Pallets or dunnage that keep cartons off the floor, where moisture collects first
- Wooden crates instead of cardboard for anything genuinely humidity-sensitive, even though they cost more and weigh more
The common mistake is sealing cargo inside plastic wrap without a desiccant — that traps whatever moisture is already present and gives it nowhere to go, which can be worse than no wrapping at all.
Who carries the cost when it goes wrong
Liability depends partly on the Incoterm agreed and partly on how the damage is classified. Insurers routinely treat condensation as “inherent vice” — a property of the cargo itself rather than an external event — and exclude it unless the policy specifically covers it and the packaging met a recognised standard for the route. An All Risks or ICC(A) policy is not automatically enough on its own; the insurer will still look at how the cargo was packed before accepting a claim. Who actually pays when a container arrives damaged is worth settling before the goods ship, not after the claim is filed.
On arrival, a surveyor’s report and photographs taken the moment the doors are opened matter more than anything written afterwards. Claims tied to condensation are argued on the state of the cargo at that exact point, and most policies set a short window to notify the insurer once damage is discovered.
Before the container is sealed
- Match desiccant quantity to the actual route and season, not a standard figure used for every shipment
- Check the container is cargo-worthy (CWO) — dry and odour-free — not just visually clean
- Avoid stuffing during or just after rain; moisture trapped at loading stays trapped for the whole voyage
- Confirm ventilation settings if the unit is a reefer rather than a standard dry box
- Have packing method confirmed as part of the wider pre-shipment inspection, rather than leaving it to whoever happens to be stuffing the container that day
None of this adds much to the landed cost. A few desiccant bags and the right liner are cheap next to the value of a container of goods. What is expensive is discovering container sweat has ruined a shipment after it has already sailed, when the only options left are a disputed insurance claim and a buyer who is no longer answering emails.
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