What is container rain?

Container rain is condensation that forms inside a sealed shipping container and drips onto the cargo. Moisture in the air and hygroscopic packing evaporates when the container heats by day, then condenses on the cold steel ceiling when the container cools below the dew point at night, and drips down. Over a long voyage it can deposit tens of litres of water onto the cargo.

How do you prevent container rain?

By removing water vapour from the container air so the dew point never reaches any surface temperature. High-capacity calcium chloride desiccants absorb 300 to 600 percent of their weight and keep absorbing for the whole voyage, sized by the DIN 55473 method. Silica gel saturates too early to keep a long voyage dry.

At what humidity does container rain damage cargo?

Atmospheric corrosion of steel accelerates above roughly 60% relative humidity, and mould growth on textiles, leather and food becomes active above about 65% RH sustained for a day or more. The goal of a desiccant programme is to hold container humidity below these thresholds throughout transit.

Technical Reference

Container Rain, Explained

Q: What is the difference between container sweat and cargo sweat?

Container sweat (container rain) is condensation on the container's cold steel that drips onto cargo, typical of warm-to-cool routes. Cargo sweat is condensation on the cargo itself when cold goods meet warm humid air, typical of cool-to-hot routes. Both are the same physics, a surface falling below the dew point of the surrounding air, and both are prevented by removing moisture from the container.

It rains inside shipping containers. Cargo that left the factory dry arrives streaked with water, rusted or mould-spotted, and the cause is not a leak, it is physics. This is the engineering explanation of container rain, the difference between container sweat and cargo sweat, the dew-point cycle that drives it, and the only reliable way to stop it.

Reviewed by the BENZ Packaging technical team · Updated June 2026

Container Desiccants →

What It Is

A Sealed Box Carries Its Own Weather

A shipping container is a sealed steel volume holding a fixed mass of air and water. The air carries water vapour, and so does everything hygroscopic inside it, wooden pallets and dunnage, corrugated cartons, kraft paper, textiles, and many cargoes themselves, can each hold litres of bound moisture. The total water is locked in when the doors close. It does not leave; it simply moves between the air, the packing and the steel as the temperature changes. That movement is container rain.

The key number is the dew point: the temperature at which air becomes fully saturated and water must condense out as liquid. Warm air holds far more water than cold air. As long as every surface in the container stays above the dew point of the enclosed air, the water stays as vapour and nothing happens. The moment any surface, usually the steel ceiling at night, falls below the dew point, water condenses on it. Enough of it, and it drips: it rains.

The 24-Hour Dew-Point Cycle

Why a single day at sea moves litres of water onto your cargo.

How one daily temperature cycle creates rain

1 · Day, heatingSolar load heats the container skin 15–25°C above ambient. Bound moisture evaporates from wood, paper and cargo, driving the enclosed air toward saturation.

2 · Air saturatesRelative humidity climbs toward 100%. The air now holds far more water vapour than it did at sealing, and the dew point rises with it.

3 · Night, coolingThe vessel enters cooler air or colder currents; the steel ceiling and walls cool fastest, dropping below the dew point of the moist enclosed air.

4 · CondensationWater condenses on the cold steel, beads, runs and drips down onto the cargo. The cycle repeats every 24 hours for the length of the voyage.

On a 25–45 day route through equatorial or monsoon zones, this cycle can deposit hundreds of millilitres to several litres of liquid water per day during peak events, accumulating into tens of litres over the voyage, all of it onto the cargo below.

See the Cycle: Temperature vs Dew Point

When the steel ceiling cools below the dew point of the trapped air, water has nowhere to go but onto the cargo.

Condensation forming on the steel ceiling of a shipping container, known as container rain or cargo sweat — Container rain: condensate beading on the cold steel roof before it drips onto the load below.

Keep the air below its dew point all voyage long with a high-capacity calcium chloride desiccant. Size your requirement →

Container Sweat vs Cargo Sweat

Two distinct failure modes that buyers often confuse.

Container Sweat (Container Rain)

Condensation forms on the container's cold steel ceiling and walls, then drips onto the cargo. Driven by warm, moist internal air meeting cold steel, typically on routes from warm origins to cooler destinations or across cold currents.

Cargo Sweat

Condensation forms on the cargo itself when cold goods (loaded in a cool climate) are surrounded by warm, humid air, as on routes from cold origins to hot, humid destinations. The cargo surface sits below the dew point of the surrounding air.

Common Cause

Both are the same physics: a surface falling below the dew point of the air around it. The fix is the same: reduce the absolute moisture in the container so the dew point never reaches any surface temperature.

Why Ventilation Is Not Enough

Standard dry containers are sealed; passive vents cannot keep pace with the moisture load, and opening to humid sea air can add water. Removing moisture with a desiccant is the controllable variable.

The Damage

What Container Rain Actually Costs

The damage is rarely a single dramatic event; it is sustained high humidity and repeated wetting. Steel and machined surfaces flash-rust once container humidity holds above roughly 60% RH, the threshold where atmospheric corrosion of ferrous metal accelerates. Electronics corrode and short. Cartons and kraft packaging soften, collapse and stain, causing load shift and rejected retail packaging. Textiles, leather and food cargoes grow mould once humidity stays above about 65% RH for a day or more. Labels lift, and pharmaceutical and powder products clump or go out of specification. A single under-protected container routinely produces damage exceeding the value of the desiccant programme by a factor of fifty to a hundred.

How to Stop It: Remove the Water

You cannot change the temperature cycle. You can remove the moisture that feeds it.

High-Capacity Desiccant

Calcium chloride absorbs 300–600% of its weight and keeps pulling water vapour out of the air, holding the dew point below every surface temperature for the whole voyage. BE DRY ›

Sized by DIN 55473

The quantity is calculated from container volume, packing-material moisture and voyage length, so the dew point is suppressed for the entire transit. The standard ›

Ceiling-Mounted, Installed Right

Pole-mount units hang where condensation forms and are fitted just before sealing, never on the floor. Installation ›

Why Not Silica Gel

Silica gel holds only ~25–40% of its weight and saturates early, so it stops absorbing while the dew-point cycle continues. It cannot keep a long container voyage dry.

Worried About Container Rain? Let Us Calculate the Protection

Send your container, cargo and route, and our technical team will calculate the desiccant units needed to suppress the dew point for the whole voyage, free of charge, worldwide.

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