What causes moisture inside a shipping container?

The water is already inside at loading, held in the air, the pallets and the cargo. As the container heats by day and cools at night, that moisture evaporates and then condenses on the cold steel when it drops below the air's dew point, dripping back onto the cargo as container rain, or forming on cold cargo as cargo sweat.

What is the best way to prevent container rain?

Load dry, then hang high-capacity calcium chloride container desiccants sized by the DIN 55474 method to keep the air below its dew point for the whole voyage. For valuable cargo, add a barrier liner and humidity indicator cards. Silica gel alone is not adequate for a full container on a long ocean route.

Does ventilation stop container condensation?

Only sometimes. Passive ventilation helps when the outside air is drier than the air inside the container, but on humid or tropical routes it can let in more moisture than it removes, so it is a supporting measure rather than a primary defence. Desiccant is the reliable active control.

Practical Guide

How to Prevent Moisture in Shipping Containers

Cargo does not arrive wet because rain got in. It arrives wet because the water was already inside — in the air, the pallets and the goods — and the day-night temperature swing at sea condenses it onto the steel, where it drips back down as “container rain.” This guide explains exactly what causes it and walks through the five proven ways to stop it, ranked by how well they actually work.

Reviewed by the BENZ Packaging technical team · Updated June 2026

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The Cause

It Is Condensation Physics, Not a Leak

A sealed container holds a fixed mass of air and water. The air carries vapour, and so does everything hygroscopic inside it — wooden pallets, cartons, kraft paper and many cargoes themselves can each hold litres of bound moisture. When the sun heats the steel by day, that moisture evaporates into the air; when the hull cools at night or the ship enters colder water, the steel drops below the air’s dew point and the water condenses on the ceiling and walls. Enough of it, and it rains onto the cargo. Two related terms describe this: container rain (condensation dripping from the roof) and cargo sweat (condensation forming on cold cargo when warm humid air meets it).

The fix is never to fight the water after it forms but to keep the air below its dew point for the whole voyage, and to stop moisture entering or being trapped at loading. No single tactic does all of that perfectly, so the right answer is usually a combination — load dry, absorb what remains, and where the cargo is valuable, add a barrier and a way to verify the result. The five methods below are how it is done in practice.

See the Problem

Condensate beads on the cold steel roof, then drips onto the load below.

Container rain: condensation forming on the cold steel ceiling of a shipping container before dripping onto cargo — Container rain on the steel roof — the moment moisture protection is meant to prevent.

The Five Ways to Stop It, Ranked

From the non-negotiable basics to the highest-value protection.

1 · Load Dry

Use dry pallets and dunnage (kiln-dried, not fresh timber), avoid loading in rain, and never seal a warm, humid container at the dock. This costs nothing and removes the biggest source of trapped water before you start.

2 · Container Desiccants

High-capacity calcium chloride units (BE DRY / ULTRA) hung along the walls and ceiling pull vapour out of the air and lock it into a leak-proof gel, holding the dew point down for the whole voyage. The single most effective active measure. How they work ›

3 · Barrier Liners

A foil or film container liner isolates the cargo from the steel and from outside air exchange, and works best combined with desiccant inside the liner.

4 · Ventilation

Passive vents help only when the outside air is drier than the inside — useless or harmful on humid tropical routes, so it is a supporting tactic, not a primary one.

5 · Monitor & Verify

Humidity indicator cards and data loggers confirm the dry environment held and give you evidence if a claim is ever disputed. Indicator cards ›

Avoid: Silica Gel Alone

Silica gel holds only ~25–40% of its weight and saturates early; it cannot keep a full container dry across a multi-week ocean voyage. Use calcium chloride for containers.

Getting the Quantity Right

The Right Method, the Wrong Amount, Still Fails

Containers still arrive wet despite carrying desiccant — almost always because the quantity was guessed. The amount needed depends on the container volume, the weight of hygroscopic packing, the humidity at sealing and the voyage length, which is exactly what the DIN 55474 Desiccant Unit method calculates. As a rough guide a 40ft container of hygroscopic cargo needs about 10–12 one-kilogram calcium chloride units on a typical route, and a 20ft about 5–6, fewer with higher-capacity BE DRY ULTRA. For your exact shipment, the free desiccant calculator sizes it in seconds, and our engineers will verify the figure at no charge.

Protecting a Container Shipment?

Send us the container, cargo and route and we will recommend the right combination of dry loading, desiccant quantity, liner and monitoring — sized to DIN 55474, free of charge, worldwide.

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