Container Desiccant: A Complete Guide

A container can leave a port at 30 °C and be 10 °C by the time it crosses an ocean. That swing is what puts water on your cargo — and desiccant is how you stop it. This guide covers the whole picture: which type to use, how much, and how to install it so it actually works.

Why a Shipping Container Needs Desiccant

The cargo is rarely the problem. The container is.

Steel absorbs and releases heat faster than the goods inside it. Through a day–night cycle, the roof cools first, and when it drops below the dew point of the air inside, water condenses on the ceiling. It then drips. This is what the trade calls container rain, and it explains most of the mould, rust and collapsed cartons found on arrival.

Two things make it worse:

• Wet cargo. Timber, textile, paper and food all carry water in. Sealed inside a container, that water has nowhere to go.
• The route. A voyage that crosses several climate zones produces far more condensation than a short one in a stable climate.

The Three Types of Container Desiccant

Calcium chloride is the default for container work because a single hanging bag holds a lot of water and the format suits the inside of a box. Silica gel is the better answer where the cargo itself is sensitive or where you want a reusable, indicator-based system. Clay is the cheapest per kilo and the usual choice for large volumes of ordinary goods.

We cover each in detail elsewhere — see our comparison of silica gel against calcium chloride, and our guide to calcium chloride container desiccant specifications.

How Much Desiccant Do You Need?

The quantity is set by four things, and the container's volume is not one of them on its own:

• The moisture already in the cargo — how much water the goods will release during the voyage
• Vapour ingress — the container seal is not perfect, and air exchange brings water in continuously
• The duration and route — longer voyages and bigger temperature swings mean more condensation cycles
• The target — the relative humidity you need to hold to protect that particular cargo

Because the ingress term keeps accumulating, dosage scales with voyage length, not just with box size. For worked figures on 20ft and 40ft containers, see our guide to how many desiccant bags a container needs.

How to Install Container Desiccant

This is where most shipments go wrong. The right quantity, badly placed, protects almost nothing.

• Hang it from the top. Water condenses on the ceiling and falls. Desiccant at floor level is downstream of the problem — it has to wait for the water to reach it. Hung from the lashing rings along the top, it intercepts the moisture where it forms.
• Space it along the length. One unit in the middle leaves both ends unprotected. Distribute along the container so no section of cargo is more than a short distance from a unit.
• Do not block the ventilation slots. Containers are designed to breathe. Desiccant placed over the vents stops the airflow that keeps the cargo stable — and can make things worse, not better.
• Keep it off the cargo. A bag resting on cartons or machinery protects the top surface and little else, and it can stain or crush what it touches.
• Secure it so it cannot swing. A hanging bag that shifts in transit can damage cargo. Tie it off so it stays where you put it.
• Mind the doors. If cargo is loaded close to the doors, that end is the most exposed to air exchange every time the container is opened. Do not leave it bare.
• Count the roof, not the floor. As a sanity check, walk the length of the container and look up. Every part of the ceiling should have a unit near it.

What to Specify When You Order

• Type and capacity rating — calcium chloride, silica gel or clay, and the water-adsorption figure it is tested to
• Format — hanging bag, blanket, canister or sachet, and the hanging method
• Quantity per container — and whether you want it supplied as a per-container kit
• Any end-use certification — food contact where the cargo is edible, and the heavy-metal documentation that EU-facing packaging buyers now ask for

Ask for the test report for the actual batch you receive. A capacity figure is a typical value tied to a stated temperature and humidity; it varies by batch, and the receiving inspection will not.

Frequently Asked Questions

How many desiccant bags does a 20ft or 40ft container need?

It depends on the cargo, the route and the voyage length more than on the container size. Our dosage guide sets out how to work through it for both container sizes.

Is calcium chloride better than silica gel for containers?

For capacity per unit and for long ocean voyages, usually yes. For sensitive cargo, reusable systems or where an indicator matters, silica gel is often the better answer. It is a trade-off, not a ranking.

Where should container desiccant be hung?

Along the top, from the lashing rings, spaced down the length of the container, clear of the ventilation slots and not resting on the cargo. Water condenses on the ceiling first, so that is where you intercept it.

Can I reuse container desiccant?

Silica gel and clay both adsorb physically and can be regenerated with heat. Calcium chloride cannot — it absorbs into a gel and is spent. In practice, regenerating at container scale is rarely worth the labour compared with fresh desiccant.