⚡ QUICK ANSWER
A moisture absorber for a shipping container is usually a calcium chloride desiccant placed inside the container to stop condensation (“cargo sweat”) from damaging goods. Hang or stand them where condensation forms first — high on the walls and in pallet voids. As a starting point, plan for roughly 1–1.5 kg per 20ft of space in normal conditions, and 2–3× that for humid cargo or long voyages, then confirm the exact quantity with your supplier.
📊 KEY FIGURES
• Air inside a container can swing from ~20°C to over 60°C during transit, driving repeated condensation.
• Warm air holds far more water vapour than cold air — every night-time cool-down can produce fresh condensation.
• Calcium chloride absorbers hold up to ~200–300% of their weight, vs ~20–30% for silica gel.
• A 20ft container ≈ 33 m³ and a 40ft ≈ 67 m³ of air to keep dry.
What is a container moisture absorber and why do you need one?
A container moisture absorber is a desiccant product placed inside a shipping container to remove water vapour from the air before it can condense on the ceiling, walls or cargo. Without one, temperature swings during transit cause “container rain” or “cargo sweat” — condensation that drips onto goods and causes mould, rust, caking, label lift and corrosion.
Because a sealed steel container heats and cools repeatedly over a voyage, moisture is released again and again. A moisture absorber’s job is to keep pulling that vapour down for the whole journey.
What types of moisture absorber are used in containers?
- Calcium chloride absorbers — the workhorse for containers; highest capacity, best for long or humid voyages.
- Silica gel — better for small sealed packs inside the container than for the whole space.
- Clay / bentonite — low cost, low capacity; short trips only.
- Container liners and barrier bags — used with desiccants for very moisture-sensitive cargo.
For most exporters, calcium chloride absorbers in pole, hanging or blanket form do the main work, with in-box sachets adding local protection where needed.
Where should you place moisture absorbers in a container?
Placement matters as much as quantity. Condensation forms first on the coolest surfaces — usually the ceiling and upper walls at night — so absorbers should intercept moisture there and in the spaces where air moves:
- Hang strips high on the side walls, clipped to the lashing rings, spaced evenly down the container.
- Stand pole desiccants in the void spaces between pallets.
- Lay blanket desiccants over or around the top of the cargo for full loads.
- Keep absorbers from directly touching moisture-sensitive surfaces, and never block container doors or vents.
How many moisture absorbers do you need? (20ft & 40ft)
Quantity depends on container size, how long the voyage is, how humid the route is, and how much moisture the cargo itself carries. Use this table as a planning starting point and confirm with your supplier for your route and season:
| Container | Normal conditions | Humid cargo / long voyage |
|---|---|---|
| 20ft | ~1.0–1.5 kg total | ~2.0–3.0 kg total |
| 40ft | ~2.0–3.0 kg total | ~4.0–6.0 kg total |
Indicative planning ranges. Timber, textiles and agricultural goods carry high internal moisture and usually need the higher figure.
The most common mistake is under-dosing. If you are unsure, choose the higher end of the range — the cost of extra desiccant is tiny next to a moisture-damaged shipment.
Frequently Asked Questions
How many desiccant bags do I need per container?
As a planning guide, about 1–1.5 kg of calcium chloride absorber per 20ft of space in normal conditions, rising to 2–3 kg for humid cargo or long voyages (double these for a 40ft container). Convert to bags using your product’s per-unit weight, and confirm the final quantity with your supplier.
Where do you put moisture absorbers in a shipping container?
Where condensation forms first and where air can reach them: hung high on the side walls, stood in pallet voids, and laid over the cargo for full loads. Keep them off moisture-sensitive surfaces and clear of doors and vents.
What causes moisture inside a shipping container?
Temperature swings during transit. Warm air holds more water vapour; when the steel container cools at night, that vapour condenses on the ceiling and walls and drips onto cargo — known as container rain or cargo sweat. Moisture in the cargo itself and in the loaded air adds to it.
Can I reuse container moisture absorbers?
No. Calcium chloride absorbers are single-use — once the calcium chloride has converted the absorbed water into gel, the unit is spent. Use fresh absorbers for each shipment.