The short answer: container rain happens when warm, moist air inside a sealed container cools below its dew point and the water condenses on the roof and walls, then drips onto the cargo. Coco peat is a plant material that holds and releases water, so a lot loaded damp feeds the cycle all voyage. The fix is mostly at origin: load coir and pallets dry into a dry, sound container, and only then add desiccants, liners or ventilation as support. Nothing you add to the box will rescue a wet lot.
Key facts
- Goods of plant origin are the most intensive source of sweat in a container, and ventilation can't remove the quantities of water they release.
- Plant-origin cargo can lose up to 0.5% of its mass to evaporation in transit, depending on its moisture at loading.
- Ambient sea air over open oceans normally runs above 80% relative humidity.
- Container-dry timber has a water content of 12 to 15%, and mould growth is expected above 20%.
- Relative humidity where a container is packed should not exceed 60%.
Two kinds of sweat
Cargo people distinguish two kinds of condensation, and they happen on different routes.
Container sweat (ship sweat). The air inside the box has been warmed and moistened by the cargo. As the ship moves into cooler water or the night gets cold, the steel cools first. When the inside surface drops below the dew point of the air, water condenses on it. On the walls it runs down to the floor. On the roof it gathers and drips. This is classic container rain, and it's worst on routes from warm to cold climates with a damp, plant-based cargo.
Cargo sweat. The reverse. Cold cargo meets warm, moist air, and water condenses on the cargo itself. For coir this mostly happens at the far end, when a container that crossed cold seas is opened in a warm, humid port or stripped into a warm warehouse.
The physics behind both is the dew point. Warm air holds more water vapour than cold air, and the amount rises faster the warmer it gets. So a box of warm, damp air has far more water to drop when it cools than a cool, dry one.
Why coco peat is a problem cargo
Coir is hygroscopic. It takes up water from humid air and gives it back when the air is drier or warmer. In a sealed container, the cargo largely sets the humidity of the air around it, pulling it towards its own moisture balance.
That means the water in the blocks is the water in the air. Load the coir damp and every hot day drives vapour out of the blocks into the air, and every cold night condenses it back on the steel. A standard dry container has no way to get rid of it. Only ventilated containers can, and even then the quantity a damp plant cargo releases is more than ventilation can carry away.
The results arrive as:
- Wet top tiers under drips from the roof.
- Mould on block surfaces, starting where drips land or where blocks touch a wet wall.
- Soft, collapsed cartons of briquettes or retail packs, which then crush the tier below.
- Wet bottom tiers where water runs down the walls and pools on the floor or soaks into pallets.
- Musty smell inside the box, a sign of moisture damage before you see it.
The mechanics of what moisture does to block weight and mould are covered in coco peat moisture content and weight. For what to do with mouldy coir once it's arrived, see mould, algae and fungus gnats in coco coir.
Moisture targets that actually prevent it
Start with the cargo, because that's where most of the water comes from.
| What | Target | Why |
|---|---|---|
| Coco peat blocks and briquettes | A written maximum moisture, measured on a wet-weight basis by an agreed method | Supplier specifications range from under 15% to about 20%. The number matters less than having one and testing to it |
| Wooden pallets and dunnage | Container-dry, 12 to 15% water content | Mould growth is expected above 20%, and wet timber feeds water into the bottom tier |
| Air where the container is stuffed | 60% relative humidity or below | Packing in humid air traps that air in the box |
| Packaging | Dry cartons, no wet wrap | Plastic sealed around a damp block holds the water against it |
Cargo guidance also warns against drying plant goods far below their recommended lower limit. Aim for a consistent, agreed level rather than "as dry as possible".
Loading-day checklist
Most condensation problems are settled on loading day. Before the first pallet goes in:
- Inspect the empty container. Dry floor, doors that close tightly on good seals, and no holes or cracks. Stand inside with the doors shut. If you see daylight, reject the box.
- Check the cargo. Spot-check moisture on a few blocks from different pallets, and look for wet or warm patches in the stack.
- Check the pallets. Dry timber, ISPM 15 marked, no bark or soil. Our guide to fumigation and pallets for coco peat covers pallet choices.
- Load under cover. Not in rain, and not from a yard where pallets have stood in the wet.
- Keep cargo off the walls. A small gap lets condensed water run down the steel rather than into the blocks.
- Protect the top tier. A sheet of absorbent paper over the top catches drips.
- Fit desiccants and close promptly. Don't leave the box standing open in humid air.
Desiccants, liners and ventilation
These are the support measures. Each has a place, and none replaces a dry load.
Desiccants
Container desiccants, usually calcium chloride in bags or poles hung along the top rails, pull water out of the air before it condenses. They keep a dry load dry through repeated day and night cycles. Published dosing recommendations vary widely between makers, because the right amount depends on transit time, the route, the season and how much water the cargo and pallets will release. Go by the maker's guidance for your route, and dose up for long voyages and monsoon loading. Desiccants can't dry a lot that was loaded wet.
Liners
Two different products get called liners:
- Thermal liners are reflective blankets that line the walls and roof. They act as a radiant heat barrier, slowing the build-up of heat by day and buffering the day and night swing that drives condensation. Some have a breathable membrane ceiling meant to reduce container rain. They suit routes with high heat exposure but are passive and won't control temperature.
- Absorbent or protective sheets such as paper dunnage, nonwoven fleece hung under the roof, and plastic sheets with condensation reservoirs. These don't stop condensation but keep it off the cargo.
For coir, a roof-level absorbent fleece or paper over the top tier is the cheap first step. Thermal liners make most sense on long, hot routes where blocks have been damaged before.
Ventilation
Ventilation helps only when the air outside is drier than the air inside. On ships, the rule is to ventilate if the dew point inside the hold is higher than outside, and not otherwise. Where dew points can't be measured, ventilate when the outside air is at least 3°C cooler than the cargo was at loading.
A standard container's small vents do very little either way. Ventilated containers move more air and are used for damp agricultural cargo, but for a properly dry coir load they're usually unnecessary. For a dry load, a sealed box with desiccant is the simpler choice.
At the destination
Arrival is the second risk point. If the container crossed cold water and is opened in warm, humid weather, moist air hits cold blocks and wraps and condenses on them.
- Open, ventilate, then unload. Coir can use up oxygen in a closed box, so let the container air before anyone walks in.
- Inspect the top tier and the door end first. That's where drips and leaks show.
- Don't unwrap cold pallets in a humid warehouse. Let them reach room temperature first.
- Photograph wet patches, roof droplets and stains before moving anything. Insurers and suppliers will ask.
- Weigh a sample of blocks. A clear gain over label weight means the lot took on water.
Then move the coir into dry, covered storage. Our guide to storing coco coir covers the warehouse side.
FAQ
What is container rain?
It's condensation that forms on the inside of a container's roof and walls when warm, moist air inside cools below its dew point. The water collects on the roof and drips onto the cargo. It's worst with damp, plant-based cargo on routes from warm to cold climates.
Why does coco peat get mouldy in shipping containers?
Usually because it was loaded with too much moisture. Coir releases water vapour as the container heats by day, and that water condenses on the cold steel at night and drips back. Wet blocks and wet pallets give mould everything it needs.
What moisture content should coco peat blocks have for shipping?
Supplier specifications range from under 15% to about 20% on a wet-weight basis. Put a maximum in writing, agree the test method, and check it on blocks from several pallets before loading.
Do desiccants stop condensation in coco peat containers?
They help a dry load stay dry by absorbing water vapour before it condenses. They can't dry a lot that was loaded wet. Follow the desiccant maker's dosing for your route and transit time.
Should I use a ventilated container for coco peat?
For a properly dry load, usually not. Ventilation only helps when the outside air is drier than the inside air. A sealed standard container with desiccants and dry pallets is simpler and works well.
Do thermal container liners help with coir?
They can on hot routes. A reflective liner slows heat build-up and buffers day and night temperature swings, which reduces condensation. It's passive protection and doesn't replace loading the coir dry.
Why were my cartons of briquettes wet when the blocks were fine?
Cartons absorb drips and wall condensation quickly and lose strength when damp. Cartons at the top or against the walls take most of it. Keep cargo off the walls and cover the top tier with absorbent paper.
Ship it dry from the start
CVM Fiber and Industries makes coco peat blocks and briquettes in Kanyakumari, Tamil Nadu, India, washed with water to reduce EC, with no calcium nitrate or other chemicals. Contact us to discuss moisture limits and packing for your route.