A sealed steel box crossing the tropics is a small climate machine. Understand the three numbers that govern it and most moisture damage becomes predictable — and preventable.
The three thresholds
Inside a closed container, relative humidity below:
- 100% prevents condensation
- 75% prevents mould growth
- 40% prevents corrosion
Those thresholds set the whole design problem. Keeping a container under 100% is usually achievable. Under 75% takes deliberate effort. Under 40% requires a sealed barrier and engineering.
Why the box gets wet
Containers stowed on top of the deck can see daily temperature swings of more than 25 °C. Paint colour matters more than you would think: at 25 °C ambient, the air inside a brown container reaches around 50 °C, while a white one reaches about 38 °C.
Two distinct failures follow.
Container sweat is the familiar one. Morning warming drives moisture out of the cargo and packaging into the air. Late afternoon cooling brings the boundary layer at the roof down to dew point, forming hanging droplets that fall onto the cargo — literally rain inside the box. Because condensed water re-absorbs slowly, repeated day-night cycles accumulate liquid water rather than reaching equilibrium.
Cargo sweat is the opposite mechanism and catches people out: goods carried from a cold climate to a warm humid one, unpacked while still cold, have ambient moisture condense onto them. This is particularly damaging on machinery and bare metal, where corrosion starts immediately.
Worth knowing: standard containers are not vapour-tight. Wear around the door area routinely produces leaks, and every leak is a condensation source.
The uncomfortable fact about wooden packaging
Here is where we have to be straight with you, because it concerns our own product.
Dry dunnage, pallets and packing timber sit at roughly 12–15% moisture content. Their sorption behaviour means that once the doors close, relative humidity inside the container will settle at around 60–75% — regardless of what you do elsewhere. Wooden packaging is usually the largest single moisture reservoir in the box.
That is fine for many cargoes: under 75% keeps you clear of mould. It is emphatically not fine if you need under 40% for corrosion protection, and it explains why the desiccant calculation so often produces absurd answers.
A published worked example makes the point better than any argument. A machine shipped to Brazil in a sealed barrier: internal volume 57.75 m³, aluminium composite barrier, 100 days transit, target 40% RH. The moisture load breaks down as:
- water in the enclosed air: 797 g
- water diffusing through the barrier over 100 days: 679 g
- water bound in six pine bracing timbers (324 kg at 15% MC) inside the barrier: 48,600 g
The calculation returns 8,346 desiccant units — not a practical proposition. Move the bracing timber outside the barrier layer, sealing any fasteners that pierce it, and the same shipment needs 246 units.
Two orders of magnitude, from one design decision. If you take one thing from this article: get hygroscopic timber out of the sealed volume. Brace outside the barrier, or use kiln-dried timber at a specified low moisture content if it must be inside.
Desiccants — and when they are wasted money
A desiccant unit is defined by DIN 55473: the quantity that adsorbs at least 3.0 g of water vapour at 20% RH, or 6.0 g at 40% RH, at 23 ± 2 °C. Bags are sold in fractions and multiples of that unit.
The materials differ more than most buyers realise:
- Silica gel — around 30% of its own weight at 50% RH; efficient at lower temperatures, loses capacity as temperature rises.
- Clay — around 25%, but thermal stability drops significantly above 50 °C, at which point it gives moisture back into the container. In a brown box in the tropics, that is a real scenario.
- Molecular sieve — around 20% at 20% RH but very fast, uniquely effective at low humidity, and stable to high temperature.
- Calcium chloride — 150–300% of its own weight, because it deliquesces into brine, and it holds efficiency at elevated temperature.
The critical rule: desiccant only works behind a water-vapour-impermeable barrier that is heat-sealed. Without one, outside vapour keeps arriving, the bags saturate quickly and the humidity inside never drops. Desiccant thrown loose into a container is close to money burned — the widely quoted rules of thumb of 4–6 kg per 20 ft and 8–12 kg per 40 ft are vendor practice for ventilated boxes, not an engineered calculation.
Placement matters too: hang bags in the upper part of the package, never in direct contact with the contents or with metal surfaces, and use many small bags rather than a few large ones. Never put hygroscopic void fill — wood wool, straw, waste paper — inside a climate-controlled package.
VCI — the alternative when you cannot seal
Volatile corrosion inhibitors work differently. The active substance passes into the gas phase, deposits a molecular film on metal surfaces, and suppresses the corrosion reaction. Because it is a gas, it reaches cavities and recesses that no coating would.
The practical figures: roughly 40 g of active substance per m³ of enclosed air, with the VCI material no further than 30 cm from the item being protected. Crucially, airtight sealing is not required — the enclosure just needs to be reasonably tight so the gas is not swept away. Protection can last up to two years.
Limitations worth knowing: it does not suit all metals, it can damage plastics and other non-metallic items, and metal surfaces should be as clean as possible for it to work. Confirm compatibility with the manufacturer rather than assuming.
Choosing between them
- Need a guaranteed internal humidity, typically under 40% for corrosion? Barrier plus desiccant — and keep timber out of the sealed volume.
- Complex machined assembly with internal cavities, or a package you cannot practically seal? VCI.
- Mould rather than corrosion is the risk? Getting under 75% is usually achievable with dry packaging, ventilation and sensible loading, without a sealed system at all.
Tell us the route, transit time and destination climate along with the item, and we will build the packaging around the protection system rather than leaving you to add it afterwards.
