Container Rain and Cargo Sweat Testing

Part of our iHaaS deep-dive series. New to HaaS? Start here.

Quick answer: Container sweat is condensation on the container's walls and ceiling when they cool below the dew point of the air inside. Cargo sweat is condensation on the cargo itself when its surface is colder than the dew point of warmer outside air. "Container rain" is what people call it when ceiling sweat builds up enough to drip. The water almost always comes in with the cargo, packaging or dunnage, so you test for it by simulating the voyage and measuring air and surface temperatures at several layers.

A container leaves port dry. It arrives with stained cartons, caked bags and a mold smell. Nobody spilled anything. The water was in the box the whole time, moving from the cargo into the air, onto the ceiling, and back down.

At ATMOSIScience®, we make fiber desiccant and anti-condensation materials, and we test how they perform in simulated shipments using our HaaS sensors. This article explains what causes container rain and cargo sweat, and how we set up a test that shows where the moisture goes.

Container sweat vs cargo sweat: two different problems

The GDV Container Handbook, published by the German Insurance Association (GDV) as a cargo loss prevention reference, separates the two clearly. They form under opposite temperature changes, so the fixes are different too.

	Container sweat	Cargo sweat
Where water forms	Container walls and ceiling	The surface of the cargo
Trigger	Air temperature drops; the wall cools below the dew point of the air inside	Air temperature rises; the cargo surface is colder than the dew point of the outside air
Typical voyage	Warm to cold, for example leaving the subtropics for temperate latitudes in winter	Cold to warm, for example a voyage into the tropics
Where the moisture comes from	Mostly the cargo, packaging and dunnage, evaporating into the container air	Warm, humid outside air reaching cold cargo through vents, door leaks, or when the doors open

Container rain is the end stage of container sweat. The handbook notes that condensation is most severe on the ceiling, and that after a few days it can build up until it drips onto the cargo. Sweat on the walls can run off. Sweat on the roof falls straight down (GDV Container Handbook, sweat classes).

Where the water actually comes from

It's tempting to blame humid air. The numbers say otherwise. The handbook states that the water vapor held in the container air itself is small and doesn't generally cause sweat damage, though it can still drive corrosion. Relatively large quantities of sweat in closed containers are always attributable to the cargo or its packaging, or a wooden container floor (GDV Container Handbook, water vapor in the container).

Worked example: wet dunnage

The handbook's own example is worth walking through (GDV Container Handbook, packaging and dunnage):

A container carries 20 metric tons of general cargo, including 2 metric tons of wooden dunnage and case lumber.
At the usual 15% water content, that wood holds 2,000 kg × 0.15 = 300 kg of water.
If the lumber wasn't dried properly, or the cases sat outside in the rain, water content can be 30%: 2,000 kg × 0.30 = 600 kg.
That water evaporates until the container air is saturated. At night, the container cools, and the vapor condenses on the ceiling and walls. It can repeat day after day.

Same cargo, same route, and the packaging alone decides whether you're managing 300 kg of water or 600 kg. That's why moisture-laden packaging is part of every test we run.

The daily pump

In sunny weather, the roof heats the container air during the day. Relative humidity inside falls, so the cargo and floor give off water vapor. At night, the walls and ceiling cool. Once they drop below the dew point of the inside air, sweat forms. The handbook reports that sweat coverage grows in stages and peaks after a few days (GDV Container Handbook, radiation weather). A single snapshot at loading tells you almost nothing. You need the curve.

The dew point math behind both kinds of sweat

Both problems come down to one comparison: a surface temperature against a dew point. (If you want the formula, we walk through it in our enclosure condensation guide.) Two examples built from conditions in our climate-room test plan:

Scenario	Air	Dew point	Surface	Result
Container sweat before unloading	10 °C, 94% RH inside	9.1 °C	Ceiling 1 °C colder than the air, at 9 °C	Margin -0.1 °C. Sweat forms with almost no temperature drop
Cargo sweat at opening	26 °C, 83% RH outside	22.9 °C	Cargo still at 8 °C from a cold leg	Margin -14.9 °C. Every exposed carton gets wet

The first row shows why high-humidity holds are so touchy: at 94% RH, a 1 °C dip is enough. The second shows why the handbook advises against opening a container immediately at a warm destination port. Cargo that has been sealed off from outside air needs time to reach the local temperature before it meets that air (GDV Container Handbook, cargo sweat).

How bad is it? The sweat class scale

When a container is opened, you need a common way to describe what you see. The handbook gives a seven-class scale, with the water quantity per square meter of surface:

Class	What you see	Water, g/m²
0	No sweat, dry	0
1	Surface feels moist, not just cold	0 to 40
2	Sweat can be swept together by hand and runs off	40 to 120
3	Distinct droplets; some run down vertical walls	120 to 300
4	Surfaces densely covered with large droplets that often run off	300 to 540
5	Isolated dripping from the ceiling; constant run-off on walls	540 to 760
6	Severe dripping from the ceiling; puddles at the base of walls	760 and above

Classes 5 and 6, with water dripping from the ceiling, are what people usually mean by container rain. It's a useful scale for unloading inspections, but it's a verdict after the fact. Sensors tell you when and why you got there.

How we test it: a three-stage shipment in the climate room

Our climate room is about 20 m² with a 3 m ceiling and a temperature range of -20 to 60 °C. It can run refrigeration, heating, humidification and dehumidification on a programmed schedule, so we can play out a whole voyage in a few days.

Climate room used to simulate a long-haul shipment for cargo sweat and condensation testing — Our climate room: about 20 m², 3 m ceiling, -20 to 60 °C · ATMOSIScience

Stage	Conditions	What it simulates
1. Moisture exposure (24 h)	26 °C at 83% RH, rising to 90% RH in the last 2 hours. 5 mL/m² of water mist sprayed over 9 zones on each face of the sample. A damp substrate with a set moisture content placed between the middle layers	Cargo caught in light rain, with moisture-affected packaging
2. Long-haul gradient (72 h)	26 to 18 °C and 83 to 50% RH; then 18 to 8 °C and 50 to 35% RH; then an 8 to 15 °C day-night cycle with RH swinging between 35 and 70%	A warm, humid southern origin, a cold, dry northern leg, then day-night swings at the port
3. High humidity before unloading (48 h)	10 °C ±2 °C day-night, constant 94% RH	Conditions that speed mold growth on moisture-affected cargo

Stage 1 loads the cargo with water the way real shipments do: from rain and damp packaging. Stage 2 cools it down, so the core of the stack stays cold. Stage 3 is where container sweat and mold risk peak, because a 94% RH hold has almost no dew point margin.

For monitoring, we put one waterproof HaaS Box Plus terminal on each wall at 1.5 m height. A wired gateway sits outside the chamber on Ethernet, with its Bluetooth antenna routed into the room, and uploads every 10 minutes so no part of the curve is missing.

The 1:3 container test: four layers, two protection schemes

A climate room tells you how a packed sample behaves. A container test tells you how a load behaves inside a steel box. For flour transport, we built a test container based on a standard 20 ft container, scaled down at a 1:3 ratio.

1:3 scale shipping container anti-condensation test for flour transport with a standard protection group, a differential protection group and door-gap sealing — The 1:3 container test: standard group, differential group and door-gap sealing · ATMOSIScience

Standard protection group. Interior walls cleaned and dried, kraft paper fixed with adhesive tape (no overlapping seams), then covered with PE film. This is the control.
Differential protection group. The protection scheme being validated, run side by side against the control.
Door-gap sealing. Sealing strips on the door gaps, so outside moisture doesn't skew the comparison. The data then reflects the protection scheme, not a leaky door.

Each group gets the same sensor layout:

Layer	Position	T&H sensors	Thermocouples	Why there
Surface	Top center, touching the PE film or the differential protection layer	1	2	Under the ceiling, where container rain lands
Side face	Middle of the side face, at the edge opposite the door	1	2	Next to a cooling wall
Middle	Center bag of the 2nd stacked layer	1	3	The cold core that drives cargo sweat
Bottom	Near the center of the floor, 5 cm up	1	2	Floor moisture and run-off

That's 4 temperature and humidity sensors and 9 thermocouples per group. One HaaS DTU collects and uploads data from every sensor. The DTU takes up to 255 RS485 channels over Modbus RTU, which is how wired sensors join the cloud.

What to measure, and what to do with it

If you're planning your own shipment test, here's the checklist we work from:

Air temperature and RH at every layer. That gives you the dew point of the air each surface is exposed to.
Surface temperatures. Ceiling-side film, side wall, cargo core, floor. Condensation is decided at the surface, so this is non-negotiable.
Dew point margin over time. Surface temperature minus air dew point, per layer. The minimum and the time spent below zero are the two numbers that matter most.
A control. Without a standard group, you can't say what the protection changed.
The moisture you put in. Record spray volume and the moisture content of any damp substrate, so the test is repeatable.
Enough duration. Sweat peaks after a few days, so short tests understate it.

The deliverables from a full validation project, including the benchmark report and the selection guide, are covered in From Pilot to Proof.

Protection works in layers

No single fix covers every route. Think of it as a stack:

Keep water out of the box at stuffing. Dry dunnage and dry packaging. The 300 kg versus 600 kg example shows how much this one decision is worth.
Line the walls. Kraft paper and PE film, the standard protection in our container test, help keep wall sweat off the cargo.
Absorb vapor before it condenses. This is where our materials come in. Fiber desiccant takes up water vapor from the air inside the load, and our anti-condensation materials absorb moisture first in enclosed spaces. Fiber desiccant can be made in any shape and in custom sizes to fit the load. See our desiccant overview for formats.
Manage the opening. Give cold cargo time to warm up before the doors open at a warm port.
Measure. A test with sensors at four layers tells you which of these layers is doing the work, so you pay for the ones that matter.

Frequently asked questions

What is the difference between container sweat and cargo sweat?

Container sweat forms on the container's walls and ceiling when they cool below the dew point of the air inside. Cargo sweat forms on the cargo when its surface is colder than the dew point of warmer outside air, typically on a voyage into warmer climates or when doors open at a warm port.

What causes container rain?

Moisture from the cargo, packaging and dunnage evaporates into the container air on warm days and condenses on the ceiling as it cools at night. After a few days, ceiling sweat can build up until it drips onto the cargo.

Can humid air alone cause container rain?

Not usually. The GDV Container Handbook notes the water vapor in the container air is small. Large quantities of sweat come from the cargo, its packaging or a wooden floor.

How long should a container sweat test run?

Long enough to include the cycles that matter. Our climate-room plan runs 24 hours of moisture exposure, 72 hours of transport gradient and 48 hours of high humidity before unloading, because sweat builds over several days.

Where should sensors go in a shipping container test?

At several layers: top surface under the ceiling, a side face, the middle of the stack, and near the floor. Pair each air sensor with surface temperature probes so you can calculate the dew point margin at each layer.