Moisture damage 101 – What is container rain?

Moisture damage 101 introduces beginners to the basics and how to get them right. In the second of our beginner’s guides to the world of moisture prevention, we take a look at container rain. What is it and why don’t they just keep the roof closed?

The first thing to say about container rain is that it isn’t like normal rain. You won’t open a container door to see that it’s turned into the showers at the gym. But the moisture-damaging effect on precious cargo is certainly like leaving goods out in the rain.

In actual fact, container rain is more like the dewy moisture that collects on the outside of a cold glass of beer or lemonade on a hot summer’s day. And just like your glass, you’ll want to put some protection like a coaster down so that the moisture doesn’t damage the antique table that’s been in your partner’s family for three generations…

Dewly noted

The science goes like this: when warm air meets the cooler surface of a chilled glass or steel container walls and ceilings that cool at night, the moisture in the air condenses on that surface. This condensation creates small water drops, which are drawn to each other to create larger drops. And finally these drops get so big that they fall down onto your shipment of high-tech, sensitive electronics or finely graded, powdered chemicals.

So how much of a temperature drop is needed to make it “rain”? A drop of 5°C is often all it takes. Which, with the warm days and cool nights of shipping by sea, effectively means there’s always a big risk.

Container breathing as well?

Unfortunately, it gets even worse for our sea cargos. Because these long-haul voyages take weeks, there may be 20, 30 or even more of these day/night temperature changes. And each time, the air inside warms and expands during the day, spilling out of the container. Then contracts as it cools at night drawing in more humid sea air from the outside. This is known as container breathing, and it means new moisture (and rain) to protect against every time the container “breathes in”.

What can you do about container rain?

Quite a lot is the reassuring answer. All it takes to protect expensive cargo is a little extra care and a simple, cost-effective moisture prevention strategy:

1. Make sure the container doesn’t have holes or damaged doors

A container that’s in good condition only allows air and moisture to move in and out slowly. Which will make a huge difference. But you can also tape over the ventilation holes for almost all shipments – except certain organic materials like cocoa that contain a lot of moisture that will easily evaporate into the air and can cause container rain from within.

2. Make sure the container and its contents are dry at the very start

Containers are normally washed before loading. But to avoid leaving the container too wet, avoid high-pressure washing and make sure excess water is dried up.

And remember – the pallets, wooden dunnage and sometimes the goods themselves will also bring in extra moisture. When the temperature rises in the container during the day, this moisture on the floor and in the pallets, packaging material and goods will evaporate into the air. Increasing the humidity and, in turn, greatly increasing the risk of container rain once the air cools at night.

3. Take care of any extra moisture with high-quality, reliable desiccants

Once you’ve minimized the moisture in the container at the start and avoided too much air flow during the voyage, you just need the final, yet all-important touch – desiccants that you can rely on and that will give you and your cargo a generous buffer. A top-quality, cost-effective desiccant solution from a trusted supplier (cough Absortech) means you can remove excess moisture from the air. And prevent the build-up of moisture to levels where it causes container rain and damage to goods.

In the first of our beginner’s guides, we take a look at What is moisture damage?.
Read it here…