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Storage containers: damp, condensation and what you need to know
Updated 17 October 2026 · 4-minute read
It’s the number one question about containers. The short answer: water doesn’t get in, but the air inside the container holds moisture that can condense on the cold steel. Here is how it happens, and the simple steps that solve the problem.
Watertight? Yes: it’s a shipping container
A 20 ft ‘dry’ container is built to cross oceans on the deck of a ship: Corten steel walls, rubber door seals, marine plywood floor. It is rated watertight and airtight (ISO 1496 standard). It doesn’t let in rain, nor water from the ground when it stands on a drained platform, as at Box Flambère.
Where condensation comes from
Warm air holds more water vapour than cold air. On a mild day, the air inside picks up moisture, especially if the stored items bring some in (mattresses, textiles, boxes carried in from the rain). At night, the steel roof cools faster than the air: the vapour settles as droplets on the ceiling, sometimes on the walls. This is known as ‘container rain’. It mostly appears in autumn and spring, when the difference between day and night temperatures is greatest.
What we do
- Drained, raised platform: no moisture rising through the floor.
- Ventilation: air vents high up that refresh the air and limit temperature swings.
- New or one-trip containers, with door seals checked on delivery.
What you can do
- Don’t store anything damp: dry your textiles, let the mattress air, avoid boxes brought in from the rain.
- Put your belongings on pallets or battens, never directly against the walls: leave a 5 cm air gap.
- Cover furniture with a breathable dust cover (fabric, not sealed plastic, which traps moisture).
- Put in one or two moisture absorbers (sachets or refillable tubs): a few euros, to be replaced every two to three months.
- Keep the most sensitive items in the middle, and the hardiest ones (garden furniture, tools, crockery) towards the walls.
What is better kept in an indoor unit
Old papers, photos, valuable books, musical instruments, paintings, leather or silk clothing, high-end electronics: for these items, an indoor unit inside the closed building offers a more stable temperature. Many customers combine the two: a container for the bulky items, a small indoor unit for the sensitive ones.
And what about the heat in summer?
A container in the sun can exceed 40 °C inside in July. Furniture, crockery, tools and boxes come to no harm; candles, cosmetics, vinyl records, chocolate and some plastics do. Same rule: sensitive items go in an indoor unit.