Why Containers Sweat, and How to Keep Them Dry

Why steel boxes sweat

Warm air holds more water vapour than cold air. At 20 °C a cubic metre of air can hold up to 17.3 g of water; at 10 °C only 9.4 g. When air touches a surface colder than its dew point, the surplus condenses on that surface. Air at 20 °C and 70 % relative humidity has a dew point of about 14 °C, so any steel colder than that gets wet.

A container's skin is thin steel with almost no thermal mass, so it follows the weather closely. On a clear night the roof radiates heat to the sky and falls below the temperature of the air, while the air and the contents inside cool more slowly. The GDV Container Handbook, published by the German insurance industry, describes exactly this cycle: condensation is heaviest on the roof, does not dry off there during the day the way it does on the walls, and builds up from day to day until it drips.

The handbook also shows how small the trigger is. With the air in the box at around 60 % relative humidity, the walls need to cool by about 8 K before condensation starts; at 75 %, about 5 K is enough. That is a modest swing between day and night.

Where the water actually comes from

The air itself is a smaller reservoir than most people assume. A 20ft container holds about 33 m³ of air. At 20 °C and 70 % humidity that air carries roughly 400 g of water; cool all of it to 10 °C and about 90 g has to condense. If that were the whole story, the problem would be minor.

It is not, because the air is constantly topped up. Hygroscopic materials — timber, cardboard, paper, textiles, grain, and the container's own plywood floor — take up moisture and release it again as the box warms and the relative humidity drops. Every warm day moistens the air, every cold night strips the water out onto the steel. Add wet goods, a damp lawnmower, a wet tarpaulin or damp ground under the floor, and the supply is effectively unlimited.

So the first rule is to put things in dry, and the second is to keep the container off wet ground — see container foundations.

Ventilation, desiccants and fleece

Standard dry containers have small passive vents high in the side walls. They let the box breathe but move very little air. Additional vents help, particularly low on one side and high on the other, so warm moist air can leave at the top and drier air enter lower down. Use proper weather-hooded container vents sealed into the steel, not open holes.

Ventilation has limits. It can only bring the inside air towards the condition of the outside air, and in fog or drizzle the outside air is damp too. It reduces condensation; it does not abolish it.

• Passive vents — cheap, no power, and the minimum for any container used for storage.
• Desiccants — container desiccant bags absorb moisture until they are saturated and then need replacing. Hang them high, where warm moist air collects. They suit a closed box with known contents; in a box that is opened often they saturate quickly.
• Electric dehumidifier — effective in a closed box and the right answer for an archive or a workshop, but it needs a power supply installed by an electrician, and a drain or regular emptying.
• Condensation fleece — an absorbent fleece fixed under the roof catches drips rather than preventing condensation. The GDV handbook cites fleece that absorbs up to 3 litres per m². Useful where the damage comes from water dripping onto goods.

Shade the roof, raise the goods

Because condensation concentrates on the roof, what happens above the roof matters. A separate lightweight roof or shade structure over the container, with a ventilated gap between the two, reduces both the daytime heating that drives moisture out of the goods and the night-time cooling that pulls it back onto the steel. It also sheds rain and snow away from the door seals. A permanent roof structure can itself need permission, so check before building one.

Underneath, the container should stand on pads with air moving below it. Inside, keep goods on pallets or shelving, never directly on the floor or tight against the walls, so air moves around them and any drips land on something that is not your stock.

Insulation: options and honest trade-offs

Insulation is the reliable fix, because it keeps the inner surface warm enough to stay above the dew point of the air inside. It is also the expensive fix, it takes internal space, and done badly it simply moves the condensation somewhere you cannot see it. The rule is that moist inside air must never reach cold steel. That means either insulation that is itself vapour-tight and bonded to the steel, or a continuous vapour barrier on the warm inside face.

• Closed-cell spray foam — sprayed onto cleaned steel, it bonds to the corrugations, leaves no air gap and acts as its own vapour retarder. The most effective way to stop condensation on the shell. Against it: it is permanent, it hides the steel so rust cannot be inspected, it is combustible and needs a covering wherever there is heat or sparks, and it has to be applied by a specialist to properly prepared steel. Open-cell foam is not a substitute here.
• Insulation boards or batts on a frame — mineral wool, PIR or wood-fibre between battens, lined with plywood or OSB. Cheaper materials, easy to fix things to, removable. Against it: it costs width and height on every face, and it only works with a continuous vapour barrier on the inside, taped at every joint and penetration. A gap lets moist air in behind the lining, where it condenses on the steel out of sight and rusts the box from the inside.
• Anti-condensation coating — a thick coating with absorbent or insulating fillers applied to the inside of the steel. It buffers light condensation and stops occasional drips, takes almost no space, and suits a store that only needs to be less wet. It is not insulation in any meaningful sense and will not make a container comfortable to work in.

Insulation eats headroom as well as width. A standard container has 2 393 mm of internal height; a 20ft High Cube has 2 698 mm. If you plan to line the floor, walls and ceiling, starting from a High Cube leaves usable height after the build-up. Insulating a container for people to work or stay in also raises building-regulation questions — see planning permission.

What is at risk, and what copes

• At risk — paper, files and cardboard; textiles, upholstered furniture and mattresses; leather; wooden furniture and instruments; electronics; bare steel tools and machined parts; seed, feed and grain; artwork and photographs.
• Usually fine in a ventilated box — plastic garden furniture, tyres, painted or galvanised equipment, building materials that live outdoors anyway, goods in sealed plastic boxes.
• Never put away wet — a lawnmower straight after use, wet tarpaulins and tents, freshly cut timber, damp firewood. Each is a moisture source that feeds condensation onto everything else in the box.

Matching the measures to the contents

• Garden tools, tyres, building materials — passive vents, pallets and dry loading. Nothing more.
• Household goods, furniture, stock in cartons — extra vents, pallets or shelving, desiccants, a fleece or coating under the roof, and a shade roof on exposed sites.
• Archives, electronics, machinery, anything valuable and moisture-sensitive — insulation with a proper vapour control layer, plus a dehumidifier.
• Workshop or office — insulation, heating, mechanical ventilation and an electrician-installed supply. See container as store, garage or workshop.

The condition grade changes none of this. A wind and watertight container keeps rain out, and so does a one-trip container; neither is condensation-free, because the water is not coming in from outside.