Why Cabins Rain Indoors?

Anyone who has slept in a converted container knows the morning: the duvet against the outer wall is damp, there are fine beads of water in the corner, rust spots on the steel. The trade calls it container rain. The cause is not mysterious. At 20 °C and 60 percent relative humidity the dew point is about 12 °C; the moment the inner steel surface drops below that, moisture in the air condenses onto it. Steel has very little thermal mass, so two or three hours after sunset the surface is already under the dew point, and it stays there all night.
Water itself draws no power; dealing with it does. Put a 300 W dehumidifier in the cabin, run it continuously, and it uses 7.2 kWh a day. A small cabin's total daily consumption is usually 3–6 kWh, so dehumidification can take more than half — more than lighting, television and phone charging combined. Worse, the load is invisible: it never appears on the appliance list, it gets omitted from the load calculation, and the shortfall only shows up on site.
The order cannot be reversed. Keep vapour out of the insulation first: the vapour barrier goes on the warm side, every seam, back box and penetration is sealed, and no convective cavity is left inside the wall. Then add active drying, and size it for the wet season rather than the annual average. Get the barrier right and the dehumidifier drops from continuous running to occasional starts — a five- to eight-fold difference in energy for the same machine.