I have just opened my 20′ container to find ice on the ceiling (not water drops). Where has that come from and what can I do to stop it?
The ice is from condensation that has frozen and was in the container and may have dripped onto your contents.
You may not have noticed it if you do not visit the container frequently, but during this freezing cold spell, when a bright sun on the container roof warms the air in the container and causes condensation which quickly freezes at night. If the sun does not appear for a few days the ice will remain.
What to do
If a Dampstick had been placed into the container this could have been avoided. The condensation would have been caught and trapped before ice formed on the ceiling of your shipping container.
Do not chip the ice off unless you can capture it easily. Place two Dampsticks in a 20′ container (4 in a 40′) and leave it to thaw out. It can defrost even on the coldest day when the sun is out. When the ice has melted, dry the ceiling using a soft wide broom and an old towel draped over it to wipe the ceiling to remove the moisture and prevent it re-circulating. You may have to do this a few times. Evening is the best time when the condensation reforms due to the temperature drop. After a few days of wiping down it should slow or stop appearing and then you can rely on your Dampsticks to do the work.
The reason I emphasised the fitting of Dampsticks earlier is they, as do all de-humidifiers, work best in milder temperatures above 5 degrees centigrade (the same as the electric powered type). If fitted earlier, before you noticed the condensation or ice, they would have removed the moisture and ice before it developed.
If fitting Dampsticks in the cold weather they will take longer to work, as cold temperatures stop them working as fast. If you do not remove the moisture in the ceiling manually, it will take the Dampsticks quite a while before you notice them working and during that time you could have a lot of damp goods! The old adage ‘prevention is always better than a cure’ certainly applies in this case!