Container dehumidifier?

[Old Mill Tree Care]

I have a 20ft shipping container. It doesn't have a Grafotherm ceiling but I do have one of those Absorbapole moisture collectors in it. That works but with the type of atmosphere we have (in Surrey) I think I need 100 of them to deal with condensation on the ceiling!

 

Does anyone use an electric dehumidifier to do the job?

[treequip]

Insulate it, gets rid of most of the problem

[Stephen Blair]

I lined the roof of mine with a tin foil stuff full of bubble wrap., a joiner friend gave me a roll of it years ago, it's just jammed with 4 bits of wood , works great.

[Old Mill Tree Care]

I lined the roof of mine with a tin foil stuff full of bubble wrap., a joiner friend gave me a roll of it years ago, it's just jammed with 4 bits of wood , works great.

 

How does that work? Bubble wrap won't absorb water.

[difflock]

I do not properly understand the "dew point" idea.

But the basic idea of insulation is to raise the inner surface temperature so that air borne water vapour will not condense.

 

Any water absorbent material will only work until saturated.

 

Thick old lime walls absorbed from the damp air inside and wicked it to the outside where the moisture was given up/evaporated on a windy drying day.

PS

I have the same problem under my bare tin roofed sheds, bloody annoying.

drip drip drip

I wish I had used the fibre cement sheeting instead (but unusual here in NI)

[Woodworks]

A dehumidifier should work well but the container needs to be well sealed or your going to be trying to dry out Surrey. After all a container with dehumidifiers is the basis for most kilns. Insulation should also work but if going down this route it needs to be good fit or the warm damp air gets behind the insulation condenses on the cold steel and then dribbles back out were it can.

[Old Mill Tree Care]

I do not properly understand the "dew point" idea.

But the basic idea of insulation is to raise the inner surface temperature so that air borne water vapour will not condense.

 

Any water absorbent material will only work until saturated.

 

Thick old lime walls absorbed from the damp air inside and wicked it to the outside where the moisture was given up/evaporated on a windy drying day.

PS

I have the same problem under my bare tin roofed sheds, bloody annoying.

drip drip drip

I wish I had used the fibre cement sheeting instead (but unusual here in NI)

 

So will insulating the roof do the job or do I need to insulate all the walls as well? I just don't know.

 

I'm aware of the saturation point issue and that's a good point Difflock. I've been told that it doesn't matter because every time the doors are opened, moisture can escape to the outside. However, that will only happen if the outside environment is much drier than the inside.

[Rob D]

As well as insulating it I'd just have a small dehumidifier in there with a hose to drain any water outside.... this is what I have in the sheds/containers where I keep all my gear.

[openspaceman]

I do not properly understand the "dew point" idea.

But the basic idea of insulation is to raise the inner surface temperature so that air borne water vapour will not condense.

 

The issue is that air reaches an equilibrium with water vapour from any surrounding wet surfaces. Warm air will absorb and hold more water vapour than cold air. So as the surroundings cool air next to them cools too, the cooler air can no longer contain the water vapour so water settles out as dew.

 

A metal as in a car or container) roof is a special case, it tends to be thermally isolated from the ground and is a good absorber of solar radiation, so it gets hotter than the surrounding objects but it is also good at radiating heat. It also has a low thermal mass compared with the surrounding land. As it is thermally isolated and radiates heat to the sky it becomes cooler than the surroundings, any air passing by is cooled more than elsewhere so it reaches the point where it is more than saturated with moisture and this settles out as condensation on the surface. Incidentally in receiving the dew, as condensation, the surface also receives the latent heat from the conversion from vapour to liquid and thus becomes less cold.

 

Cars are basically sealed so there is little air exchange with the outside, my landrover has condensation on the roof in the morning but by midday the air inside has warmed up to re absorb the moisture. If it had a roof lining, as a car does, then there is little air flow through the lining and the lining is in near direct contact with the metal, so although the metal gets cold little air touches it and it doesn't get very damp, below the lining the air is insulated from the roof by the lining which stays about as warm as the rest of the interior, except the windows, so the only condensation tends to be on the colder glass.

 

The open or poorly sealed barn is a difficult one to deal with as the air inside is constantly replenished with air from outside that is warmer than the metal roof, and consequently it is always receiving more water as it cools the ambient air. Again an insulator in direct contact with the metal will combat this but is expensive. I have used Tyvek vapour permeable membrane over the rafters and below the metal and this works in that any condensation runs down the membrane and out whilst moist air can still rise through and the woodwork does not get wet (beware embedding woodwork in foam, it will rot). Also as the air spaces under the metal are reduced there is less air movement to carry in fresh moisture laden air.

[morten]

I use a 20' container as a burglar-resistant workshop, and need to prevent condensation as it will cause tools to corrode. I have reduced the humidity considerably and prevented condensation by mounting a small air extraction fan, driven by a solar panel (one unit, used on yachts). This will pull air through the container whenever the sun shines, which incidentially is when the outside humidity is lowest.