Open a shipping container on a cool morning and you may find water dripping from the ceiling, even though it hasn't rained in days. The same thing happens in metal sheds and in attics, where it shows up as wet roof sheathing and damp insulation. Shippers call it container rain. It isn't a leak. It's condensation, and once you understand where it comes from, you can do a lot to stop it.
What causes container rain: the dew point
All air holds some water vapor. The dew point is the temperature at which that air can't hold its moisture anymore, so the water turns back into liquid. The cargo insurers' Transport Information Service uses a cold drink bottle as the everyday example: take it out of the fridge, and water beads up on it because the air touching it is cooled below its dew point.
Daniel Rheaume of Solar Blaster calls container rain "the beer can effect." Think of a cold can of beer sweating on a hot Arizona afternoon. A metal roof does the same thing. When the inside surface of the roof drops below the dew point of the air inside, water forms on it. The drops grow, then fall. Water forming on the ceiling and walls is often called container sweat, and when it drips onto the goods below, it's container rain. When the moisture forms on the goods themselves, it's called cargo sweat.
Why the roof gets so cold at night
Temperature swings drive the whole problem. In the daytime, sun heats the roof and the air inside, and that warm air picks up moisture. At night, especially under a clear sky, the roof radiates its heat straight up to the cold sky. That's called night-sky radiant cooling. The USDA Forest Products Laboratory notes that at night a roof "may be as much as 10°F to 15°F (5°C to 8°C) colder than the ambient air as a result of radiation to the sky."
Thin metal roofs on containers and sheds have almost no thermal mass or insulation, so they cool fast. That's why metal shed condensation and steel container condensation are such common complaints.
Where the moisture comes from
Condensation needs water vapor, and there are plenty of sources:
- Humid outdoor air. Warm, damp air that gets in during the day carries moisture with it.
- Wet cargo and packaging. The International Institute of Marine Surveyors warns to use only dry pallets and dunnage, because pallets stored outside, even under tarps, "can quickly absorb significant moisture."
- The ground. A shed or container sitting on bare soil or a slab with no vapor barrier picks up moisture from below.
- People and activity. Breathing, working, washing, or drying gear inside a shed or container home adds water to the air. In houses, moist indoor air leaking into the attic does the same.
The damage it causes
Container rain isn't just a nuisance. It leads to:
- Rust on the roof, tools, equipment and metal goods.
- Mold and mildew on wood, cardboard, fabric and stored furniture.
- Ruined cargo or stored goods. Cardboard goes soft, labels peel and products spoil. The Bubblepack guide explains that insurers often treat condensation as an expected event, so standard cargo policies may not pay for it.
- Wet attic insulation and roof sheathing. RDH Building Science found condensation on the underside of roof plywood when night-sky radiation dropped it below the dew point, and mold grew on the sheathing of every test roof within the first year.
How to stop shipping container condensation and shed condensation
There's no single fix. The best results come from combining these steps.
Ventilate and exchange the air. Moving damp air out and bringing drier air in keeps the inside air closer to outdoor conditions, so there's less trapped moisture to condense on a cold roof. This works best with intake low and exhaust high, so air enters near the floor or soffits and leaves at the top, where warm, moist air collects. Our solar vent fans are built to assist passive vents that way, not overpower them. You can read more about that approach in Why Solar Blaster Bets on Balance, Not Brute Force.
Match the fan to the space. In a 1,000-cubic-foot space with proper intake, Solar Blaster lists about 10.4 minutes per air exchange for the Mega RoofBlaster and about 28 minutes for the standard RoofBlaster. Bigger isn't automatically better. The DOE's Building America Solution Center warns that high-volume attic fans create negative pressure and can actually make attic moisture problems worse.
Keep moisture sources out. Load cargo dry, use dry pallets, don't store wet gear, put a vapor barrier under sheds and containers on soil, and in homes, seal ceiling air leaks so moist indoor air stays out of the attic.
Use desiccants. Moisture-absorbing bags or dehumidifiers help in closed containers, especially during shipping or long-term storage. Both the IIMS and Bubblepack recommend them.
Add insulation and vapor control. Insulating the roof keeps its inner surface warmer, ideally above the dew point. Anti-condensation coatings and liners can help too. Pair insulation with proper vapor control so moisture doesn't get trapped behind it.
Being honest about the limits
Ventilation helps, but it's not a cure-all. When outdoor air is already humid, bringing more of it in on a clear night can still leave a cold roof below the dew point. The Forest Products Laboratory notes that depending on climate, ventilation air "may occasionally carry moisture into the attic rather than remove it." RDH's field research in the Pacific Northwest found moisture problems even in attics with code-required ventilation, and its authors say that wetting "cannot be prevented by ventilation alone."
In damp or coastal climates, plan on ventilation plus moisture control, desiccants, and insulation. In dry climates like ours in Arizona, ventilation and air exchange usually do more of the heavy lifting, but big day-to-night temperature swings can still cause container rain.
The bottom line
Container rain is physics: moist air, a cold roof and a drop below the dew point. Cut the moisture, keep the air moving gently from low to high, and keep the roof surface warmer, and most of that indoor "rain" goes away.
As Daniel Rheaume of Solar Blaster puts it, "Our products move air, which is a natural cleanser. Clothesline fresh."
Questions about your container, shed or attic? Call us at (480) 747-7097 or email sales@solarblasterfans.com.