Shipping container vents are cheap, and most storage boxes need more of them than they arrive with. Plan on at least two on a 20ft, one high at each end, and four on a 40ft. Without them, a sealed steel box in a humid climate sweats on the inside and drips on your things. Two welded louver vents fitted before delivery solve the problem for most storage. A whirlybird on the roof helps in hot places, and a powered fan is for people, dust and fumes rather than boxes of furniture.
Below is why steel sweats, and how many vents of each kind to fit and where.
Why a sealed steel box needs vents
Air holds water vapor, and the warmer it is the more it holds. During the day the container heats up and the air inside pulls moisture out of the floor, the cardboard and any damp gear. After sunset the steel cools faster than the air, drops below the dew point, and water condenses on the roof and upper walls. By morning it can be dripping off the ceiling.
Vents let the moist air out before the steel gets cold enough to condense it. Warm air rises, so a vent high in the wall lets it escape, and that draws outside air in through a low vent at the other end. It needs no fan and no power, only a path for the air. Keep the box on blocks or gravel so air moves under the floor too. The stopping condensation guide covers the rest, including desiccants, dehumidifiers and what not to store in steel.
A properly fitted louver shouldn't let rain in. It has angled slats and a rain hood, and it goes in the upper wall, never in the roof seam.
How many vents and where
As a rule of thumb, you want air crossing the box at height, plus a low intake if you can fit one.
| Container | Minimum | Better | Where |
|---|---|---|---|
| 10ft or 20ft | 2 louver vents | 4 louver vents | One high at each end, diagonally opposite, with the extra pair low on the same ends |
| 40ft or 40ft high cube | 4 louver vents | 6 louver vents, or 4 plus a whirlybird | Two high at each end on opposite sides; extra pair low or at mid-length |
| Any size with people, dust or fumes inside | Powered exhaust high at one end | Exhaust plus a low louver intake at the other end | Fan on the end away from the door you use |
High vents go in the top third of the wall, in a flat section of corrugation, at least a foot below the roof so the cut stays clear of the top rail. Put intake and exhaust at opposite ends and on opposite sides so air crosses the whole box. Don't mount vents on the door leaves, because the doors flex and the seals suffer. A double-door container gives real through-flow with both ends open on dry days, but you still want fixed vents for the days you're not there.
If you want a number, work from air changes. One air change an hour in a 20ft (about 1,170 cu ft) is roughly 20 cubic feet per minute, so two to four changes an hour comes to about 40 to 80 CFM. A 40ft (about 2,390 cu ft) needs about 80 to 160 CFM for the same. Passive louvers can manage that with a breeze or a good temperature difference, but they move much less on still, muggy days. A workshop with people, dust or fumes needs more air than that, which means a fan.
Passive louver vents
The standard aftermarket louver vent is a small steel plate, often around 12 in wide and 6 in tall, with angled slats, a rain hood and a bug screen. The tiny factory vents on most boxes are there to equalize air pressure in transit and move very little air on land.
There are two ways to get proper ones. Welded vents are fitted before delivery and painted to match. A bolt-on container vent kit costs less: a louvered plate with a gasket and bolts, plus your time with a jigsaw, and you'll need to prime and paint every cut edge or it will rust.
For storage containers nobody works in, the louvers in the table above and a gravel pad are usually all the ventilation you need.
Whirlybird and roof vents
A turbine vent pulls air out whenever the wind turns it, and rising warm air helps keep it turning on still days. In a hot climate a 12 in whirlybird for a shipping container usually moves more air than a louver, and it's installed on a flashed curb.
It needs a low intake, because a whirlybird on an otherwise sealed box has nothing to draw air from, so pair it with two low louvers. The cut is in the roof, and the roof is one of the places container leaks most often start, which is why we'd rather a fabricator weld in the curb than have it siliconed on site. Skip it in cold, wet regions, where it pulls heated air out all winter and can ice up.
Powered exhaust fans
Once people spend time inside, or there's dust, paint, welding fumes or fuel, passive vents aren't enough. The workshop setup is a wall-mounted exhaust fan high at one end with a louver intake low at the other.
- A solar vent fan with its own panel needs no wiring and runs when the sun is out, which is when the box is hottest.
- A wired 10 or 12 in shutter exhaust fan moves several hundred CFM and needs an electrician. It's the one for a workshop or studio.
- An inline duct fan with a filtered intake suits a grow room, where you control air changes, temperature and smell together. See airflow for a grow room.
Any fan needs make-up air. If you seal the box with insulation and run an exhaust fan with no intake, the fan works against a vacuum and the door whistles.
Vent kits vs vents fitted before delivery
If the container hasn't shipped yet, have the vents welded in before delivery, so the edges are sealed and painted and the cost is in your quote. List vents in the quote form and we'll check what's possible near you. The modifications guide covers what else is worth doing before the truck leaves.
If the container is already in your yard, a kit is the practical route. Cut in the flat of a corrugation, deburr and prime the edge, bolt the vent through with the gasket on the outside, and seal the bolt heads.
Vents for storage vs for people inside
For storage, louvers are enough: two to four on a 20ft and four to six on a 40ft, with blocks or gravel under the floor and desiccant tubs for the first humid month. Don't insulate a storage box instead of venting it, because you'll spend far more and still trap damp air.
With people inside, you need vents and insulation together. Insulation keeps the steel above the dew point, and the fan removes the moisture you breathe out and the dust you make. A heater, stove or anything else that burns fuel inside turns the vent plan into a code question, with a sized fresh-air inlet and a carbon monoxide alarm.
Frequently asked questions
Do shipping containers need ventilation?
Yes, unless the climate is desert-dry. A sealed container sweats inside whenever the steel cools faster than the air in it, and that water lands on your things. Two louver vents high at opposite ends of a 20ft, or four on a 40ft, stop most of it.
Where should vents go on a shipping container?
Put them high in the walls at opposite ends and on opposite sides so air crosses the full length, in a flat section of corrugation at least a foot below the roof. Add a low pair on the same ends for a proper stack effect, and keep them off the doors.
Do vents make a container less secure?
They make very little difference. A louver opening is only about 12 by 6 in, it's backed by slats and a screen, and it sits around 7 ft up. If it worries you, ask for a vent with a welded security grille behind it.
Is a whirlybird better than louver vents?
In a hot climate it usually moves more air than a louver, but it needs low intakes and a well-flashed roof cut. In cold or wet regions, louvers alone are the better choice.
How much do shipping container vents cost?
Bolt-on kits cost the least, plus your labor. Welded louvers fitted before delivery come next, then a whirlybird, a solar fan and a wired exhaust fan, which also needs electrical work. Anything we fit is priced in your quote.
Ask for vents when you request a price and we'll check what can be fitted before delivery, with the cost itemized in your quote. Start my quote.
