Also called vapor retarder, poly barrier
A vapor barrier is a layer, most often polyethylene sheeting or a foil-faced facing, installed in a wall assembly to stop moist interior air from migrating through and reaching a cold surface where it would condense. In a stick-built wall it is a separate step. In a steel container it usually is not, because the insulation method itself can do that job.
A vapor barrier is not something our shop quotes as its own modification, and if you are pricing a build-out you will not find it on the sheet next to insulation and electrical. That is not an oversight. It is because a shipping container is not framed lumber, and the two ways we insulate a container each handle vapor a different way built into the method itself.
So the flat answer is no, but the useful answer is what replaces it, and that changes depending on which of the two systems goes into your box.
Spray foam goes on wet and cures directly to the corrugated steel, following every ridge. Done right, there is no gap between the foam and the metal anywhere, which means there is no path for interior air to reach a cold surface and no separate membrane needed to stop it. The foam is doing the vapor barrier's job as a side effect of how it bonds, not as an added step.
Rigid panel is different. It fits into a framed interior rather than bonding to the steel, so the seal depends on the framing being tight against the panel and the panel joints being sealed, not on a membrane behind it. A gap at a seam or an unsealed penetration in a rigid-panel build is the same failure a missing vapor barrier would cause in a house wall: interior air finds the cold steel, and it condenses there, hidden behind the finished side.
Neither method adds a poly sheet as a separate product on this site. The performance comes from how the insulation itself is installed, which is why our shop treats the choice of system, not the addition of a barrier, as the decision that matters.
A house wall has cavities, sheathing and siding between the inside air and the weather. A container wall is one sheet of fourteen-gauge steel with nothing on the other side of it but outside air. That steel tracks the outside temperature closely, so on a clear night it can sit well below the dew point of the humid air trapped inside, and any interior moisture that reaches it condenses fast, the same mechanism behind container rain.
That is also why we do not treat insulation as a stand-alone product. We spec insulation and ventilation together on every build that gets one, because sealing moist air away from the steel and giving whatever moisture remains somewhere to go are two halves of the same job. A vapor barrier, where one would apply, is one more piece of that same picture rather than an item you buy separately.
Insulation, ventilation and the rest of a build-out are quoted together, in writing, before any cutting starts.
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