An Aluminium Layer Is the One Thing Plastic Cannot Do.
Ordinary polyethylene sheeting slows water vapour but never stops it; over weeks, moisture diffuses straight through. An aluminium-foil laminate barrier — a sandwich of polyester, aluminium and polyethylene — is effectively impermeable to water vapour and oxygen. Heat-sealed around the cargo, it forms an envelope the outside humidity cannot enter. Inside that sealed envelope you place desiccant sized to the trapped air and any hygroscopic packing, and the contents stay at a safe humidity for months, even years.
“Vacuum” packing adds a step: drawing the air down before the final seal, which shrinks the barrier onto the cargo, removes most of the moisture-carrying air, and reduces the desiccant needed. The two approaches — sealed barrier with desiccant, and vacuum barrier with desiccant — are chosen by how long, how hostile and how sensitive the job is. For most export machinery a heat-sealed barrier bag with calculated desiccant is the workhorse; vacuum is reserved for the longest storage and the most moisture-critical contents.