Why wrapped pallets sweat — and how to stop it

Nothing on the pallet is producing water. It was already there — as vapour, in the air and in the product — and the wrap simply gave it a cold surface to collect on. Air holds moisture in proportion to its temperature. Cool it below its dew point and that moisture stops being vapour and becomes liquid on the nearest cold surface.

Wrap a warm load and you seal warm, moist air inside a plastic skin. Move that pallet into a chilled store, a night trailer or a cold loading bay and the film cools first. Its inner face becomes the coldest surface in the package, so water forms there, runs down and drips back onto the cartons. A cold pallet moved into a warm, humid yard does the same in reverse.

Usually three sources at once. The product’s own heat: hot-filled cans and bottles, bread still giving off steam, cooked product going out before it has cooled through, blast-chilled stock that is cold outside and still warm at the core of the pallet. Then residual water: washed produce, ice glaze on frozen goods, board that took up moisture in the packing hall.

And finally the ambient air itself: whatever the humidity is in your plant when you wrap is sealed in and travels with the load. Fresh produce adds a fourth source. It is still alive after harvest — it respires, giving off heat and water vapour for as long as it sits in the box, and a solid wrap turns that into a sealed, humid chamber.

A solid film is not a passive bystander. It blocks vapour, so the water that forms has no way out, and it blocks air movement, so the load holds its heat far longer and spends longer in the band where condensation keeps forming. That is why a load can arrive wet after a full night in a chilled trailer: the cold air never reached the middle of the pallet, and the extra plant hours needed to get it there cost real money.

Water on bare or coated metal starts rust within hours. Can crowns, closures, lids and tinplate speckle first, and that alone is enough for a customer to refuse the pallet even where the product inside is untouched. Wet labels lift: the adhesive softens, corners curl, print runs, and the pack can no longer be sold at full price.

Corrugated board is worse. It takes up moisture and loses stiffness, so the columns that were carrying the stack stop carrying it — the load sags, leans, and the top layers move. Then mould on bread, cheese and produce, ice bridges fusing frozen blocks into a lump that has to be broken apart, and slower freezing across the batch because cold air cannot get in.

In principle, yes. In practice it means floor space you do not have, a queue of pallets between the line and the door, and a stage that has to be staffed and controlled. Most plants wrap at the end of the line because that is where the pallet is finished — and the pallet is warm because the product is.

Even a fully cooled load does not settle it. Wrapping seals in that moment’s air, and the pallet still has its journey ahead: a cold trailer, a warm ferry deck, a chilled depot, an unheated yard. Every temperature swing on that route is another chance for water to leave the air.

The fix is to stop sealing the load. Aerosonic is a breathable pallet wrap with engineered perforations — hole size and count set against the load, not a standard pattern — so air moves through the pallet instead of stopping at the skin. Warm air and vapour leave, the load cools inside the wrap, and no warm wet pocket forms for condensation to collect in.

Breathability costs nothing in containment. In Toreto’s own wrap tests — same pallet, same machine, same power stretch and tension — Aerosonic and Supersonic at 16µ held 10 kg per square inch on 66 g of film per pallet, against 9.9 kg per square inch on 200 g for 23µ solid film. Cost per pallet varies with film price and pallet size. Chilling and freezing also run faster, because cold air can pass through the load.

Apply the inside face of the film to the goods; it adheres better. Two revolutions at the base to lock the load to the pallet, then spiral up with a 20–30 mm overlap and two revolutions at the top. Tension enough to secure the load fully, but not so much that you crush it — a crushed carton is a wet carton waiting to happen.

On heavy or unstable loads, spiral back down to the pallet: the criss-cross adds stability on slippery loads. Finish with a clean cut or tear and swipe the tail down by hand. And do not undo the work afterwards — a hood, a second wrap or any sealed layer over a breathable film puts the vapour barrier straight back.