A moisture barrier is a film, sheet or coating placed between a load and a source of moisture, such as a wood pallet, a wet floor or condensation in a container. Common forms are poly top sheets, pallet covers, slip sheets and container liners. Barriers slow moisture transfer; they do not dry wet wood.
Where moisture comes from
Wood pallets hold water. Pallets built from green lumber, or stored outdoors, can carry high moisture content that migrates into cartons and encourages mold. Ocean containers add condensation as temperatures swing, and refrigerated loads can sweat when they leave cold storage.
Barrier options
A plastic or coated paper sheet between the pallet deck and the first layer of product stops direct contact. Top sheets and full pallet covers shed water from above. Stretch wrap adds some protection but is not sealed. For export, desiccants and container liners are often combined with dry, heat-treated pallets.
Buyer implications
The most effective step is often a drier pallet: kiln-dried lumber or pallets stored under cover. Heat treatment for ISPM-15 reduces some moisture but is not the same as kiln drying to a target moisture content, and treated pallets can take on water again outdoors. Where mold is a recognized risk, specify a maximum moisture content and covered storage in the purchase order.
Related Terms
- Moisture Content (MC)
- Kiln Drying
- KD-HT (Kiln-Dried Heat-Treated)
- Green Lumber
- Heat Treatment
- Stretch Wrap
- Plastic Pallet
- Export Pallet
- Browse all glossary terms
Frequently Asked Questions
Does heat treatment stop pallets from getting moldy?
Not reliably. ISPM-15 heat treatment targets pests by heating the wood core, and it may remove some moisture, but it does not guarantee a low final moisture content. Treated pallets stored outdoors can absorb water again. To reduce mold risk, specify kiln-dried lumber or a maximum moisture content, store pallets under cover, and consider a barrier sheet between pallet and product.
Is a slip sheet a moisture barrier?
It depends on the material. Plastic slip sheets resist moisture, while plain fiberboard slip sheets can absorb it. When the goal is to keep pallet moisture away from product, a plastic film or coated sheet between deck and load works better. Slip sheets used in place of pallets are mainly a handling and cube-saving method and need push-pull attachments on lift trucks.