Spec guide
Water is the only thing that genuinely kills a box.
- Strength loss
- Large in humid air
- Onset
- Long before it feels damp
- Best fix
- Dry storage, then a liner
Ask about a liner
Product, fill weight, box size and whether it travels cold. Send them in a quote request and a partner supplier follows up.
The answer, first
Corrugated is hygroscopic: it exchanges moisture with the air around it until it reaches equilibrium. In dry air, board performs to specification. In very humid air, its moisture content climbs and it loses a large share of its compression strength. Nothing looks different. Nothing feels wet. The stack simply starts creeping.
Liners solve product-side moisture and contamination. Only storage — a roof, a dry floor, air movement and stock rotation — solves ambient moisture. Doing the second without the first is common; doing the first without the second is where money disappears.
| Relative humidity | Board moisture | Retained compression | Practical consequence |
|---|---|---|---|
| Low to moderate | Low | Close to rated | Rated performance. Ideal storage band |
| Moderate | Moderate | Slightly reduced | Small in practice; typical of conditioned warehouses |
| High | Higher | Clearly reduced | Derate stacking. Typical summer, unconditioned |
| Very high | High | Much reduced | Avoid stacking loaded more than two high |
| Condensing or wet | Saturated | Severely reduced | A common reason for rejecting a box |
| Direct wetting | Saturated | Very low | Unrecoverable even after drying |
Choosing a liner
Gauge follows abuse, not product value. Fine, abrasive or heavy material tears thin film during filling and discharge; dry, gentle product does not.
| Gauge / type | Typical application | Handles | Watch for |
|---|---|---|---|
| Thin poly | Dust control, dry non-abrasive product | Light filling | Tears on discharge with granulate |
| Medium poly | General purpose, resin, regrind | Most operations | The default; specify gussets |
| Heavy poly | Abrasive, heavy or auger-filled product | Rough filling and tipping | Cost; often unnecessary |
| Foil / metallised | Hygroscopic powders, ingredients | Moisture and light barrier | Sealing method matters more than gauge |
| Vented / breathable | Warm product that must equilibrate | Condensation control | Not a moisture barrier — the opposite |
Condensation in transit
The most frequent moisture failure is not rain — it is warm product loaded into a box that then sits on a cold trailer floor overnight. The dew point moves inside the container, water condenses on the liner or, worse, on the board, and the box arrives structurally compromised with product that looks fine.
Fixes, in order of effectiveness: let the product equilibrate before filling; use a sealed liner; put a slip sheet or pallet between the box and a cold floor; and avoid sealing a warm load completely airtight when it is about to be chilled.
- 01
Store under a roof
Strongly recommended. Outdoor storage under wrap or tarpaulin is worse than open air, because it traps humidity against the board.
- 02
Get it off the concrete
Slabs wick. A pallet or slip sheet under every band is cheap and prevents the classic damp bottom course.
- 03
Rotate stock
Moisture uptake and creep are cumulative. First in, first out protects strength as much as it protects inventory.
- 04
Meter, do not guess
A pin-type moisture meter is a small cost next to a pallet of boxes, and it is worth using for anyone storing corrugated at scale.
Questions
Moisture, answered.
How much strength does corrugated lose when damp?
Does a poly liner protect the box or the product?
What causes condensation inside a bulk box?
Diagnosis
Nine moisture problems and what each one actually is.
Damp board looks the same whatever caused it. The cause decides the fix, and the fix is almost never a thicker box.
| What you see | Cause | Confirm by | Fix |
|---|---|---|---|
| Damp bottom course only | Wicking from a concrete slab | Meter the bottom band vs the top | Pallet or slip sheet under everything |
| Damp stripe down one side | Condensation on an exterior wall | Check which face was against the wall | Leave clearance from the wall |
| Whole stack uniformly soft | High ambient humidity | Meter anywhere; readings match | Dehumidification or faster stock rotation |
| Water inside a sealed box | Condensation in transit | Warm product, cold trailer floor | Equilibrate before filling; sealed liner; slip sheet |
| Damp under a tarpaulin | Trapped humidity | Compare with open-air stock | Store indoors; open air beats a tarpaulin |
| Staining with no dampness now | Previously wet, since dried | Meter reads normal but board is soft | Downgrade — strength does not come back |
| Liner sweating inside | Product loaded warm | Check fill temperature | Vented liner, or cool before sealing |
| Mould smell, board looks fine | Extended damp storage | Smell test at close range | Reject it |
| Soft only where banded | Strap trapping moisture | Look under the band | Corner board under strapping; looser banding |
Storage
A storage specification you can hand to a shift supervisor.
Ten lines. Everything on it is cheap, and collectively it protects the value of every box you eventually sell or reuse.
- Store flattened and banded, never loose
- Keep band stacks to a sensible height
- Every stack on a pallet or slip sheet — never on the slab
- Clear of exterior walls
- Under a roof; open air beats a tarpaulin, and both lose to indoors
- First in, first out — creep and moisture uptake are cumulative
- Keep relative humidity as low as you can influence it
- Meter the bottom course of any incoming load before accepting it
- Never stack anything wet, even 'it will dry out'
- Sell accumulations promptly if you have no use for them
Specify the liner with the box, not after it.
Send the product and the journey in a quote request, and a partner supplier follows up. Availability depends on partner suppliers.