Key takeaways
- Corrugated at 90% relative humidity retains roughly half the compression strength it has at 50%.
- Nothing about damp board looks or feels different until well past the point where strength has gone.
- Board that has been wet and dried does not recover its original figure — the fibre bonds do not re-form.
- The most common moisture failure is not weather but condensation from warm product on a cold trailer floor.
In August 2022 a customer in southern Missouri had a stack of loaded boxes fail in a warehouse where nothing had changed. Same boxes, same product, same stacking plan as the previous February. The only variable was that it was August in southern Missouri.
Corrugated is hygroscopic and nobody mentions it
Board exchanges moisture with the surrounding air until it reaches equilibrium. At 50% relative humidity it settles around 7% moisture content and performs to specification. At 90% it climbs past 12% and loses roughly half its compression strength.
This happens without any visible change. The board does not feel damp at 11% moisture content. It feels like cardboard. The strength has simply left, and the only way to know is to measure it — which is why a pin-type moisture meter is standard equipment on every one of our grading benches.
| Relative humidity | Board moisture | Retained strength | What it means in practice |
|---|---|---|---|
| 35 – 50% | ≈ 6 – 7% | 100% | Rated performance. Conditioned warehouses |
| 50 – 65% | ≈ 7 – 9% | ≈ 90% | Negligible. Most of the year in most buildings |
| 65 – 80% | ≈ 9 – 11% | ≈ 75% | Derate stacking. Typical unconditioned summer |
| 80 – 90% | ≈ 11 – 13% | ≈ 55% | Two high loaded, maximum |
| Above 90% | Over 13% | ≈ 45% | Automatic reject in our grading |
| Direct wetting | Saturated | Under 25% | Unrecoverable, even after drying |
It does not fully come back
This is the part that surprises people. Board that has been wet and then dried does not return to its original figure. The fibre bonds that were broken by water do not re-form, and permanent set from any load carried while damp stays permanent.
We grade dried-out boxes on their current condition, not on what they used to be, and a box that spent a weekend in the rain will not grade above 4A no matter how convincingly dry it feels on Monday.
People send us boxes stored outside under a tarpaulin and cannot understand the grade. The tarpaulin was the problem. It kept the rain out and the humidity in.
Condensation is the sneakier version
The most frequent moisture failure we see is not weather. 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 board, and the box arrives at its destination compromised while the product inside looks perfect.
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 airtight immediately before it is chilled.
What to actually do
- Store under a roof. Outdoor storage under wrap is worse than open air, because it traps humidity against the board.
- Get everything off the concrete. Slabs wick, and the bottom course is where strength matters most.
- Rotate stock. Moisture uptake and creep are cumulative and largely irreversible.
- Derate stacking in summer. The multipliers are published; apply the honest one rather than the comfortable one.
- Buy a moisture meter. It costs less than a pallet of boxes and it will settle arguments for years.
Questions