Mould on new wooden packaging is one of the most common complaints in the industry, and one of the most misunderstood. It is rarely a manufacturing defect. It is almost always a moisture and storage problem — and the single most common cause is something the receiving warehouse does, not the maker.
The numbers that matter
Lumber for building boxes should be at 12–15% moisture content, which matches the equilibrium moisture content at normal relative humidities. EPAL caps pallet moisture at 22% at the time of manufacture.
Note the tension between those two figures. A pallet at 21% is perfectly compliant; export box timber at 21% is too wet for the job. "It meets the standard" and "it is dry enough for your shipment" are different claims.
The threshold that governs mould is 19%. Mould spores colonise wood surfaces above roughly 19% moisture content; keeping packaging at 19% or below is the single most effective preventive measure. On the air side, the equivalent is relative humidity below 75%.
Decay is a different and more serious process requiring much wetter wood — fungi establish near fibre saturation, around 28–30%, and continue above about 20%. If you have decay rather than surface mould, the wood has been wet for a long time.
Mould or blue stain? A five-second test
These get confused constantly, and the commercial consequences differ.
Mould grows on the surface. Rub it and it smears or brushes off. It is a hygiene and appearance problem, and it can be remediated.
Blue stain is a sapstain fungus that penetrates deep into pine, spruce and fir sapwood. Rub it and nothing happens — it cannot be removed, because it is inside the wood. Importantly, blue stain does not destroy wood fibre; strength is essentially unaffected. It is a cosmetic issue.
Blue stain grows best at 21–32 °C in wood above 22% moisture content, is dormant below 10 °C, and is killed above 54 °C. It is not on the ISPM 15 target pest list and is not prohibited by the standard — but it has caused sporadic shipment stoppages in China and Australia, and some importers specify blue-stain-free packaging. If you ship to those markets, raise it at the specification stage rather than discovering it at the border.
The heat treatment myth
This one costs people money. Heat treatment does not prevent mould.
ISPM 15 heat treatment is an insect and nematode measure — 56 °C at the core for 30 minutes. It will kill mould present on the surface at the time, but it does nothing to prevent recolonisation afterwards. Spores are everywhere; the moment treated wood is stored damp, mould returns.
Kiln drying does help, because it lowers moisture content — but only if the packaging is then kept dry. A kiln-dried pallet left in the rain is a wet pallet.
Worth restating: heat treatment and kiln drying are different processes with different endpoints, even when done in the same chamber. HT specifies a time and temperature; KD specifies a moisture content. If mould matters to you, specify moisture content as a separate line in your order.
How moisture is measured
Three European methods, in descending order of accuracy:
- EN 13183-1, oven-dry. Weigh, dry at 103 ± 2 °C until mass changes by no more than 0.1% in two hours, re-weigh. This is the definition of moisture content and the only precise method. Destructive and slow.
- EN 13183-2, electrical resistance, with insulated hammer electrodes driven into the face along the grain, 300 mm from the end, at a depth of 0.3× the thickness. Realistic accuracy is ±2% — better for characterising a batch than an individual board.
- EN 13183-3, capacitance. Non-destructive but surface-biased and disturbed by nearby metal. The guidance is explicit that it should be used for checks, not for determining moisture content.
One subtlety that explains a lot of disagreements: moisture gradients. Immediately after drying to a 16% average, the surface can be below 10% while the centre is still above 20%. Two people measuring the same board with different instruments at different depths will get different answers, and both can be right.
Why this is a structural issue too
Moisture is not only about appearance:
- Compared with lumber at 10% moisture, lumber at 20% has roughly half the compressive strength and about two-thirds the flexural strength.
- Nail and screw holding power falls as moisture rises. Worse, timber that is too wet when assembled loses holding power as it dries, because shrinkage enlarges the nail holes.
- Shrinkage happens across width and thickness; length is virtually unchanged. Too-wet timber opens gaps between boards that initially fitted tightly. Too-dry timber later swells and levers joints apart.
So a crate built from wet timber is not merely at risk of staining — it is measurably weaker and its joints will loosen in transit.
Preventing mould in practice
Most of this happens after the packaging leaves us:
- Store covered, dry, ventilated and well lit. Do not store packaging in a trailer — dark, warm and humid is an incubator.
- Elevate stacks off the ground so water cannot wick up into the bottom boards.
- Space rows for airflow. Even a light air movement helps significantly.
- Remove stretch wrap immediately on receipt. This is the most common self-inflicted wound we see. Wrapping timber that is still above 19% traps moisture against the wood and guarantees mould.
- Rotate stock, and remove mouldy material promptly — it seeds fresh lumber next to it.
If mould has already appeared, remediation is labour-intensive but possible: a 10:1 water-to-bleach solution or 3% hydrogen peroxide, scrubbed or pressure-washed, followed by proper drying. Drying is the part people skip, and it is the part that determines whether it comes back.
If your product or market cannot tolerate staining, tell us before we quote. Specifying a moisture content, and agreeing how packaging is stored at your end, costs far less than a rejected consignment.
