Covington GA & Sacramento CA
UNITED STATES
The short answer: uncoated fiber loses roughly 45% of its compression strength at 85% relative humidity — a condition that is routine in Gulf Coast and Southeast warehouses in summer and normal inside an ocean container. Coatings such as Nomar and poly, water-resistant adhesives, and simple logistics controls close most of that gap. The mistake is treating moisture as a material defect rather than as a specifiable condition, and then blaming the packaging when a pack designed for 50% RH fails at 90%.
Honeycomb paperboard is hygroscopic: paper fibers absorb water from the air. As moisture content rises, hydrogen bonding between fibers weakens and the sheet loses stiffness. In a honeycomb panel this matters twice over, because the structure depends on thin cell walls resisting buckling — and a softened wall buckles at a much lower load.
The relationship is not linear. Below about 55% relative humidity the effect is modest. Between 65% and 75% it becomes noticeable. Above 85% it is severe.
| Relative humidity | Approximate strength retained | Where you encounter it |
|---|---|---|
| 50% RH | 100% (reference) | Conditioned facility, TAPPI standard |
| 65% RH | ~80% | Typical unconditioned warehouse, spring |
| 75% RH | ~68% | Southeast warehouse, summer |
| 85% RH | ~55% | Gulf Coast summer, unventilated trailer |
| 90%+ RH | ~45% or less | Ocean container, condensation cycles |
The practical implication: a panel specified against a datasheet figure and used in a Houston warehouse in August is operating at roughly half its assumed capacity, and nobody in the chain necessarily knows it.
Ambient humidity weakens fiber gradually. Condensation wets it directly, and that is a different failure mode.
Condensation occurs whenever a surface falls below the dew point of surrounding air. Three situations produce it reliably: a cold container entering a warm humid port, a chilled product brought into an ambient warehouse, and the classic “container rain” where moisture evaporating from cargo condenses on the container roof overnight and drips back down.
No amount of specification makes uncoated fiber tolerate standing water. The answer is a barrier where contact occurs, plus control of the condition itself through desiccants and ventilation.
A polyethylene layer applied to the liner. Excellent moisture barrier, good abrasion resistance, and the most robust option for sustained high humidity. The consideration is repulpability: modern poly-coated grades are widely accepted in the OCC stream, but confirm this with your supplier and, if relevant, with the receiving mill.
Surface treatments such as those in the Michelman Nomar family provide abrasion resistance and a degree of moisture resistance while remaining fully repulpable. The right choice where surface protection of the packed product matters as much as the board’s own strength — for example against painted or polished surfaces.
Independent of the face treatment. Standard PVA bonds soften when wet, at which point the sandwich delaminates and the panel loses its structure entirely regardless of how strong the liners are. On any humid application this is the first thing to specify, and it is frequently omitted.
Historically common, increasingly avoided because waxed fiber is rejected by most recycling streams. Where a customer or a state program requires recyclability, wax is a liability rather than a solution.
| Situation | Recommended specification |
|---|---|
| Conditioned facility, domestic dry van | Standard construction, no coating |
| Unconditioned Southeast warehouse | Water-resistant adhesive, derate 25% |
| Refrigerated or frozen chain | Coated face on cold-contact surfaces |
| Ocean container, 3–6 weeks | WR adhesive + coated faces + desiccant + shroud |
| Outdoor staging (unavoidable) | Coated, plus a covered pallet; treat as compromised if wetted |
| Product with painted or polished surface | Nomar-type abrasion treatment at contact points |
Specification changes are the visible answer. Three operational controls often deliver more.
Storage. Fiber components stored on an outside dock for three weeks arrive at the packing station already weakened. Moving pallet storage indoors is free and frequently resolves a seasonal damage pattern nobody had connected to weather.
Shrouding. A simple pallet cover on a loaded unit protects against condensation drip in a container and against dock exposure. It costs cents and is the single most effective control on ocean lanes.
Desiccants. Container desiccants are standard practice for moisture-sensitive cargo and are inexpensive relative to the value of a container load. They address the cause rather than the symptom.
The professional approach is not to hope for dry conditions but to design against the wet case.
Establish the worst realistic relative humidity your pack will experience, using facility data rather than assumption — a logger left in a warehouse for a month costs almost nothing and settles the argument. Apply the corresponding strength retention factor from the table above. Then apply the sustained-load creep factor and a safety factor as normal.
The result is often a thicker panel or a smaller cell size than the datasheet suggested, and that is the correct outcome. A pack that works in July is a pack that works.
Three tests answer the practical questions.
Conditioned compression at high humidity. Specimens equilibrated at 85% or 90% RH before crushing, rather than at standard conditioning. This gives you the real derating factor for your material rather than a generic one.
Cyclic humidity exposure. Alternating high and moderate humidity reproduces the container environment far better than a steady state, and it reveals delamination that a static test misses.
Water drop or spray on coated faces. A crude but informative check of whether a coating is doing what it claims where condensation contacts the surface.
Ask your supplier which of these they can run or source. A supplier who quotes only standard-conditioned figures for an ocean application is not engaging with the problem.
Coatings improve performance and can compromise end of life, so the choice deserves explicit attention rather than being left to the converter.
Repulpable coatings — including modern poly grades and Nomar-type treatments — are accepted in the OCC stream and preserve the favorable EPR fee position of an all-fiber pack. Wax and heavy laminates generally are not, and a pack that becomes non-recyclable to solve a humidity problem has traded one cost for another.
Where a barrier is genuinely required, specify repulpability alongside the barrier requirement and ask for documentation. Both your customer’s sustainability team and your state EPR filing will eventually want it.
Humidity exposure is not uniform across the country, and packaging specified for one region frequently underperforms in another without anyone identifying why.
Southeast and Gulf Coast. Unconditioned warehouses routinely run above 70% relative humidity from May through September. Programs specified in a conditioned plant and stored in a third-party warehouse in this region see a distinct summer damage pattern.
Pacific Northwest. Sustained moderate humidity rather than peaks. The effect is gradual softening over long dwell rather than acute failure, which makes it easy to attribute to something else.
Southwest and Mountain. Very low humidity for much of the year, which is generally favorable for strength but can make board brittle and can affect dissipative treatments that rely on moisture content.
Northeast and Midwest. Large seasonal swings. The specification challenge is that a pack qualified in February is operating in a different material state in August.
A single humidity logger placed in each significant storage location for a month costs very little and converts this from speculation into a design input.
Three signals distinguish a moisture problem from a design problem, and confusing the two leads to over-specifying the pack year-round for a seasonal condition.
First, seasonality: damage rising in summer and falling in winter on the same lane is a humidity signature, not a handling one. Second, geography: damage concentrated in shipments routed through one warehouse or one region. Third, appearance: crushed rather than fractured components, with the crush occurring progressively rather than at a single impact point.
Where all three are present, the answer is a coating and a storage change rather than a heavier panel applied to every shipment nationwide.
Roughly 20% at 65% RH and 45% at 85% RH for uncoated construction. Coated construction with water-resistant adhesive retains substantially more, but no fiber material is unaffected.
No. Coatings slow moisture uptake and resist surface wetting; they do not make the panel immune to standing water or prolonged saturation. Treat a soaked panel as compromised regardless of coating.
Repulpable coatings, yes, and they are widely accepted. Wax coatings, generally not. Specify repulpability explicitly rather than assuming it.
Yes, and it handles cold better than most plastics, which turn brittle. The issue is condensation on the return to ambient, addressed with a coated face at cold-contact surfaces.
Moisture is not a defect in fiber packaging; it is a design condition. Measure the humidity your pack will actually see, apply the corresponding derating factor, specify water-resistant adhesive before anything else, and add a coating where contact or condensation demands it. Then handle the operational side — indoor storage, shrouds, desiccants — which frequently matters more than the specification.
Premier Packaging Products supplies honeycomb with water-resistant adhesives, poly and Nomar coatings, and white liners for printing, from Georgia and California. Tell us your humidity exposure and we will specify against it.