This is Part 2 of a three-part series on summer packaging hazards in the LTL environment.
In Part 1 of this series, we examined how summer humidity degrades paper-based corrugated packaging. Wood pallets and crates present a different, yet equally serious, set of risks. While corrugated loses structural strength when exposed to moisture, wood actively absorbs and releases ambient moisture, creating its own humidity problem within the trailer environment. Green hardwood in particular can release significant moisture into a hot trailer, raising the humidity level and accelerating the corrugated strength loss described in Part 1. Beyond the impact on surrounding packaging, that absorbed moisture creates the conditions for mold growth.
Background: Moisture Content of Wood
To understand why summer humidity is so damaging to wood pallets and crates, it helps to understand the relationship between wood and water. Wood is hygroscopic, meaning it continuously exchanges moisture with the surrounding air. At roughly 30% moisture content, wood reaches its fiber saturation point, the level at which free water between cells has largely evaporated. That threshold is significant, but it is not the safety line for mold prevention. Mold can establish itself on wood well below 30%, which is why both structural lumber standards and wood packaging guidelines target 19% or below as the functional benchmark for stability and mold resistance.
For softwoods, that standard is well established. United States (U.S.) building codes and wood packaging guidelines require softwood lumber to be dried to 19% moisture content or below to ensure structural stability and inhibit mold germination. The industry designation KD19, kiln-dried to 19%, confirms that standard has been met, and the vast majority of softwood pallet and crate lumber is produced to KD19 for exactly that reason.
Hardwoods follow a different commercial path. Much hardwood pallet lumber is sold green, meaning it has not been kiln-dried and is frequently at or above the 30% fiber saturation point at the time of purchase. Hardwood is generally denser than softwood and tolerates slower drying, which is why green sales are commercially common. That commercial norm comes with a meaningful tradeoff: the higher the moisture content at the time of purchase, the greater the mold exposure risk during summer transit and storage. When loaded into a sun-baked trailer on a summer afternoon, green hardwood actively releases that stored moisture into the trailer air, turning the interior into a high-heat humidity chamber that threatens every piece of freight sharing the space.
The Moisture Content Ebb and Flow
Lumber dried to KD19 at the mill does not stay at 19% indefinitely. Wood is hygroscopic, and in sustained high-humidity environments it continuously reabsorbs moisture from the surrounding air. In the heat and humidity of a southern summer, a pallet or crate can rehydrate past the 20% threshold at which mold spores actively germinate. That rehydration happens gradually and invisibly, with no outward warning before the conditions for mold growth are fully in place.
The trailer environment then takes that elevated moisture content and amplifies it further. Extreme temperature swings between cool nights and high humidity sun-baked mornings drive cycles of condensation inside trailer walls and floors, a phenomenon sometimes called “trailer sweat”. That condensation deposits additional moisture directly onto pallets, packaging, and freight, pushing wood that may already be approaching the mold threshold past it. Wood naturally provides the organic material mold requires, and when warmth, humidity, and condensation converge inside a closed trailer on a summer run through the South, the conditions for mold establishment are close to ideal. Once mold takes hold on a pallet or crate, it spreads quickly and can transfer to the freight, the packaging, or adjacent shipments sharing the trailer.
What Shippers and Carriers Can Do
Managing summer mold risk in wood packaging comes down to three practical areas: procurement specification, preventative treatment, and storage and staging discipline.
For softwood pallets and crate lumber, KD19 should be the minimum acceptable moisture content in any procurement specification. For operations where mold has been a recurring problem, particularly in humid southern corridors, tightening that requirement to KD15 provides a meaningful (albeit expensive) additional safety buffer before transit rehydration pushes moisture content back toward the danger zone.
Hardwood presents a more graduated challenge. Because hardwood pallet lumber typically enters the supply chain green, the practical approach is to work moisture content down in stages. Targeting air-dried lumber in the 20% to 25% range eliminates most of the free water that would otherwise vaporize into the trailer environment. For operations where sourcing dried hardwood at that specification is difficult or cost prohibitive, softwood at KD19 often becomes the more practical and economical alternative.
For high-risk lanes or operations with a history of mold claims, a preventative fungicide treatment applied during pallet manufacturing adds a layer of protection that moisture content specifications alone cannot provide. PQ80 is a concentrated, water-soluble fungicide engineered specifically for mold control on both hardwoods and softwoods. PQ80 is permitted by the FDA as a preservative for wooden articles used in packing and transporting food and agricultural products, making it appropriate for regulated supply chains including food, beverage, and pharmaceutical.
Storage and staging conditions matter as much as the lumber specification itself. Wood pallets and crates held in open environments with good airflow and natural light dry more effectively and are less likely to develop mold than those stacked tightly in dark, enclosed spaces where humidity accumulates (trailer or warehouse corners). Air circulation is the simplest and least expensive moisture management tool available. For freight that must wait in a trailer, carriers should open the rear doors whenever safe and feasible to encourage cross-ventilation. A sealed trailer sitting in summer heat becomes a humidity chamber. Even partial ventilation meaningfully reduces the moisture accumulation that drives mold growth.
Conclusion
Summer mold risk in wood packaging is not a mystery. The conditions that produce it are predictable, the thresholds that matter are well established, and the tools to manage it are available and cost effective. What the summer season demands is that shippers and carriers treat wood moisture content as a managed specification rather than an assumed given.
A crate that left the mill at KD19 in April is not necessarily at KD19 when it is loaded in July. Green hardwood sourced without a moisture content requirement is a humidity source, not just a platform. And a trailer sealed against the summer heat without ventilation is an incubator. Recognizing those realities and building procurement specifications, treatment protocols, and staging practices around them is what separates a summer shipping program that holds up from one that generates claims.
For more technical insights and industry compliance guidelines, visit the NMFTA’s LTL Packaging resource page. If your organization is evaluating wood packaging specifications for seasonal performance or looking to understand where your current program may be vulnerable, contact us.
We work with shippers and carriers across the country to build packaging programs that account for the conditions freight actually faces, including the ones that only show up in summer.
