Definition
Lumber that has been seasoned by exposure to ambient air rather than in a kiln. Air-dried lumber is stacked with spacers (stickers) to allow airflow and is left to dry naturally over a period of weeks or months. While air drying is less expensive than kiln drying, it produces lumber with higher and less consistent moisture content, typically around 19-25%. Air-dried lumber alone does not meet ISPM-15 requirements for export pallets.
Related Terms
Kiln Dried
Lumber that has been dried in a heated kiln to reduce its moisture content, typically to 19% or below. Kiln drying is distinct from heat treatment for ISPM-15, though some kilns can achieve both purposes simultaneously. Kiln-dried lumber (marked KD) resists mold, warping, and shrinking better than green or air-dried lumber. While KD lumber is preferred for pallet construction, kiln drying alone does not satisfy ISPM-15 requirements unless the specific temperature and duration thresholds are met.
Moisture Content
The amount of water present in lumber, expressed as a percentage of the wood's oven-dry weight. Moisture content is a critical quality parameter in pallet manufacturing, affecting weight, strength, dimensional stability, mold susceptibility, and heat treatment effectiveness. Kiln-dried pallet lumber typically has 19% or less moisture content, while green lumber may exceed 50%. Excessive moisture leads to heavier pallets, mold growth, and product contamination risks.
Green Lumber
Freshly sawn lumber that has not been dried, retaining its natural moisture content which can range from 30% to over 100% of the wood's oven-dry weight depending on species. Green lumber is heavier and more susceptible to mold, staining, warping, and shrinkage as it dries. While green lumber is less expensive and often used for economy pallet production, it requires careful handling and is not suitable for applications requiring dimensional stability.
Heat Treatment
A phytosanitary treatment method approved under ISPM-15 in which wood packaging material is heated until the core temperature reaches a minimum of 56 degrees Celsius (132.8 degrees Fahrenheit) for at least 30 continuous minutes. Heat treatment kills insects and pathogens without chemicals, making it the preferred method for treating export pallets. Treated wood is stamped with the HT mark as part of the ISPM-15 compliance marking.
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