Definition
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.
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.
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.
Air-Dried
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.
Dry Kiln
An enclosed chamber used to dry lumber by controlling temperature, humidity, and air circulation. In the pallet industry, dry kilns serve the dual purpose of reducing lumber moisture content and providing heat treatment for ISPM-15 compliance when operated at the required temperature and duration. Kiln schedules are tailored to the wood species, thickness, and initial moisture content to achieve target dryness without causing defects like checking or warping.
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