Kiln drying is the controlled drying of lumber in a heated chamber to reduce moisture content to a target level, commonly 19 percent or less for softwood dimension lumber sold as KD.
Detailed Definition
Kiln drying helps reduce moisture-related problems such as staining, shrinkage, mold growth and dimensional change. It differs from ISPM 15 heat treatment in its goal: heat treatment kills pests by holding the wood core at 56 C for 30 continuous minutes, while kiln drying reduces moisture content. A pallet can be kiln dried (KD) without being heat treated (HT), and the reverse. Pallets are often built from green or air-dried lumber, so KD must be specified when it is needed. See moisture content.
How kiln drying works
A dry kiln controls temperature, humidity and airflow to draw moisture out of wood at a rate that limits checking, cracking and warping. Operators follow a drying schedule for the species and thickness and monitor moisture as the charge dries. Hardwoods generally dry more slowly than softwoods of the same thickness. Moisture content is the weight of water as a percentage of the oven-dry weight of the wood, so very wet green lumber can read above 100 percent. Thick blocks and stringers take longer to dry than thin deck boards.
Specifying and checking kiln-dried pallets
- State a maximum moisture content rather than just KD, and say whether it applies to boards, stringers or blocks.
- Agree on the measuring method; pin and pinless meters need species and temperature corrections.
- Check a sample at receiving, since dried wood regains moisture in wet storage.
- Specify heat treatment separately if pallets will be exported.
Dry pallets weigh less than the same pallets built green, which can matter for freight weight. Building with dry lumber also avoids the loosening that can occur when green boards shrink around nails after assembly.
Authoritative Sources
- National Wooden Pallet & Container Association (NWPCA): the US trade association representing wood pallet manufacturers, recyclers, and component producers, founded in 1947.
- FDA Food Safety Modernization Act (FSMA): the 2011 federal food safety law whose FDA rules include preventive controls for human food and the Sanitary Transportation of Human and Animal Food rule covering vehicles and transportation equipment used for food.
Related Glossary Terms
Frequently Asked Questions
What moisture content counts as kiln dried?
There is no single figure for every product. For softwood dimension lumber graded under US rules, KD commonly means 19 percent moisture content or less, and some products are dried further, such as KD15. Furniture and millwork lumber is dried much lower. For pallets, the buyer should state the target, for example a maximum percentage at time of shipment, and the method used to measure it.
Why do food and cold-chain users ask for kiln-dried pallets?
Wet wood can support mold growth, stain packaging and add weight. In refrigerated or closed environments, moisture from the pallet can also condense on loads. Drying lowers moisture content and with it these risks, though it does not sterilize the wood or stop it regaining moisture later. Users who need dry pallets usually combine a moisture limit with clean, covered storage and inspection at receiving.