Kiln drying removes water from sawn lumber in a controlled chamber, using heat, humidity control and air circulation to bring boards to a target moisture content, typically 19% or less for pallet and packaging stock. It is faster and more predictable than air drying, reduces mold and blue stain, and is the usual first step before the heat treatment required for export pallets.
Why pallet lumber is kiln dried
Freshly sawn softwood can hold as much water as dry wood weighs, or more. Drying matters for several practical reasons:
- Biological stability. Blue-stain fungi grow best in warm wood above roughly 22% moisture content, according to the Northeastern Lumber Manufacturers Association; they do not thrive in wood dried to 19% or below. See mould and blue stain.
- Dimensional stability. Boards shrink as they dry. Drying before assembly prevents loose nails and gaps in finished pallets.
- Lower weight. Dry pallets are lighter, which cuts freight cost.
- Compliance. Lumber marked "KD" (kiln dried) is dried to 19% maximum moisture content; "KD-HT" indicates it has also been heat-treated. See kiln-dried vs heat-treated wood.
Moisture content is the weight of water as a percentage of oven-dry wood weight, and how to measure it is covered in wood moisture content.
Main kiln types
| Kiln type | How it works | Strengths | Limits |
|---|---|---|---|
| Conventional (steam or hot-water heated) | Heated air circulates through the stack; humid air is vented and replaced | Fast, flexible, proven for all species | High energy use because hot humid air is exhausted |
| Dehumidification | A refrigeration-type heat pump condenses water from the air and recycles the heat | Energy-efficient, lower temperatures, simple to operate | Slower, limited temperature range, less suited to very fast schedules |
| Vacuum | Reduced pressure lowers the boiling point of water, so drying proceeds fast at low temperature | Very fast, gentle on thick or valuable hardwoods | High capital cost, small charge sizes |
| Solar | Sun heats a glazed chamber with fans | Very low operating cost | Weather dependent, slow, hard to control |
| High-temperature (HT) conventional | Dry-bulb temperatures above 100 °C for softwood | Drying and set of pitch in a day or two | Not suitable for many hardwoods, high stress risk |
According to Oklahoma State University Extension, dehumidification kilns are the type most commonly used today for small and medium operations, because most of the water is condensed on the coils and removed as liquid instead of being vented. Vacuum kilns are several times more expensive but dry very quickly.
How a drying schedule works
A schedule is a series of steps defining dry-bulb temperature (the actual air temperature) and wet-bulb temperature (reflecting humidity), changing as the wood dries. The difference between the two, the wet-bulb depression, sets how aggressively moisture is drawn out.
The USDA Forest Products Laboratory's Dry Kiln Operator's Manual describes typical conventional schedules that start near 110 °F (about 43 °C) at high humidity for green lumber and rise to 160 °F (about 71 °C) at low humidity as moisture content falls, with the exact steps depending on species and thickness. Softwoods are tolerant of higher temperatures than most hardwoods, which is why pine and spruce pallet stock can be dried much faster.
Typical stages in a schedule:
- Warm-up. Heat the load and wood to temperature at high humidity so surfaces do not dry too fast.
- Main drying. Raise dry-bulb temperature and lower humidity in steps tied to the moisture content of sample boards.
- Equalizing. Raise the humidity so that wet and dry boards converge to a uniform final moisture.
- Conditioning. Briefly steam or humidify to relieve drying stresses.
- Cooling and unloading. Let the load cool before it meets cold air, to avoid condensation and checking.
Kiln operators track moisture with sample boards, in-kiln resistance probes, or by weighing, and verify with handheld meters at unloading.
Drying for heat treatment
For ISPM 15, wood must reach a core temperature of at least 56 °C for 30 continuous minutes. Many pallet plants combine drying and heat treatment in one kiln cycle, with the load heated well above the minimum temperature. The process and how it is verified are explained in heat treatment kiln operation, and kiln equipment is described in heat treatment kiln equipment.
Drying defects and how to avoid them
Defects arise from moisture and stress gradients. The Oklahoma State fact sheet describes the sequence: early in drying the surface shrinks while the core is still wet, creating tension in the shell. If tension is too great, surface checks appear. Later the core dries and shrinks against the already-set shell, creating casehardening, with compression on the outside and tension inside, which can cause cupping and warp after sawing.
Common defects include:
- Checks and end splits: cracks from too-fast drying, especially on ends. End coating and slower schedules help.
- Warp (bow, crook, twist, cup): caused by uneven drying and poor stacking. Good sticker alignment and top weights reduce it.
- Honeycomb and collapse: internal cracks or cell collapse, mainly in dense hardwoods dried too hot.
- Wet pockets: uneven final moisture, often in thick pieces or resinous areas.
- Stain and discoloration: sticker stain and chemical stain, especially if wood is not dried quickly.
Stacking quality matters as much as the schedule. Stickers should be dry, uniform in thickness and aligned vertically; load edges should be supported, and airflow must pass evenly through the stack.
Energy and costs
Evaporating water takes heat. Conventional kilns use thermal energy (steam, hot water or direct firing) and electricity for fans. Many pallet and sawmill operations burn their own wood waste, such as bark and sawdust, to produce heat, which makes kilns economical; see pallet wood waste and biomass. Dehumidification kilns use electricity but recover most latent heat. Pre-drying by air before kilning reduces energy consumption considerably; see air drying lumber.
Frequently asked questions
What moisture content is kiln-dried lumber?
Lumber stamped KD is dried to a maximum of 19% moisture content. Many pallet specifications use values around 18–20%, and furniture-grade hardwood is dried much lower.
How long does kiln drying take?
It depends on species, thickness, kiln type and starting moisture. High-temperature softwood schedules can run in about a day or two, while conventional hardwood schedules take weeks.
Is kiln drying the same as heat treatment?
No. Kiln drying targets moisture content; ISPM 15 heat treatment targets a core temperature of 56 °C for 30 minutes. The two are often combined, but a kiln-dried board is not automatically ISPM 15 compliant.
Which kiln type suits a small pallet plant?
Dehumidification and conventional kilns are the common choices. The right one depends on energy supply, volumes and whether heat treatment is done in the same chamber.
Sources
- Fundamental Aspects of Kiln Drying Lumber — Oklahoma State University Extension
- Dry Kiln Operator's Manual — USDA Forest Products Laboratory, via Woodweb
- Dry Kiln Operator's Manual (PDF) — USDA Forest Products Laboratory, via Northern Arizona University
- Beware of Blue Stained Wood in Wood Packaging Materials — Northeastern Lumber Manufacturers Association