Kiln Drying Schedules Explained: How Temperature and Humidity Control the Drying Process

What a Drying Schedule Actually Controls

A kiln drying schedule is a set of instructions that tells the kiln how to change temperature and humidity at different stages of the drying process. The two key variables are dry-bulb temperature (the air temperature in the kiln) and wet-bulb temperature (the temperature of a thermometer with a wetted wick, which reflects the humidity in the kiln). The difference between dry-bulb and wet-bulb readings — the wet-bulb depression — determines the relative humidity inside the kiln and therefore the rate at which moisture can leave the timber.

A high wet-bulb depression (large difference between the two readings) means low relative humidity and fast drying. A low wet-bulb depression means high relative humidity and slow drying. A kiln schedule manages this relationship throughout the drying cycle to balance drying speed against the risk of stress-related degrade.

Why the Sequence of Conditions Matters

The most common mistake in kiln drying is applying aggressive (low humidity, high temperature) conditions too early in the drying process. When timber first enters the kiln, the surface moisture content is high and the surface dries quickly under any conditions. If the kiln humidity is set low from the start, the surface loses moisture much faster than the core can supply it. The surface dries and tries to shrink while the core is still wet and unable to shrink. This puts the surface in tension and causes surface checking — small cracks along the face and edges of the board.

The solution is to start the schedule with relatively high humidity — a small wet-bulb depression — to slow the rate of surface drying and allow the drying front to move inward more evenly. As the average moisture content of the load falls toward the fibre saturation point, the schedule progressively increases the wet-bulb depression to increase the drying rate. By the time conditions become aggressive, the gradient between surface and core is smaller and the risk of stress cracking is much lower.

• Stage 1 (high MC): low wet-bulb depression — slow surface drying to match core
• Stage 2 (mid MC): moderate wet-bulb depression — balanced drying rate
• Stage 3 (low MC): higher wet-bulb depression — accelerated final drying
• Final conditioning: steam injection to equalise MC across the load and relieve residual stresses

The Role of Temperature in Drying Rate and Timber Quality

Higher temperature increases the vapour pressure of water in the wood, which drives moisture toward the surface and out of the timber faster. It also reduces the viscosity of free water in the cell cavities, making it easier to move. The combined effect is that a kiln running at 80°C dries timber significantly faster than one running at 60°C.

Temperature also affects the physical properties of the timber during drying. At temperatures above approximately 50°C, the lignin in the cell walls begins to soften — the wood becomes more plastic and less prone to fracturing under drying stress. This is why high-temperature (HT) drying schedules can sometimes be used for species that would check badly under the same drying rate at lower temperatures.

However, excessive temperatures cause their own problems. Temperatures above 90°C can cause resin bleed in softwoods, colour change in light hardwoods, and a reduction in the equilibrium moisture content of the wood (the timber ends up hygroscopically stiffer than it was before drying, affecting how it responds to humidity changes in service). For most hardwoods, kiln temperatures above 75°C require careful justification.

Conditioning: The Final Stage That Most People Skip

At the end of a drying cycle, timber that has been dried to the target average MC will typically have a moisture gradient from the surface (drier) to the core (slightly wetter). It may also have residual drying stresses — the surface was put into tension early in the drying process, and even though the stress has reduced as the core dried, some residual stress often remains.

The conditioning stage addresses both of these issues. Steam is injected into the kiln to raise the humidity, which allows moisture from the slightly wetter core to redistribute outward — equalising the MC gradient across the cross-section. At the same time, the elevated humidity and temperature relieve residual stress by allowing the cell walls to creep slightly. Properly conditioned timber has a uniform MC across its cross-section, no residual stress, and is far less likely to warp or crack during subsequent machining.

Skipping conditioning saves a few hours of kiln time and significantly increases the risk of problems in manufactured products. It is a false economy.

How Schedules Are Determined for Different Species

Standard drying schedules for common timber species have been developed and published by timber research organisations and are used as starting points by most kiln operators. These schedules specify initial temperature, initial wet-bulb depression, and the step changes to make at different MC stages. They are based on decades of testing and represent a reasonable starting point for common species.

In practice, schedules are often modified based on the specific kiln equipment, the origin and quality of the timber, the thickness of the load, and the required final MC. A kiln operator who dries the same species regularly will develop empirical knowledge of what works in their specific equipment — adjusting schedules based on observed degrade rates and drying times.