Air-dried wood vs kiln: what is better. A pro perspective

When a tree is freshly chopped down, it is considered to have a 100% moisture content (MC), and it will take a year, depending on your location to dry it out properly. Wood with around fourteen percent moisture content is used for the construction of buildings, decks, and furniture. When wood is dried too quickly, usually through a kiln method, it undergoes a process called equilibration.

Around ninety-five percent of sawn wood is nowadays are dried in kilns. Only two percent is air-dried in stacks at the sawmill during the summer season, known as timberyard seasoning, while three percent is delivered from the sawmills undried, primarily to the furniture and joinery industry, which dries the wood itself.

Air-dried wood vs kiln, what is better?

Both Kiln and air drying both have there use cases. Kiln-drying is a fast and effective method of drying and protecting wood, and you can reach low moisture level independent of the environmental conditions. Kiln drying can be costly, and needs a lot of energy. Air-drying is more environmental friendly as it does not use additional energy, but it takes much more time, and it is difficult to reach low moisture levels, particuraly in more humit locations.

Kiln-drying and Air drying

Kiln-drying is the fastest and the most effective method of drying and protecting the wood against termite, mold, mildew, and insect infestation. If you turn up the kiln to one hundred and fifty degrees on the final days of drying, it will kill all the eggs, larvae and insects. Kiln-drying also “sets the pitch,” which means the sap of the freshly cut green trees is fully crystallized and will never seep out later on after the construction of furniture or decks.

Kiln-drying helps you reach the desired moisture content (MC) level within 10 days maximum.

Air-dried wood may settle more, depending on its moisture content, than kiln-dried wood though since it usually has a higher moisture content. Air-dried wood is less expensive, up to twenty percent less than kiln-dried wood since lots of investment and the high cost of running an automatic modern-day kiln are eliminated.

The drying of newly sawn wood is the most energy-intensive step in the sawmill process. The kilns are heated primarily by burning bark and possibly shavings, while the fans run on electricity.

Most commonly used sealants need dry fibers to cling onto and create a layer or barrier over the wood. There is an alternate wet sanding process, where initial fibers grasp onto the finish are then shaved down, and another coat of sealant or finish is applied. If you want multiple coats of sealant, you should make a cocoon for moist wood to continually rot in, even if the sealant did stick to it the first time.

Kilns use natural gas, electricity, or steam-heated heat exchangers. Solar energy is also an economical option. In kiln-drying, deliberate control of temperature, relative humidity, and air circulation create variable conditions to achieve specific drying profiles. Wood is stacked in piles, which are fitted with equipment to control atmospheric temperature, relative humidity, and circulation rate.

Kiln-drying provides a means of overcoming the limitations imposed by the harsh weather conditions. With kiln drying, as is the case with air drying, unsaturated air is used as the drying medium. Almost all commercial wood in the world is dried in industrial kilns.

Why is it so important to dry wood?

If your wood has a moisture content (MC) level above eleven percent, you will run into many problems. This is why wood that is to be painted on the construction site should be primed as quickly as possible to protect against ultraviolet radiation, and the surface moisture content should be a maximum of sixteen percent when painting. It is recommended that exposed wood such as external cladding and interior cladding should be industrially surface treated with film thickness for the primer of at least sixty micrometers as a dry layer.

When wood has been dried down to sixteen percent in a kiln, the surface of the wood becomes very dry, often six to seven percent moisture content, while the moisture content in the middle of the wood may lie at around nineteen to twenty-two percent. Depending on the time between drying and packaging, the outdoor temperature and relative humidity (RH), this difference in moisture content will remain in place to a greater or lesser degree.

The low surface moisture content provides good protection against microbial growth. A very accurate way is to first weigh a piece of wood, then dry it in a kiln at one hundred and three degrees Celsius and weigh the fully dried wood again.

Basic principles of air-drying wood

The air’s content of water vapor, its vapor concentration, is stated in grams water/cubic meter air and it varies over the year. Vapor concentration outdoor is at its highest in the summer season (nine to eleven grams per cubic meter) and lowest in the winter season (three to five grams per cubic meter) – while relative humidity (RH) and wood’s equilibrium moisture content is lowest in the summer season (sixty-five percent to seventy-five percent and eleven percent to fifteen percent respectively) and highest in the winter season (ninety percent to ninety-five percent and nineteen percent to twenty-three percent respectively).

Wood should have a surface moisture content of a maximum of sixteen percent if being given surface treatment. Painting outdoor must take place in May-August, but it is important to check the surface moisture content and the moisture content of the wood, for instance using an electrical resistance moisture meter with insulated hammer electrodes.

The relative humidity figure lies between the water vapor’s partial pressure and its saturation pressure at the temperature in question. The relative humidity of the air in a heated room is, therefore, highest in the summer season (forty-five percent to sixty percent) and lowest in the winter season (ten percent to twenty-five percent). The colder it is outdoors, the drier the air indoors.

The moisture content in wood, both indoors and outdoors, adapts to the relative humidity and temperature of its surroundings. In heated homes in mid continents, the moisture content in wood averages out across the year at more than seven percent, with the highest figures in the summer season (seven percent to twelve percent) and the lowest in the winter season (two percent to six percent).

Comparison Between air drying and conventional kiln

 By kiln drying, wood can be dried to any desired low-moisture content, but in air drying, moisture contents of less than eighteen percent are very difficult to achieve for most hilly and remote locations. In conventional kiln drying the drying times are considerably less, followed by air drying. This means that if the capital outlay is involved when air drying is used, this capital sits for a longer time.

On the other hand, it is very expensive to install, operate, and maintain an industrial kiln. Wood that is being air-dried takes up a lot of places, which could also cost money.

There is little control over the drying conditions, so drying rates cannot be properly controlled in air drying. In kiln drying the temperatures employed typically kill all the bacteria, fungi, and insects in the wood if a maximum dry-bulb temperature of above sixty degrees is used for the drying schedule. In air drying, this is not guaranteed.

If air drying is done improperly involving exposure to direct sunlight, the rate of drying may be overly speedy in the dry summer season, causing cracking and splitting, and too slow during the winter season.

Compartment-type kilns are most commonly used in the furniture industry. A compartment kiln is filled with a static batch of wood through which air is circulated. In these types of kiln, the lumber remains stationary.

The drying conditions are successively varied according to the type of lumber being dried. This drying method is well suited to the needs of the furniture industry, which have to dry lumber of varied species and dimensions, including refractory hardwoods that are more liable than other species to check and split.

Humidification is commonly accomplished by introducing live steam into the kiln through a steam spray hose. To limit and control the humidity of the air inside when large quantities of moisture are being rapidly evaporated from the lumber, there is normally a provision for ventilation of the chamber in all kinds of kilns.

Air circulation is the means for carrying the heat to and the moisture away from all parts of the wood. Forced circulation kilns are most common now, where the air is circulated employing electric fans or blowers, which may be installed outside the kiln chamber or inside the kiln chamber.

Final remarks

To sum up, the major advantages of conventional kiln drying include higher throughput and better control of the final moisture content. Conventional kiln drying enables the wood to be dried to any moisture content regardless of weather conditions. For most large-scale drying operations, conventional kiln drying is more effective than air drying.