Freshly cut timber is classified as green lumber, which does not mean it is green in colour, it means it has a moisture content greater than 60%. All timbers are hygroscopic material which means they have the ability to absorb moisture and release water like a sponge. The amount of water timber can hold varies, but it can be as much as 200%.

This moisture is not bad, during life it is what allows the tree to take up nutrients from the ground. High moisture levels can also make timber more flexible and easier to work with. But from a firewood point of view, high moisture levels pose several environmental and health difficulties, which is why you will often find kiln dried wood being sold as firewood. In this ultimate guide to the drying of kiln dried wood, we'll explore the step-by-step process of creating kiln dried logs, why this process is so important, the various types of drying, and the equipment involved.

Editors Note 19th August 2023. This article was originally published in July 2021. Today, we have added major new areas including many new images, added an infographic, definitions for air and kiln drying, an explanation as to why low moisture is important, types of kiln drying, a section about vacuum kilns and backing facts and statistics.

Why burning wet wood is bad

Burning wood with a high moisture level will produce heavy black smoke which contains coal tar (creosote). This residue builds up in the chimney and flue increasing the risk of a chimney fire. If left unchecked, it can also lead to structural damage to the chimney. Burning wet wood is also much less efficient than dry wood, so you will need more wood to heat your home. And because green wood doesn't burn at such a high temperature it produces more particulate matter.

Burning wet wood on a BBQ

Burning wet wood on a BBQ

These tiny soot particles eventually find their way into the atmosphere where they contribute to global warming. So, before firewood can be burned it must be dried to remove moisture to a level below 20%. There are several methods of doing this, but kiln drying is by far the fastest and most convenient. But what exactly is kiln drying and how does it compare with other timber drying methods such as drying by air?

What is kiln dried wood?

Kiln dried wood is wood that has been heated in a purpose-built kiln. This heats the timber without burning away fibres, thus persevering more fuel for burning. As the timber heats the excess moisture held within the wood evaporates, thus drying out the wood from the inside.

Kiln drying is typically used to bring the wood's moisture content down to less than 20%, making it suitable for burning. The kiln drying of wood process also helps to remove impurities taken up by the wood during the growth phase, thereby further improving burning efficiency.

For more information, please read on here.

Kiln drying of wood – the ultimate infographic guide to how kiln dried logs and firewood are made

We have created an infographic describing many aspects of drying wood with a kiln. A preview of the infographic can be seen below. As you can see, it provides a fantastic visual explanation but there is far too much to display here, so to view the whole infographic, simply click on the image below (or click here) and you will be able to read everything in our detailed visual guide!

How Does the Kiln Drying of Wood Work Infographic

A preview of our "how does the kiln drying of wood work" infographic. Click to see the whole thing.

To learn more, view our infographic – kiln drying of wood

What is air drying?

Air drying is a natural process where logs are left in the open air to reduce moisture content. The air dried wood process takes between 8-16 months and results in a non-uniform drying process, with the wood often being moister in the middle compared to the edges. This process continues until the air dry wood reaches its equilibrium moisture content.

What is kiln drying?

Unlike drying by air, the kiln drying process takes place in a closed chamber where airflow, temperature, and humidity can be controlled. The primary goal of kiln drying is to move the moisture to the surface of the timber and let it into the atmosphere. This process usually takes 6-8 weeks and results in uniformly dried wood.

Kiln drying vs air drying

We have known for centuries that wood needs to be dried before burning, but we have not always used kilns. The traditional method for drying wood is to stack it in the open where air can circulate to draw out moisture. However, this process is incredibly slow, it can take anything from 8 to 16 months before the wood can be used.

Kiln drying on the other hand only takes 4-6 days to do the same thing for firewood, whereas 6-8 weeks is needed for premium grades such as furniture. Kiln drying also helps to remove impurities in the wood. making it even more efficient to burn. The heating process also removes any insects that took up residence in the timber in the forest. The table below summarises the differences between the two processes.


Kiln Drying Air dried Drying
Speed 6-8 weeks

Faster, can be controlled and optimised for different types of wood.

8-16 months

Slower, depending on the climate and weather conditions.

Quality More consistent, reduces the risk of defects like warping and splitting. Less consistent, quality can vary based on environmental conditions.
Cost Higher, due to the need for specialised equipment and energy use. Lower, as it uses natural processes and requires less equipment.
Moisture Content Can be precisely controlled, usually reduces to 20% or below [1] Less control, but properly stacked will dry to about 15-20% [3]

Why is low moisture content so important?

A low moisture level of 20% or below is a crucial factor that underpins many of the advantages of kiln dried wood or kiln dried lumber, ensuring:

  • Optimal burning efficiency
  • Reduced smoke and emissions
  • Minimal creosote build-up
  • Extended shelf life
  • Enhanced heat output
  • Decreased risk of chimney fires
  • Easier ignition
  • Eco-friendliness

To gauge the correct moisture level a moisture meter should be used.

The step-by-step process of kiln drying

Several types of kilns are used to dry timber for firewood, but the two most common types are conventional and dehumidification kilns. We summarise all of the steps for kiln drying below:

  • Harvesting - trees are cut down, and logs are delimbed and prepared for drying by species, size, or by end-use. The logs are then cut into timber or lumber as required and sorted.
  • Grouping – at the sawmill, the sorting process is vital. The trunks are then debarked and grouped by size and moisture level. This grouping process is important because the kiln drying of wood process works more efficiently if all the timber is of equal size and contains similar moisture levels. Once debarked and grouped the logs are ready for kiln drying.
  • Pre-drying - logs are air dried to reduce the initial moisture level.
  • Loading the kiln - the pre-dried logs are stacked in the kiln, a chamber where air circulation, relative humidity, and temperature can be controlled.
  • Kiln drying - the kiln has heated air to around 170°F, and moisture is slowly removed from the logs. This process usually takes 6-8 weeks.
  • Cooling and conditioning - the kiln is cooled, and the logs are conditioned to equalise moisture content.
  • Unloading and storage - once the logs reach the correct moisture level, they are removed from the kiln, planed into final dimensions, and sorted depending on the grade in a controlled climate to prevent moisture absorption and swelling.

Types of kiln drying

There are various types of kilns used for kiln drying of wood, we summarise the main three options below:


Conventional Kilns Dehumidification Kilns Vacuum Kilns
Operation Uses steam flow into the kiln through pipes and radiates heat into the kiln’s atmosphere Uses a dehumidifier to remove moisture from the air, recycling heat within the kiln Uses vacuum pressure to remove moisture from the wood
Speed Moderate Moderate Fastest
Cost Lower initial cost, but higher operating cost due to energy use Higher initial cost, but lower operating cost due to heat recycling Highest initial cost, but lower operating cost due to efficiency
Quality Good, but can vary based on conditions Good, and more consistent due to a controlled environment Excellent, due to precise control over drying conditions
Energy Efficiency Lower, due to the need to constantly heat air Higher, due to the recycling of heat Highest, due to the efficient use of vacuum pressure


We now go on to discuss these different types of kilns.

Conventional kilns

These kilns consist of a thermally efficient chamber that is heated using a gas-fired boiler. The heat is then fed across the timber using circulating fans where it draws out moisture (a process called evaporation). The excess water vapour is then discharged through a chimney. This type of kiln is very inefficient however because most of the heat generated is discharged through the chimney. For this reason, most conventional kilns are being phased out and replaced with more efficient dehumidification kilns that recycle heat energy.

A conventional kiln for drying wood

A conventional kiln for drying wood

Dehumidification kilns

Dehumidification kilns use heat pumps to generate and recycle heat. The process starts by heating the air to a temperature of around 29ºC. This warm air is then fed into the chamber where it is forced over the timber using circulating fans. This causes the water held in the timber to evaporate much like in a conventional kiln.

However, unlike a conventional kiln, this vapour is not discharged, it is fed into an evaporator coil which cools it down to a temperature of about 15ºC, thus condensing the vapour back into a liquid. Meanwhile, the heat removed during the condensation process is recycled to reheat the water. The main advantage of this kiln process is that the water gets hotter with each cycle through the pump. The water starts at around 29ºC but can reach as high as 72ºC after a few cycles.

This makes dehumidifier kilns much more efficient and environmentally friendly than conventional kiln models. As a result, even though dehumidification kilns are powered by electricity, which is more expensive than gas, they are still cheaper to operate than conventional kilns. When properly sized they can also dry timber as fast as a conventional kiln. As a result, most new kilns installed today are of the dehumidifier variety.

A dehumidification kiln for drying wood

A dehumidification kiln for drying wood

Vacuum kilns

Vacuum kilns are advanced wood drying chambers that use reduced pressure and controlled heat to accelerate moisture removal. Lower pressure lowers water's boiling point, expediting evaporation even at lower temperatures. This method significantly shortens drying times while reducing the risk of defects like warping and cracking. Vacuum kilns preserve wood colour and quality, consume less energy, and are effective for a range of wood species. Despite higher initial costs, they offer a more efficient and controlled drying process for wood, making them valuable for speciality drying applications.

A vacuum kiln for drying wood

A vacuum kiln for drying wood

Interesting facts and statistics

Here are some interesting kiln dried logs facts and statistics:

  1. In comparison to well-seasoned or kiln dried wood, the emissions from burning newly felled wood are more than twice as high. [1]
  2. Newly felled wood can contain up to 60% moisture content, while naturally dried or seasoned wood, as well as kiln dried wood, typically has an average moisture content ranging from 12% to 20%. [2]
    • Wet wood starts at ~60% moisture content
    • Generally, drying wood by air for 3 to 6 months can reduce its moisture content by approximately 30%
    • Extending the air drying period for an additional 6 to 9 months and sheltering the wood under a storage shed can further reduce its moisture level to 12% [3]
    • Therefore, kiln drying of wood saves 9 to 15 months compared to air dried wood [3]
  3. Kiln dried wood complies with the requirements of the Air Quality (Domestic Solid Fuels Standards) (England) Regulations 2020 that now apply in England. This states that wood fuel sold in volumes under 2m³ must be certified as Ready to Burn [4]

Kiln dried firewood at Cozilogs

Cozilogs offer premium firewood dried using our own specially designed carbon-neutral dehumidification kiln. This ensures that all firewood meets a minimum of 20% moisture to ensure the most efficient burn rate. And we guarantee that all firewood is sourced from sustainably managed forests. At Cozilogs, you'll find a range of kiln dried firewood, ensuring you can choose from the best timbers for burning. Ash, as an example, has a naturally low moisture content which makes it more efficient to dry than other woods. While its dense structure ensures a steady burn rate and high burning temperature.


The following information sources have been used in this post:




[4] drying-lumber.html