What Moisture Content Firewood Should Be (And How To Check)
An important aspect of having a fire in any type of fireplace, whether it’s an open fireplace or wood-burning stove, is to ensure that the firewood is dry enough to use.
For wood to burn efficiently on a fire it needs to have a low enough moisture content, so what moisture level should firewood be?
For firewood to burn efficiently it should have a moisture content of 20% or lower. As the moisture content of firewood increases above 20% it gets progressively harder to burn effectively in a fire.
So how do you know whether firewood is dry enough, how can you measure moisture content in firewood, and can firewood be too dry? We’ve explained moisture content for firewood in much more detail below.
What Moisture Level Should Firewood Be?
Many of the issues associated with poorly burning fireplaces or stoves can be the result of burning wood that is too wet. These issues can include wood that isn’t:
Catching alight properly
Generating much heat
Burning without producing smoke
You may also notice that wet wood also makes hissing and spitting noises as the fire tries to burn off the excess moisture. Wet wood is harder to burn because more energy is required by the fire to burn of the excess moisture before it can start to efficiently burn the wood.
Freshly cut wood can have a moisture content of anything over 20%, meaning that this ‘green’ wood is very high in moisture content and would struggle to burn on a fire.
The time of the year that the wood is cut can also determine how high the moisture content of the wood is. Wood cut in the winter months will typically have a lower moisture content than wood that is cut in spring.
This is important because for wood to be useful as firewood then it needs to have a moisture content of under 20% to burn efficiently.
To achieve this moisture content the wood needs to be dried out, through a process known as seasoning.
The higher the moisture content of the wood when it’s cut, the longer it can take for it to dry out to recommended firewood moisture content levels. Cutting wood when it’s already low in moisture content as possible can help to speed up the drying out process.
Seasoning wood can be a lengthy process. Certain hardwoods can take up to 2 years to dry out, while softwoods can typically take up to 1 year.
Hardwoods such as Ash or Oak are from deciduous trees, which typically take longer to grow compared to softwoods such as Pine, from coniferous or ‘evergreen’ trees.
The slower growing process of hardwoods can mean that the wood grows more densely, meaning that the seasoning process can also take longer as a result.
Although the seasoning process for firewood can take a long time, almost no involvement is required once it has been setup.
To ensure that wood dries out in the most efficient way, and to prevent it from rotting, the wood should be:
Stacked on top of some sort of impermeable ground, such as concrete, to ensure that moisture from the ground doesn’t seep up into the wood, and for any excess water to runoff into the nearby ground.
Located under some sort of canopy, to help shelter the wood from the majority of the rainfall throughout the process.
Open on one side to allow the weather to help dry out the wood, in particular the wind.
The wood also shouldn’t be directly covered over by anything, such as sheet of tarpaulin, as this can trap the moisture and cause the wood to rot rather than dry out.
We season our own firewood to be used in our wood stove, and the picture below shows how we arrange the wood so that is dries out effectively.
As the conventional seasoning process can take many months or even years, a process known as kiln drying helps to vastly speed up the drying process of firewood. High moisture content wood is simply placed in an oven-like chamber where the excess moisture content is evaporated off.
You’ll typically find that bags of logs or kindling bought from your local store will have been kiln dried as stated on the bags.
How Do You Know If Firewood Is Dry Enough?
Dry firewood ready for burning looks a lot different to wood that is freshly cut and high in moisture content.
The table below highlights the main differences between wet and dry wood
Bark peeling & cracks
Softer bark & wood
In summary, you can tell whether firewood is dry enough if the wood is:
Brown with no hints of green.
May be splitting at the ends with cracks forming.
The bark can be coming away or is easier to peel off.
Can make a hollow sound when pieces are hit together.
The images below show what you can expect a dry piece of wood to look like.
Ultimately the best way to get an accurate reading as to whether firewood is dry enough is to use a moisture meter.
A moisture meter is a handheld device that can be used to accurately measure the moisture content of any wood by giving a reading when pressed against it.
Moisture meters will typically have two metal pins that stick out the top (although some models of moisture meter can be pinless), and will also have a digital screen on the front that will give you an accurate reading to typically one decimal place.
Here’s what our moisture meter looks like:
In many cases wood can be sold in bags as ‘seasoned’ wood but the actual moisture content of the wood isn’t low enough for it to burn efficiently.
A study by the University of Tennessee (which can be found here) showed that a typically bag of ‘seasoned’ wood had moisture content values ranging from 50-90%, which is far too high to burn efficiently. The study concluded that low moisture content is key for firewood, and that the best approach is to buy and further season the firewood well before it’s needed.
We bought a bag of ‘kiln dried’ kindling and logs from our local store and used our moisture meter to find out what the actual moisture content of the logs actually were.
The bag stated that the logs were ‘less than 20% moisture content’.