Heat treatment of timber is an age-old technique we see in some tricks of Scandinavian construction. More recently in the past decades, this technique has taken the UK/Central Europe by storm. Because of its prevalence, garden designers, decking installers and manufacturers have jumped on the bandwagon. Sometimes, this enthusiasm is warranted, and other times it glosses over limitations. These limitations are especially concerning given the substantial price increase compared to other alternatives.
The Impacts of Thermal Fertilization
Pine, Birch, and Ash are some of the most common timber species to which thermal fertilization is applied. The process involves heating timber to about 160°c – 215°c in a closed, non-oxigenated system. At these conditions, the chemical structure of wood changes. The complex carbohydrates that are preferred by fungi and other insect infestations are broken down. Also, the wood is left with far less capability to absorb and lose moisture.
While these changes do occur, they also prove to have some major benefits for these wood species. The wood becomes less susceptible to insect infestations and moisture. Lastly, the wood becomes more dimensionally stable and has a darker luster. The changes that occur in the wood are a result of the heat treatment, and do not require the use of chemical preservatives. The treatment is especially beneficial for the construction of raised vegetable beds or other structures that children come in close contact with.
What people usually think of as the hardening of wood is the opposite of what is true. This is due to the fact that people believe thermally modified wood is the opposite of what is true. With thermally modified wood being even more brittle than wood that has received no treatment, since the flexible cell wall of the wood core that is responsible for the resiliency of the wood has been lost. This also means they are more likely to receive more damage to the surface due to impact, even though they are easier to cut and machine.
Best Applications
The best example of where thermally modified wood is even more justified is in deck wood. This is where the dimensional stability is the most important. This is also true because all other wood available for decking is thermally modified. This is also due to the fact that, as the ambient moisture content in the air changes, other hardwood decking wood expands and contracts. As thermally modified wood has the lowest coefficient of moisture in all other wood, it shows no changes, which means that the deck may remain more level and have less of an effect.
The best and most obvious place to put it is in exterior wall cladding, where the wood is exposed to driving rain and direct sun. This wood will face severe weather changes all across the British winter. As thermally modified wood has a lower moisture coefficient than all other woods, it causes less stress to the joints and causes splitting which all other wood end losing to exposed wood positions.
Rot-resistant wood structures such as pergolas, arbours, and large planters experience the benefits of rot resistance. Timber that comes into contact with soil, as well as timber that comes into contact with standing water, generally poses a risk of rot to timber, regardless of the wood treatment. Timber with a thermal modification is effective in certain applications, however, the reward to risk ratio is considerable.
Where Standard Treated Timber is Damaged
Not all timber applications require a thermal modification treatment, and using thermal medical applications where they are not needed than the timber is warranted passive.
The framing of a garden room or outbuilding that is built, enveloped, and protected from the weather is covered. In those circumstances, standard treated C16 or C24 framing wood performs well and is well beyond the needed expectations.
Light garden furniture, movable structures, and passive are all post leaser modifications that are out of the cost to benefit ratio. In the described passive example of a small garden planter, with a periodic maintenance of water based oil or stain, would last twenty or more years. Thermally modified wood, in that described context, would not be a recoverable cost due to the excessive cost of the wood.
Building gates has plenty of details involved. With painted timber gates, thermal modification show definite improvement for the timber’s stability. This means saunter gates will shift hammer, hinge, and latch. So, for gates that are more wooden, the texture from burnt timber is nice. For simple gates that are boarded and sauntered on the side and are more timber, the expenses are worth it.
The Colour Question
Thermally modified timber yield from the process as arduous, then will turn patina. With the right exposure the weather will turn to silver in one to three years. For some people, this is aesthetically pleasing. For more warm brown timber, this means maintenance using oil each year.
Being an owner of this timber means you will be the owner of this timber means maintenance. when presented. It means they will not have rotting timber of years. It means the timber will be the same, but the patrons will look more, and be consistently a commitment for extensive periods of time.
Cost in Context
Thermal Modified Timber can often have a price point 30-60% more than pressure treated softwoods (depending on the species); however, when you compare thermal modification to some of the more expensive hardwoods (like oak or ipe), thermal modification can even be cheaper. It is all about what the relevant comparison is.
If the relevant comparison are the expensive hardwoods, then Thermal Modified Timber can be a cost saving alternative with similar performance properties. On the other hand, if the relevant comparison is low cost pressure treated woods, then Thermal Modified Timber is more expensive, and in this case, the specific application and lifespan are the more likely justifications. These two examples show that a pressure treated pine deck that is likely to be demolished in 5 years is a different lifespan justified cost, than a Thermal Modified Timber deck likely to be costing 30% more with a lifespan justified cost of 20 years.
This material has specific, legitimate advantages. It certainly is not a cost reducing universal option, and knowing where these advantages are, and using Thermal Modified Timber only where it is justified cost wise, is a far more justified position then using it across the project.