Comparing timber choices in sustainable building: Thermally modified wood vs Black Locust

April 23, 2024

Is Thermally modified wood a truly sustainable option? In our blog, we discuss its advantages and disadvantages compared to Black Locust lumber.

It is becoming evident that there is a shift in the building industry towards sustainability. Sustainable building also affects choices in building materials such as timber. Thermally modified wood, endangered tropicals and other non-environmental types of wood are not suitable choices in sustainable architecture. In our article we will be exploring sustainable green lumber options of Thermally modified wood and Black Locust and diving into the characteristics to get a better understanding.

Thermally modified wood vs Black Locust history

Both Thermowood and Black Locust Lumber are rated as green wood products. Thermowood is a new solution developed in Finland that has been around for about two decades. Black Locust on the other hand, has been on the market for generations, the first settlers at Jamestown, which was the first European settlement in the US used Black Locust in agriculture and housing. It is a type of hardwood that has huge potential as a high-end building material but is not widely known.

Thermally modified wood production process

Thermally modified wood is heated gradually to a temperature between 180-215°C and kept there for 3 or 4 days in an inert atmosphere (either steam or vacuum). The heating process alters the wood’s chemical properties until its final moisture content is around 4-6%. This limits the ability of the wood to absorb moisture. After the thermal modification, the wood becomes dimensionally stable and less prone to twist and warp with changes in humidity. This is very different to kiln drying, which only reduces the moisture content of the timber. 

Thermally modified wood durability

Thermally modified wood can be produced from a variety of wood materials such as ash, soft maple or red oak. After the thermal modification certain types of thermally modified wood can be used for exterior landscaping projects without additional surface treatment. They are resistant to rot because the carbohydrates in the wood that provide the food sources for bacteria and mold are destroyed during the thermal process. 

Thermally modified wood disadvantages

There is a reduction in strength and stiffness that occurs due to the thermal modification process. In the case of southern yellow pine, the reduction in strength is up to 25%. Due to loss of strength, it is not advised to use thermally treated wood in ground contact, load-bearing constructions or for protection against termites. Thermally modified wood does have an increased carbon footprint due to the 3-4 days of heating which consumes a lot of energy. There is a relevant increase in the emissions to create electricity unless solar power is used. The overall energy consumption is about 20% higher than that of the sawn timber drying process. So overall you get a type of timber that is weaker than the natural product at a high price point. 

Black Locust lumber, a truly sustainable alternative

Black Locust lumber performs best when airdried to a moisture content of 18%. Once airdried, Black Locust has an exceptional natural durability of 50+ years without the help of additional toxic chemicals or oils. Black Locust wood has great stability and is prone to minimal shrinkage. Black Locust lumber belongs in the exterior due to its natural strength and density, a perfect type of wood for landscape architecture. Black Locust performs well on the Janka Hardness Scale (1700), and is stronger than Red Oak (1290). (Thermally modified wood’s  Janka Hardness depends on that of the original wood, but keep in mind that the thermal modification process does reduce its durability somewhat.)

And how about Black Locust wood’s appearance? Black Locust lumber has a beautiful light golden brown color which develops a silvery gray patina when exposed to UV light. This is a popular finish for many architectural designers looking for a minimalistic and natural aesthetic. 

To sum up, Black Locust has the advantage over Thermally modified wood due to its proven history. The truly sustainable choice is the Black Locust as it is a natural product without chemicals or energy usage. At the same time you get a stronger natural product compared to Thermally modified wood.

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