Black Locust (Robinia Pseudoacacia) and thermally modified wood as sustainable choices in green building

By Black Locust Lumber team

   It is hard not to notice the explicit signs of a global paradigm shift in the building industry. The shift towards sustainable building influences timber choice, too. Non-environmental, chemically-treated wood is no longer a fancy option. Just as using endangered tropical woods is no-longer a sign of elegance. What is going to substitute these materials in modern sustainable architecture and landscaping?

   Black Locust (Robinia Pseudoacacia) and thermally modified wood can both be referred to as evident choices in dimensionally stable domestic wood products complying with green rating credit standards. Black Locust has been on the market for generations as a hardwood used traditionally in boat building and agriculture with huge, but unrecognized potential as a high end building material.

   Thermally modified wood is, on the other hand, a new green building solution. This article aims to clarify the characteristics and applications of these two sustainable resources. In both cases, wood is carefully processed to ensure dimensional stability and durability. Thermally modified wood is heated in absence of oxygen to a temperature between 180-215°C and kept there for 3 or 4 days. It’s final moisture content is around 4-6%. The technology was developed in Finland, but is now used with minor adjustments in several other countries. The process can be applied to wood of virtually any species of lumber. The modification changes the wood’s physical and chemical structure, resulting in limited moisture absorbing ability. This makes the wood more stable and less prone to cup, warp or twist, and also increases resistance to decay, fungi and insects, prolonging the lifetime of the original wood. However, a scientific study (Čermák et al, 2015 ) pointed out that later wetting cycles could 1 decrease this initially gained stability to a level lower than that of the unmodified wood. For this reason, it is recommended to treat the modified wood for external use.

   A serious disadvantage of high heat treatment is that it results in lower strength and stiffness values, therefore load-bearing structural usage of thermally modified wood is usually not recommended.

   In case of Black Locust, air-drying and kiln-drying methods are both used, but experience shows that Black Locust wood performs best when air-dried to around 18% moisture content. If air-dried and sealed properly, it has excellent stability and is prone to minimal shrinkage. Black Locust wood has exceptional natural resistance to biodegradation, due to the concentration of extractives, such as robinetine and dihydrobine. According to DIN EN 350-2 Black Locust (robinie) is the only wood growing in Europe having a resistance class of 1-2.

   Additionally, it’s wood is very dense, durable and strong, very prone to weathering. According to the Janka Hardness Scale, Black Locust (1700) is considered to be stronger than Teak (1155) and Red Oak (1290) and just below the tropical, Jarrah (1900). Thermally modified wood’s hardness depends on the janka hardness of the original wood.

   The color of thermally modified wood also depends on the original wood. As a result of the thermal modification process, the wood becomes a darker brown tone, but due to UV light it turns silvery grey, unless applying pigmented surface protection.

   Black Locust is golden brown in color and when exposed to UV light, it patinas to a silvery grey which is a popular natural finish desired by architectural designers achieving a contemporary minimalistic aesthetic. Besides the wood’s appearance and mechanical characters, it’s environmental performance is equally important. If it is used for building projects that aim to qualify for credit under LEED, as green building credentials include (among others) renewability, low embodied energy and a light carbon footprint. In this aspect, Black Locust is definitely the golden standard.

   Black Locust is a domestic wood, both in Europe and North America, with ample resources allowing sustainable production. It is naturally rot-resistant, so there is no need to use chemical treatment. Black Locust wood products have a long lifespan (50 years+ outdoors) and are carbon positive, in addition, generally low energy consumption is related to its production. The air-dried locust is characterized by low embodied energy with extra light carbon footprint.

   In the case of thermally treated wood, the high heat consumption requires high energy useage, creating approximately four times higher embodied energy than Black Locust. It’s carbon footprint depends on the type of energy used for the thermal treatment. Thermal modification can upgrade lower class wood. This allows a longer lifecycle compared to the original wood, and the fact that it needs less chemical treatment than the original makes it more sustainable than the untreated or chemically treated wood.

Both locust and thermally modified wood can be used for external cladding and decking, for making fences and window trims or interior lining. Black Locust is ideal for making facades, terraces, pavers and site furnishing as well, while thermowood can be used for sauna benches and lining material.

   As a rule of thumb, we can say that where extreme durability is elemental or there is direct contact with soil or water, Black Locust is the ideal choice. Massive amounts of data is available proving that Black Locust can sustain for 50 years or more in outdoor conditions. Thermal modification on the other hand, is a method that is relatively recently developed, thus lifespan of wood products gained by thermal modification is based on estimations instead of real experimental data.

The ultimate question is whether the environmentally conscious customer wants to pay less for a newly developed material with uncertain durability or would like to invest in the golden standard of robust and trustworthy Black Locust, that has been proven by time.