Thermally modified wood 101: Improving the appearance & performance of wood

Wood, by nature, is a biodegradable and dimensionally unstable material. These innate characteristics present problems, such as rotting or warping, for architects and designers. One solution is an innovative, value-add process called thermal modification. The finished product is an exterior-grade domestic wood species that is typically not used outdoors.

At Intectural, we offer a thermally modified wood called Arbor Wood, which is harvested from sustainably managed North Central Minnesota forests.

History of thermal modification

Though thermally modified wood is relatively new to the U.S., it has been used in interior and exterior applications in Europe since the early 1990s. Developed by industry leaders and VTT Technical Research Center of Finland, the process was meant to bring value add to the Scandinavian country’s largest resource.

What is thermal modification?

Thermal modification uses heat and steam to fundamentally change the properties of wood, making it more hydrophobic, or less absorbent, so the wood has minimal expansion and contraction with the fluctuation of moisture. The sugars are also converted to a nonfood source to decrease rot and darken the wood for an appealing aesthetic.

Thermally modified timber process

The 3-phase kiln process uses only heat and water – no chemicals – making it one of the most natural, chemical-free ways to extend the life of wood products. A full cycle takes 24 to 72 hours.

Phase 1: Temperature increase

The temperature in the kiln is raised rapidly to around 212°F, affecting the natural composition of the wood. Steam prevents the wood from checking in the high heat. The moisture content of the wood is reduced from 15 to 20% to nearly zero.

Phase 2: Thermal modification

The temperature goes even higher to reach target color levels, around 415°F. The higher and longer the heat, the darker the wood becomes throughout, giving domestic wood the look of desirable imported wood.  Steam is used to prevent the wood from burning in the high temperatures.

The natural acids and sugars are modified from the extreme heat, which changes the physical structure of the wood. The wood’s ability to absorb moisture decreases, making it less prone to rot and decay. The equilibrium moisture content (EMC) of the wood decreases. This means the swelling and shrinking of the wood material due to moisture variations can be reduced by up to 60% compared to unmodified wood.

Phase 3: Cooling & Re-Conditioning

The temperature is reduced by spraying water onto the wood and cooling. Conditioning and remoistening bring the wood moisture content around 4 to 6%.

Final Step: Milling

The TMT (Thermally Modified Timber) raw stock is then processed by local mill workers to the desired specification. Common mill forms are siding, decking and flooring.

Interested in how Arbor Wood can be applied to your project? Contact us and we’d be happy to talk with you more about this sustainable, contemporary design solution.