Thermal modification is a scientific process that changes the properties, chemistry, structure and performance of wood at the cellular level. Thermal modification is not the same as drying. Thermally modified wood is superheated at much higher temperatures than kiln-dried wood in a very controlled manner.

a.) the wood becomes much more stable, 
b.) it becomes slightly more brittle than non-modified wood of the same species, and 
c.) the process ‘cooks’ and chemically changes the sugars in the wood, making the modified wood more rot resistant.     

The process of thermal modification helps to stabilize the wood. By ‘stabilize’ it is meant that the wood is less susceptible to shrinkage and seasonal expansion and contraction. This is especially important for exterior lumber. A more stable outdoor deck or siding means less movement for you. And that leads to better performance of the outdoor deck or siding.

Another important benefit is that the thermal change “cooks out” the sugar. This removes the food source for many types of mold, fungi and insects. For you, this means that the boards are more resistant to insects and rot and are less likely to rot.