Expert opinions suggest that when you thermally modify wood it increases the durability rating of the wood significantly. Test reports on wood that has been thermally modified wood using North American Ash are given much better durability ratings compared to other wood species that have not been thermally modified.
Durability is one of the key performance factors used to assess the suitability of a wood species for a specific application. The durability rating of a species is based on the natural ability of the heartwood of that species to resist decay and insect pests (including termites). The durability ratings are tested only on the performance of the heartwood because sapwood of all timber species has poor resistance and so the natural durability rating applies only to the heartwood of a timber species. Most testing is done by putting the species into the ground and then testing for historical data.
There are four class ratings for wood.For each of the four classes there is an expected service life range. The above ground ranges are different from the in-ground contact ranges. Class 1 rated species are the most durable and Class 4 rated species the least durable. What helps allow thermally modified wood to receive the Class 1 rating for durability is it will not hold or absorb moisture due to the thermal modification process. Also, the the high heat used during the process modifies the cell structure of the wood and makes it highly resistant to rot while greatly reducing expansion and contraction.