It may be surprising but usually the wood is not treated at all, there is really no need for that. When you heat the sauna and it gets moist from the steam, that is only for a little while and it’s recommened to throw in one more piece of firewood in the end to make sure the heat dries out the sauna. The panels are actually extremely dry, they weigh nothing, and to my knowledge these were put in 2004 and the sauna has been used probably about twice a week.