It is best to rely on the advice of those who have been building baths for a long time. Moreover, these tips are distributed for free.
1) the outer walls should be from round and high -quality forest. For the construction of baths of this type, spruce is used. I advise you to choose a spruce from the northern regions, an example of this Transbaikal Territory. Do not choose spruce from the southern regions, as it spoils quickly.