A wooden sauna is the default and for good reason: timber insulates, it does not get hot enough to burn skin the way metal and stone do, and it smells right. The choices inside that are which species, whether it is solid or clad, and how much upkeep you are prepared to do.
The species that get used
Western red cedar is the premium choice: stable, aromatic, rot resistant and expensive. Hemlock is the common mid option, clean grained and almost odourless, which suits anyone who dislikes cedar's scent. Nordic spruce is the traditional European interior. Thermally modified softwoods are heat treated to be more stable and darker, and have taken a lot of the market.
Why knots and density matter
Anything you touch at temperature should be low density and knot free, because a knot is denser than the board around it and gets hotter. That is why bench boards are graded more strictly than wall cladding and why a cheap cabin with knotty benches is genuinely worse to sit on, not just less pretty.
Solid timber against clad frame
Solid log or thick plank cabins have mass, hold heat and look substantial, and they move with the seasons. A framed cabin with insulation, a vapour barrier and thinner cladding reaches temperature faster, costs less to run and is what most indoor and prefab saunas are. Neither is better; they behave differently and cost differently.
Upkeep
Never varnish or seal the inside: a film finish traps moisture and blisters, and it is the single most common mistake. Bare timber is correct, sanded lightly when it darkens. Benches can take a dedicated paraffin based sauna oil. The outside of an outdoor cabin does want a finish, and that is where the annual work is.
Questions people ask about wooden sauna
Which wood is best for a sauna?
Cedar for the look and scent, hemlock if you want it neutral, thermally modified softwood for stability at a lower price. All three are used in good cabins.
Can you varnish the inside?
No. A film finish traps moisture and blisters in the heat. The interior stays bare, sanded occasionally, with a paraffin based sauna oil on benches if you want one.
Why do knots matter?
A knot is denser than the board and gets hotter, so knot free timber is specified where skin touches it, especially the benches.