A traditional Finnish sauna is a specific thing rather than a style: hot dry air, a stove with a real stone load, water poured on the stones, and a room built so the steam reaches you. Plenty of American cabins are sold with the word Finnish attached while missing the parts that make one work.
Heat and loyly
The defining feature is loyly, the burst of steam from water on hot stones. It is what makes a Finnish sauna feel different from a merely hot room, and it needs a stove holding enough stone to turn several ladles into steam without the temperature collapsing. A heater with a thin tray of stones cannot do it, whatever the listing calls it.
The bench geometry
Steam rises and the upper bench is where the sauna happens, so the relationship between the stove top, the upper bench and the ceiling is the design. The classic rule keeps the top bench level with or above the top of the stones, and the ceiling low over it. A cabin with a high ceiling and low benches is a warm room, not a Finnish sauna.
Ventilation
Air in low near the stove and out on the opposite side keeps the room breathable and the heat even, and it is the detail most missing from imported cabins. Without it the lower bench stays cold, the air goes stale within twenty minutes and the cabin never dries properly between uses. It costs almost nothing at build time.
Buying one in the United States
Wood fired is the traditional heat and is practical outdoors. Electric heaters with a generous stone load give you most of the same experience indoors. What to insist on is the stone capacity, the bench to ceiling geometry and the vent path. Cladding species and door glass are preferences; those three are what make it Finnish.
Questions people ask about traditional finnish sauna
Does a Finnish sauna have to be wood fired?
No. An electric heater with a proper stone load gives you loyly too. Wood is traditional and is easier outdoors than in.
What temperature is a Finnish sauna?
Hotter than most American cabins run, with low humidity between ladles. The steam bursts rather than the dial are the experience.
What is loyly?
The burst of steam from water thrown on hot stones. It is the defining feature, and it needs a heater with real stone capacity.