A sauna can be built wrong and still look beautiful. But it will feel wrong — and after a few sessions, you'll know it. Good sauna design starts with function.

Wood Selection

The interior wood must be stable at high temperatures, resistant to moisture, and free of resins that can drip or scorch when heated. The most commonly used species are:

  • Aspen (haapa) — The Finnish standard. Light in colour, low in resin, doesn't splinter, and stays cool to the touch even at high temperatures. The best choice for benches.
  • Alder (leppä) — Slightly warmer in tone than aspen, similarly resin-free. Popular for walls and ceilings.
  • Thermowood — Heat-treated pine or spruce. More dimensionally stable than untreated wood, deeper in colour. Good for exterior-facing walls and floors.
  • Cedar — North American standard. Aromatic, insect-resistant, and attractive. The resin content is higher than aspen but typically not a problem at normal sauna temperatures.

Avoid pine and spruce in their untreated form for bench surfaces. They weep resin when hot — sticky, unpleasant, and potentially damaging to skin.

Bench Geometry

The upper bench should sit 100–120 cm from the ceiling. Lower than that and the heat stratification works against you — you'll cook at head level and freeze at your feet. The bench should be wide enough to lie down on: 60 cm minimum, 90 cm is better.

A two-level bench arrangement (lower bench as a step and seat, upper bench as the main bathing surface) works well in most residential saunas. The lower bench is useful for children, those new to sauna, or anyone who wants gentler heat.

Ventilation

A sauna without proper ventilation is a sauna that feels stuffy and produces unpleasant, heavy steam. Fresh air should enter low — ideally near the stove — and exhaust high, on the opposite wall. A simple adjustable vent (10 × 25 cm) at floor level near the stove and another near the ceiling on the far wall is the traditional solution. Keep ventilation minimal during heating; open it slightly during the session.

Lighting

Bright overhead lighting in a sauna is a mistake. It makes the room feel clinical and makes it hard to relax. Use indirect, low-level lighting — a recessed fixture behind the upper bench, or low-wattage lights near floor level — to create warmth without glare. LED strips rated for high-humidity environments work well and consume almost no power.

The Stove Placement

The stove should sit in a corner opposite the bench, with enough clearance on all sides (typically 30 cm minimum, check manufacturer specs) to avoid scorching the wall. The rocks on top of the stove should be accessible from the bench — you shouldn't have to stand up or lean awkwardly to ladle water.

Done right, a sauna is a room you'll use every week for decades. Take the time to get the details right on the first build.