1. Plan the room
- Size: about 60 cm of bench per person; inside height 2.0–2.2 m. The sauna size calculator turns people into dimensions.
- Benches: the upper bench about 1.0–1.2 m below the ceiling, so heads sit in the heat (bench height).
- Heater: sized from the effective volume (heater size calculator). The heater decides the power supply, so choose it before anything is built.
- Try it in 3D: the 3D studio checks bench gaps, heater clearance and door, and gives you a materials list.
- Count the materials: the build cost calculator lists the timber, insulation, foil, panelling and bench boards for your size and adds up your shop's prices.
2. Floor and base
Indoors, a tiled or otherwise waterproof floor with a drain is easiest; removable wooden duckboards go on top. Outdoors, start with a foundation that keeps the wood off the ground (see outdoor sauna).
3. Frame the walls and ceiling
A timber frame (often 45 × 70 or 45 × 95 mm studs) with the ceiling at the planned height. Add solid blocking where the heater, benches, backrests and lights will be fixed.
4. Electrics rough-in (electrician)
The heater's supply, the control unit and the light are installed by a licensed electrician following the heater's manual: heat-resistant cable inside the hot room, the sensor at the stated height, and the circuit sized for the heater. Plan this before the walls are closed.
5. Insulation
Mineral wool between the studs (often 50–100 mm in walls and more in the ceiling, where the heat collects). It must withstand sauna temperatures; foam boards meant for houses usually do not.
6. Vapour barrier
Aluminium foil on the warm side of the insulation, shiny side facing into the room, joints overlapped and taped with aluminium tape, and as few holes as possible. It keeps steam out of the insulation. Details in sauna vapour barrier.
7. Battens and panelling
Battens of about 20–25 mm over the foil leave an air gap behind the panelling; then 12–15 mm tongue-and-groove boards (spruce, aspen, alder, thermowood or cedar). Leave a small gap at floor level. See sauna wood.
8. Ventilation
Fresh air in low near the heater, exhaust out on the far side, about six air changes an hour for an electric heater. See sauna ventilation.
9. Benches
Untreated softwood or a low-conductivity wood such as aspen or abachi, with hidden screws and gaps between the slats. Upper bench about 55–65 cm deep.
10. Heater, stones, door and light
- Heater: fixed with the safety distances from its manual, with a guard rail where people could touch it.
- Stones: rinsed and stacked loosely, filled to the maker's figure (sauna stones).
- Door: opens outwards, no lock (sauna doors).
- Light: heat-rated and low on the wall or under the benches (sauna lighting).
11. First heating
The first heat-ups usually smell of the heater's protective coating and the new wood: heat the empty room with good ventilation before the first session.
Costs
The sauna cost guide breaks down kits, heaters and materials; the running cost calculator gives the cost per session.