Where it fits
- Next to the bathroom or shower, the classic spot: the shower and drain are close by.
- Basement: often has the space and ceiling height; check the moisture and the ventilation path.
- Attic, spare room or a large closet: fine if the floor can take a wet room and the ventilation can reach outside.
Space and height
- Floor: from about 1.3 × 1.3 m inside for two people sitting; 2.0 × 1.8 m seats four and lets one lie down. The sauna size calculator works it out.
- Height: about 1.9–2.2 m inside, so the upper bench sits 1.0–1.2 m below the ceiling.
- Try it in 3D: the 3D studio has indoor and corner presets you can resize to your room.
Moisture and walls
Insulate the walls and ceiling and put an aluminium foil vapour barrier on the warm side (vapour barrier). Avoid building straight against a cold outside wall without insulation, where moisture can condense. A tiled floor with a drain makes cleaning easy.
Ventilation
Fresh air in low near the heater and exhaust on the far side, led outside or into a bathroom with an extractor fan, never into a closed space (sauna ventilation).
Power supply
Small heaters (up to about 3.6 kW) can run on a single-phase circuit; most heaters from 6 kW up need three-phase power in continental Europe or a 240 V circuit in North America. A licensed electrician installs the circuit; the electrical requirements tool shows what to ask for.
Running cost
Indoors the sauna starts from room temperature, so it heats faster and uses less energy than a garden sauna. Glass is the biggest variable:
| Room | Effective volume | Heaters that fit | Heat-up, 8 kW | Cost per session |
|---|---|---|---|---|
| Wooden door | 267 ft³ | 6–10.5 kW | 30–40 min | $1.27 |
| Glass door | 323 ft³ | 7.5–10.5 kW | 30–40 min | $1.28 |
| Glass door + 1 m² window | 366 ft³ | 7.5–13 kW | 30–40 min | $1.29 |
| Glass front (4 m²) | 436 ft³ | 9–13 kW | 30–40 min | $1.37 |
Harvia's rule adds 1.2 m³ of effective volume for every m² of glass. Heat model with the same 8 kW heater in each room, indoors at 20 °C; electricity $0.18/kWh (United States average).
Kit or custom build
Prefabricated indoor kits come as panels that bolt together; a custom build uses the room's own walls. Both need the same vapour barrier, ventilation and heater sizing. See how to build a sauna.