Sauna ventilation: how much air, and where the vents go
Good löyly needs fresh air, but every source places the vents a little differently. This page compares the Finnish building code, heater makers and research, then works out the airflow and vent sizes for your room.
Three layouts that work
Where the air enters and leaves matters more than the exact airflow. Fresh air should mix with the heat over the stones, and used air should leave low, so the whole room, including your feet, stays warm.
Where the sources differ
| Source | Airflow | Supply | Exhaust |
|---|---|---|---|
| Finnish code (decree 1009/2017, as interpreted) | ≥ 2 dm³/s per m² ≥ 6 dm³/s in a flat | – | – |
| VTT field study, 2002 | 3–4 dm³/s per m² | Above or beside the heater | Under the benches, ~30 cm up, or a ~10 cm gap under the door |
| Saunologia | 3–7 dm³/s per m² | Above the heater, midway to the ceiling | Below bench level, ideally under the benches |
| Harvia, electric | 6 air changes / h | Mechanical: high. Gravity: beside or under the heater, 50–100 mm | Near the floor, far from the heater; gravity pipe twice the supply diameter |
| HUUM, gravity | – | Near the floor under the stove, or above it no lower than 80 cm | Opposite wall, at least 20 cm above the supply and no higher than 60 cm |
They agree on the essentials: bring air in where the heat is, take it out low, and keep a high drying vent closed while you bathe. The differences are in how low the supply may go in a gravity system and how much air is enough.
During warm-up, bathing and after
| Phase | Supply and low exhaust | High drying vent |
|---|---|---|
| Warm-up | Can be turned down. In our warm-up model this saves only a few minutes, because heating the wood and stones takes most of the time. | Closed |
| Bathing | Open. This is when fresh air matters for breathing and for soft löyly. | Closed, or your feet go cold |
| After | Open | Open, or leave the door open, until the room is dry |
Common mistakes
- Supply low and exhaust high. Fresh air shoots straight up and out. Löyly stays at shoulder height and your feet stay cold; Saunologia reports head-to-feet differences of up to 40 °C in poor layouts against under 10 °C in good ones.
- Supply blowing on the heater's sensor. The heater switches off early. Harvia asks for a minimum distance between the two.
- Only a ceiling exhaust, always open. The steam leaves before it reaches you. If the exhaust has to be in the ceiling, a duct running down to bench level fixes it cheaply.
- Exhausting to the washroom without a gap. Harvia asks for at least 100 mm under the door and mechanical exhaust in the washroom.
- A wood stove in a depressurised house. If the house's ventilation pulls harder than it supplies, smoke and carbon monoxide can be drawn into the room. Give the stove its own combustion air.
Ventilation in a home is one connected system. Use these figures to plan and discuss, and have a ventilation designer or installer set the final sizes and balance the airflows. Building rules differ by country and region.
Sources
- Harvia support: Ventilation in the sauna
- HUUM: Sauna ventilation: what are the most important considerations?
- Lassi A. Liikkanen, Sauna ja koneellisen ilmanvaihdon ratkaisut, Saunologia, updated 2025
- Saari, M. et al., Terveen saunan tekijät, VTT Research Notes 2144, 2002. VTT (PDF)
- RAVA3Pro interpretations of the Finnish decree 1009/2017 on indoor climate and ventilation. KIRAHub (PDF)