By The Sauna Company · Published 10 Nov 2026
Good sauna ventilation design does two jobs: it removes the CO2 that bathers breathe out, and it circulates air so the top and bottom of the room are not wildly different temperatures. Get it wrong and the sauna feels stuffy, the top bench scorches while your feet stay cold, and people leave feeling light-headed.
Key takeaways
- A poorly designed sauna is, first and foremost, a poorly ventilated sauna.
- Exhaled breath carries roughly 40,000–60,000 ppm of CO2, against about 370 ppm in outdoor air, so a closed hot room changes quickly.
- Research has linked indoor CO2 of 1,000 ppm to measurably poorer decision-making compared with 600 ppm.
- Inlet and outlet positions, vent sizes and air change rates have to be calculated for each room and heater.
- Basement and other indoor saunas need more ventilation planning than outdoor ones.
Is the stuffy feeling CO2 or a lack of oxygen?
Each person in an average sauna has a space of roughly 6ft x 4ft x 3ft, about two cubic metres of air. Every breath they exhale carries around 40,000–60,000 ppm of CO2. The UK Health and Safety Executive puts normal atmospheric CO2 at about 0.037%, roughly 370 ppm. Put two or three people in a small sealed room, add heat that makes them breathe faster, and the CO2 level climbs steadily.
The feeling of confusion, dizziness or suffocation that people describe in a sealed space comes from too much CO2, not from running out of oxygen. The HSE lists headaches, dizziness and confusion among the effects of raised CO2. Effects show up well before those symptoms, too. In a 2012 study in Environmental Health Perspectives, Satish and colleagues found that people working at 1,000 ppm CO2 scored moderately but significantly lower on six of nine decision-making measures than at 600 ppm, with larger drops at 2,500 ppm. The study was done in an office-like room, not a sauna, and the authors asked for their results to be confirmed, but it shows that air quality matters at levels most people would not notice.
This is also why steam on stale air never feels like good sauna steam. Ladling water onto the stones adds moisture and heat, not fresh air. Our post on what makes good sauna steam goes into this.
What are the two goals of sauna ventilation?
Our primary design goal is effective ventilation for the removal of excess CO2. The second is air circulation that keeps the temperature difference between the upper and lower layers of air as small as possible.
1. Air changes. Fresh air has to come in and stale air go out at a steady rate. Heater manufacturer Harvia recommends that the air in a sauna with an electric heater changes six times per hour.
2. Circulation. Hot air rises. Without good circulation, the ceiling can be far hotter than the floor, so the person on the top bench is uncomfortable while the person below is barely warm. The airflow has to pass the heater so incoming air is warmed and mixed rather than dumped onto the bathers.
Where do the vents go?
There is no single right answer, because it depends on the heater, the room and whether ventilation is mechanical (fan-assisted) or gravity-driven. As a published example, Harvia's guidance for saunas with electric heaters says:
- With mechanical ventilation, the supply air vent goes in the upper part of the sauna.
- With gravity ventilation, the supply air comes in low, beside or below the heater.
- The exhaust vent goes near the floor, as far from the heater as possible.
- The supply vent should be 50–100mm in diameter, and the exhaust pipe about twice the diameter of the supply.
- Incoming air must not cool the heater's temperature sensor, which should sit where the heater manual specifies.
We treat manufacturer guidance as a starting point. With every sauna we supply calculations of air volume, heat loss and heater capacity, and drawings for the ventilation fittings layout, so your builder can form the openings and ducts in the right places before we arrive.
How do indoor and outdoor saunas differ?
Outdoor saunas are fairly straightforward to ventilate, because fresh air is on the other side of the wall. Indoor saunas, especially in basements, need more planning: where the fresh air comes from, where the stale air goes and how to route ducts through an existing house. Small rooms such as converted WCs or under-stairs voids hold less air, so CO2 builds up faster and the design margin is smaller. None of that rules them out. It just means the airflow has to be designed before the cladding goes up.
Can climate control even out the layers?
Even a well-ventilated sauna has some temperature layering, which is why traditional saunas use high benches. Climate control is one way to reduce it. According to Saunum, its Saunum Base captures hot steam near the ceiling and mixes it with cooler, more oxygen-rich air near the floor, giving a more even temperature. We use it in our climate-controlled saunas (Aqua Spazio Saunum), priced from £19,995 fully installed, excluding VAT and builders' work. Because it reduces the need for high benches, it suits modern single-bench layouts, and it can be retrofitted to many existing saunas with remote-controlled heaters.
How can you tell a sauna is badly ventilated?
- It feels stuffy or heavy within a few minutes, even at moderate temperatures.
- The top bench is unbearable while the floor is cool.
- People come out light-headed or with a headache.
- There is a musty smell, or mould and mildew between uses. Unpleasant odours are one of the five signs a sauna needs attention that we list on our refurbishment page.
What did we build in Notting Hill?
In Notting Hill, a disused spare WC became a two-person sauna. The design included a custom ventilation strategy to keep the temperature even, which matters all the more in a room this small. The walls are Thermo Aspen laid horizontally, with minimal warm lighting and a Harvia wall-mounted heater with built-in controls.
FAQs
Does a home sauna need ventilation?
Yes. Without ventilation, exhaled CO2 builds up in a small hot room and causes stuffiness, dizziness and headaches. Ventilation also evens out the heat between the top and bottom of the sauna and helps it dry out after use.
Where should the vents go in a sauna?
It depends on the heater and whether ventilation is mechanical or gravity-driven. Harvia's guidance, for example, puts the exhaust near the floor as far from the heater as possible, with the supply either high (mechanical) or low beside the heater (gravity). Each design should be calculated for the specific room.
How many air changes does a sauna need?
Harvia recommends six air changes per hour for a sauna with an electric heater. The right rate for your sauna depends on its volume, heater and number of users.
Why do I feel dizzy in my sauna?
The most common design cause is poor ventilation letting CO2 build up. Heat, dehydration and staying in too long also contribute. Keep sessions to 15–20 minutes, drink water afterwards and leave if you feel unwell.
Can ventilation be improved in an existing sauna?
Often, yes. Ventilation can be reworked during a refurbishment, and climate control such as the Saunum Base can be retrofitted to many existing saunas. An assessment of the room and heater is the first step.
Talk to a sauna designer
We design the airflow before the finishes. Book a free design consultation, or email photographs and dimensions of your space to info@saunacompany.co.uk for an initial assessment. Read more on our sauna design page.
Planning one for your own home? See our bespoke home saunas page for options and recent projects, or book a free design consultation and we'll talk through your space.
Sources
- Satish et al. (2012), Is CO2 an Indoor Pollutant? Environmental Health Perspectives: decision-making declines at 1,000 and 2,500 ppm CO2.
- HSE: General hazards of carbon dioxide: atmospheric CO2 about 0.037%; headaches, dizziness and confusion at raised levels.
- Harvia: ventilation in the sauna: six air changes per hour, vent positions and diameters.
- Saunum Base product leaflet: manufacturer description of mixing ceiling steam with cooler floor-level air for a more even temperature.