Barrel saunas are popular especially in North America. They are rather easy to fit on backyards and also relatively inexpensive and they look cool.
One challenge with this sauna design is ventilation and heat stratification. While the basic rule of löyly (steam) in Finnish saunas assume that your feet are above the top of the stove for better heat and steam experience, in barrel saunas you keep your feet on the floor. Another design choice in barrel saunas is that ventilation is often implemented using gravity based ventilation where air intake is on the floor level below stove and the exhaust is on the opposed wall on the top. This is probably the only meaningful way to implement proper ventilation without mechanical components but the downside is that this solution often creates draft on the floor and it further increases heat stratification.
I cannot really say that I was bothered by any of these ventilation or stratification issues but multiple threads in different sauna groups in Reddit and Facebook prompted me to look into this.
Electric Sauna Ventilation in Theory
There are multiple quality articles about sauna temperatures and ventilation available online. I’ve found these masterpieces the most useful:
- Saunologia.fi – Finnish Sauna Essentials Part 5 – Air quality
- VTT Research – Temperature and ventilation of the Finnish Sauna, 1992 (in Finnish)
Temperatures and humidity without mechanical ventilation
I have three temperature and humidity sensors in my sauna as illustrated below. These sensors write their readings to a MariaDB. The top sensor writes two values per second whereas the Bluetooth sensors write in about five-second intervals.
Experiment 1 – standard ventilation
For starters, I measured temperatures and humidity first by using regular ventilation: cool air was sucked in from below the stove and I kept the exhaust vent closed so the air escaped from the gaps as air pressure increased after adding steam.
Below you can see the temperature graphs of a typical sauna session. You should notice about 40c difference between the top and the bench level. Floor is typically 5-10c warmer than outside air and in this session outside air was a few degrees above freezing. I shut down the stove in the half way and you can see, how the temperature starts dropping quite quickly on the top sensor but not so much on the floor or on the bench level. The challenge is to get the bench level temperature to above 40c especially in winter.
Looking at the humidity graph – you can see the readings in the top sensor spiking as I add steam. The bench sensor indicates that not much happens on that level. In other words, the steam goes to the roof level and stays there until the hot and humid air escapes from the gaps.
Mechanical Ventilation Implementation
My conclusion after reading different articles of ventilation was that mechanical ventilation is the only viable way to go with electric saunas. In Finland, people don’t really think about this because electric stoves are mainly used in apartments and you must implement mechanical ventilation there anyway. You can still build it wrong, of course.
My idea was to bring fresh air above the stove mechanically using a 195 CFM fan from Vivosun ($30 investment). I then got a few WiFi plugs so that I could turn the vents on an off using my sauna app. Unfortunately, these fans are not adjustable so the only ways to control the flow would be by installing adjustable vents and by turning the fans on and off.
The fans are able to circulate 195 cubic feet air per minute which is about 90 liters per second. I haven’t found specific guidance on the ideal air circulation but some sources suggest 3-7 liters per m2 per second. This barrel sauna is about 3.2m2 in the middle so in any event 90 liters is probably way too much.
I added 4-inch vents above the stove and on the floor on the opposite side as illustrated in the pictures below. The location for the exhaust vent is a bit tricky due to the shape of the sauna. Somewhere on the lower part of the door would have been more ideal, but not very practical :).
Experiment 2 – timed ventilation
My first experiment was to keep the vents closed during the heat up and then use home automation so that both fans start running for 60 seconds when the top sensor detects steam.
While mechanical ventilation seemed to improve air quality (the air felt more fresh), there was little to none impact in sensors. However, this might also be because my bench sensor is a Bluetooth sensor and it is less reponsive than the top sensor that is hard-wired.
Experiment 3 – Continuous ventilation
The second experiment was to switch on both vents when I started saunaing. I have also tried to keep the ventilation on while heating up the sauna but that doesn’t seem to do much else than lengthen the heat-up time.
This setup has proven to make a bit more difference as can be seen in the graphs below. Temperature-wise, not much happens on the floor level because the floor sensor is below the exhaust and is probably affected by the air coming in from the gaps. However, the difference between the top and the bench sensor is now much less: my general observation has been that with this solution, I have been able to “move” about 10c temperature from the top to the bench. In other words, without ventilation, the top sensor temperature would be about 80c and after tuning on the vents, it drops by about 10 degrees and after a few loyly sessions, I am able to get the bench temperature from around 40 to around 50 c. This may feel like a small difference but percentage-wise, it is quite significant as without ventilation the bench temperature seemed to be about 50% of the top temperature and now it is about 70%!
Even more promising evidence can be seen on the humidity sensors where you can see about 10-20g/m3 increase on the bench level when adding steam. That scientifically proves that the steam circulates much better now. The next step is to acquire more responsive humidity sensors to prove this point. Getting AH to close about 50g whilst increasing temperature by ten Celsius actually makes difference because now the dew point increases to above 37c and the steam starts condensing on your skin on the bench level. The steam feels more “fresh”.
Experiment 4 – Continuous ventilation in summer
After about half a year, I’ve concluded that it is best to run ventilation when I start saunaing and keep it running until I finish. I don’t switch off ventilation during cooling breaks.
It get pretty warm and humid where I live so I performed another test in June when it was 102F/39C during the day. The barrel was about 113F/45C when I switched the stove on. When I started saunaing, the outside temperature had dropped a bit below 100F/37C and relative humidity was about 30%.
The readings were quite interesting this time as can be seen below. I managed to get the bench temperature above 65c with ventilation. The floor was still considerably colder but my top sensor is above my head so by keeping my feet on the opposite bench, I’d say the temperature difference between my head and legs was very minimal.
Looking at humidity graphs, you must keep in mind that my bench sensor is leas responsive than the top sensor so the spikes are less dramatic zduring löyly. What you can see though is that th edew point creeps above the body temperature on bench level that makes you think you sweat more (see the About Sauna Temperature and Humidity post). That high dew point temperature definitely makes the steam feel more moist.
After about of six months of use, I’d say that my $100 investment to mechanical ventilation does not seem to go in waste. I feel that I have now successfully reduced heat stratification between my head and the bench very significantly.
Other things to consider
In addition to this ventilation hack, I also added an extra bench that slides in an out. It allows me to sit higher up and keep my feet on the opposing bench. I have also lowered the stove two inches so the benches are now closer to the top of the stove.