About Sauna Temperature and Humidity

First of all, I did not study physics after high school and I am by no means an expert in physics. However, the discussion of sauna temperature and humidity has been bothering me so I thought I try to explain a few basic things in a semi scientific way. This blog mentions the word löyly about 400 times. Löyly means the steam coming out of the stove when you pour water on the stove. I say when and not if. If you are not generating löyly, you are missing out.

As I mentioned in the Generating Perfect Löyly blog post, the sauna room temperature does not always correlate with the sauna experience because:
1. You are supposed to get warm and sweaty by gentle löyly.
2. The temperature highly depends on the type of your sauna. 215F feels about right in a smoke sauna but would be a torture in a small electric sauna.
3. Sauna thermometers and hygrometers usually suck. If you think your sauna is 200 degrees warm, check the thermometer first.

Another common question in forums is the humidity. Again, this is often measured by relative humidity. That in turn highly correlates with temperature so it is not a meaningful metric.

Science project begins

I have been wondering, why is the steam and the sauna experience actually very good in barrel saunas. These saunas break about every rule in the sauna rule book: they are small, often heated with an electric stove and you sit below the top of the stove whereas you are generally supposed to keep your feet at the same level with sauna rocks and rest of your body above that.

My sauna is a regular 4-person barrel sauna (Almost Heaven Pinnacle). I have introduced it in a separate Pöllä 3 Sauna blog post. It has a 6kW heater. The inside lenght is 160 cm (5 ft 4 in) and the inside barrel diameter is 175 cm (5 ft 10 in). So the volume is approximately 3.85 cubic meters. The rest of the blog is using metric system so bear with me.

I wanted to see if something weird happens with löyly in these saunas. What happens in löyly, stays in löyly.

I found out about a Finnish startup Ruuvi (translates to a screw). They develop wireless bluetooth temperature and humidity sensors and their website talks about sauna as one use case for their product. The tag costs about 30 dollars and you can use it to alert on your phone when your sauna is warm. Wireless temperature tags are not novel but the fact that they promised the sensor to work at up to 85C is novel. They go even as far as selling custom lithium batteries that can bear higher temperatures. I thought this was a marketing gimmick but I could not find similar batteries from anywhere else. I guess I know what to bring back from Finland next time.

Anyway, finding a Finnish tech startup that mentions sauna as a use case was this “solution looks for a problem” moment to me. Shut up and take my money. Four sensors arrived in a few days from Finland to Texas with a hand-written note from the founder. Well done!

Ruuvi has a very simple app that allows you to record the readings with a smartphone and then save the log files as CSV for further analysis. The units store data locally for 10 days and you can download the dataset to your phone.

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Sauna temperature

I placed three sensors in my sauna: one on the floor, one on the bench and one on the top.

I then heated up my sauna a bit warmer than usual. I usually go in when the temperature is in the 60’s in Celsius. This time, I started when the top sensor was showing 75c. The temperature on the bench was 53c and on the floor 38c. Btw, the regular sauna thermometer next to the top sensor showed 85c at this point. I always knew that meter sucked.

Quite dramatic 50% difference between the floor and the top. Why?

Hot air is lighter than cold air so hot air elevates to the top. Barrel saunas are small and the air circulates relatively well. The ventilation is based on the idea of having air vents below the stove. Warmer air escapes from the sauna via the top vent on the opposite side or from above the door (remember the The “Ovi Kii” Rule). This lowers the air pressure and creates a small vacuum that sucks new colder air from those vents. This is very simple yet brilliant mechanism that ensures that oxygen is added to the room and that sweaty smelly air escapes the room. You can control the flow by opening or closing the top vent on the wall. Another way to control the ventilation is to pour water on the stove.

Humidity

Most people refer to humidity via the relative humidity metric. The percentage tells you “the amount of water vapor present in air expressed as a percentage of the amount needed for saturation at the same temperature” (definition by Oxford Languages). This dew point increases as temperature increases. The correlation is very steep: if the air has 50 grams of vapor in a cubic meter, the relative humidity is 100% at 20c but less than 20% at 80c. 20% humid sauna at 80c has about the same amount of vapor it the air than 50% humid sauna at 60c.

Why would humidity matter then? Because that is the basic principle of löyly. You convert water from the bucket to vapor, which mixes with the air and hits your body and turns back to water again. And.that.feels.good.

Only a portion of that moisture on your body in sauna is actually your sweat. Most of it is that löyly that finds a colder element (like your heart) in a sauna and hits your body, your body cools the steam down and the steam converts back to water. This has to do with the dew point being very different on your 37c skin vs. 80c air. Fascinating topic and a good reason to learn Finnish is to read more about this in for example Saunologia.

What does löyly do to the temperature and humidity?

Common sense says that sauna gets warmer when you add steam to the room. It definitely feels so. People that read books could claim that this is not true because you the energy in the room stays the same: you move water from the bucket on the sauna rocks to the air. Temperature should not change. But does it?

I started the session by pouring about 200-300 grams water on the stove. I then enjoyed the steam, waited for a while and added another 200 grams or so. The chart below shows how temperature kept increasing. Part of that was due to the stove being on but the change seemed to be accelerated as I added löyly. The graph below also highlights, how dramatically the humidity increases in a small sauna by adding a ladle of water. That was expected, right?

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Another way to understand whether the steam actually increases sauna temperature is to look at the temperature changes in different sensors. Your can see that the temperature increases fastest during this löyly session on the top sensor so I’d say that löyly increases sauna temperature in the steam area (above the bench) if the bucket is on the floor.

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Back to humidity. In the first chart, you could see how the humidity increased by adding water. At first, it may seem that my second löyly was weaker than the first one. During the first round, the humidity rapidly went from 5% to 17% and then to 22% as I added a bit more löyly. On the second time it went from 4% to 10% to 14%.

However the temperature in the first round was 70c and on the second round 80c so relative humidity is a poor metric for comparison. Let’s look at this through absolute humidity. The chart below shows that absolute humidity peaked at about 45 grams / m3 on both times. Now remember that my sauna is about 3.85 m3 big and I added maybe 200-300 grams of water on the stove. Some of that did not convert to steam and some steam escaped but the numbers sort of add up.

Another interesting thing to look at is “where does that löyly go” in a sauna. Again, vapor is about 100c warm (can be actually a bit warmer) so common sense says that it goes up because it is warmer than the air in my sauna. Because there are those vents under the sauna stove and because the opposite wall is not airtight, the steam travels to the top of the opposite wall. You can see this in the chart below that show absolute humidity on different sensors. Humidity on the floor doesn’t change, on the bench level it almost doubles and on the top it quadrupled.

Conclusions

• I will continue my day job in IT and leave physics to someone else. Read saunologia for more scientific articles and the Finnish Tiede magazine forum.
• The above sources are a good reason to learn Finnish
• Sauna temperature doesn’t matter. Löyly is always about 100c and that gently pats your skin. Löyly is btw about 100c even if the sauna temperature is above 100c. A nice way to cool down
• Relative humidity is very poor metric in sauna but more humid air generally feels better and the best humidifier is the ladle
• You sweat much less in a sauna than you think. Most of that “sweat” on your skin is actually löyly that cools down on your relatively cold skin and converts back to water. You will still sweat a lot so stay hydrated. This topic is worth another blog post.
• Sauna IoT is the new black – throw your old thermometers away and transform your life with a data-driven saunology.