In a traditional dry sauna, the temperature and humidity levels interact and influence each other directly. Both can be controlled by splashing water on the rocks, creating steam, or löylyä that is essential for a true Finnish sauna experience. When you splash water on sauna rocks, water vapor transfers heat energy from the rocks to the surrounding air, thereby raising both the heat and humidity levels throughout the sauna.
But how hot should a sauna be? What’s the best humidity level for an authentic Finnish sauna? We recommend the ‘Rule of 200’ to determine the perfect balance of heat and humidity for a comfortable sauna experience with all the health benefits.
Ideal Temperature For a Sauna – Use the Rule of 200
Due to their interdependence, the ideal sauna temperature and the ideal humidity level need to be achieved together. We recommend using an old Finnish ‘Rule of Thumb’ to achieve the perfect temperature including humidity – it’s known as the ‘Rule of 200.’
The “Rule of 200” says the sweet spot for most people is when the Temperature and Relative Humidity equals 200 degrees fahrenheit. The theory rests on the idea that a bathers comfort is a function of temperature and humidity.
So if the temperature is 165 F, there could be up to 35% humidity before some sauna users feel uncomfortable. As the temperature rises, the humidity should go down, which will happen naturally.
You may notice that 200 (t+rh) is a natural sweet spot for your sauna. As the temperature rises, you will need to make a lot of steam to keep the relative humidity above 30% (if you have proper ventilation). The amount of water vapor air can hold increases significantly as you approach 200 degrees.
Sauna Thermometer and Hygrometer
You need an accurate sauna thermometer and hygrometer combo to find the sweet spot. This one works well:
The the Hygrometer, it becomes clear why some saunas seem way hotter or cooler than they are. It will become easy to have a perfect sauna at a lower temperature. And once you figure out your perfect sauna conditions you’ll be able to dial it in every time, and even automate it.
Hot and Dry Sauna Benefits
Hot and Dry saunas are better for aiding the recovery of a workout by making muscles relax.
Very hot sauna sessions can also help condition the cardiovascular system in a way that mimics actual exercise. If you’ve read about how sauna sessions have the same effects as working out, they are probably talking about hot, dry sauna sessions.
Hot dry sauna sessions are also going to be the best way to get healthy skin benefits of sauna. Simply, hot dry saunas create the perfect environment for sweating.
Sweating improves blood flow to the skin, removes dead skin cells, and can remove toxins.
How hot is too hot? Well, the World Sauna Championships were discontinued after one finalist died and the other was sent to the hospital after spending 6 minutes at 110C (230F). Listen to your body, and know your first few sessions might be short. Sauna is not a competition.
Steamy, Less Hot Sauna Benefits
Sauna is not a competition, and many consider the very hot, dry sauna to be abrasive. They would prefer a gentler heat that they can stay in for longer and really relax.
The löyly at softer temperatures is considered gentler and more therapeutic than in extremely hot saunas, where the water seems to explode as it hits the rocks.
Soft, steamy heat is also best if you’re using skin care products that you want to penetrate deep into your skin. The steam opens pores.
Gentler heat is also excellent for meditation, yoga, and other spiritual practices where heat is not the only focus.
If you have children or folks with medical conditions in the sauna, you will also want to start with a very soft heat.
About Relative Humidity
Relative humidity is a measurement of the ratio of water vapor at a given temperature. The amount of water vapor it can hold increases as temperature rises.
People often confuse relative humidity with absolute humidity in saunas. Since we are measuring relative humidity, the sauna may not feel (or be, in absolute terms) any less humid as temperature rises, even though relative humidity drops.
Getting it Right With Electric Stoves
It’s important that the sauna rocks are heated first, and the room is heated by the rocks and steam. If you or the room are being heated directly by the heating coils or fire, it will likely feel harsh or unpleasant.
Wood burning saunas typically heat up faster and provide additional sounds and glows that add to the experience. You can find the best wood burning sauna heaters here.
Infrared saunas are unable to reach 200 as a function of temperature (f) and relative humidity (rh). The maximum temperature range is 105-120 F and they do not offer humidity control.
Steam Bath Temperature
Steam baths are completely different than infrared or traditional dry saunas – they are extremely high humidity and relatively low temperature (around 130 degrees F). Learn more about steam baths and how they differ from saunas here.
Is the rule of 200 an exact science? Absolutely not. But it is a good way to think about how heat and humidity interact in the traditional sauna. HELO even includes it in in their sauna heater manuals and guides.
Get on the bench and experiment.