The Hidden Physics of Löyly: Why Steam Changes Everything in a Sauna
Depending on where you are in the world, the sauna conversation sounds very different. In Finland, the question that defines a sauna session is a simple one: shall we throw more löyly? Löyly — water thrown onto hot stones to create steam — is not an optional extra. It is the ritual. It is what separates a good sauna from a great one, and experienced bathers will happily debate the merits of one ladle versus two, of birch-infused water versus plain, of the perfect moment to throw. Some of the images in this article are AI generated.
In newer sauna markets — much of the United States, large parts of Asia, and many places where sauna has only recently arrived — the conversation is different. People talk about temperature. They ask how hot it should be and how long they should stay. In some traditions, throwing water on an electric heater is actively discouraged, or simply not part of the culture at all. Sauna, in these contexts, is understood primarily as dry heat.
This is not a small cultural difference. It means that a significant share of the world's sauna users are having a fundamentally different experience from what Finnish tradition considers the real thing — and may not even know what they are missing.
It also means that sauna science, which has largely followed the same assumptions as the broader culture, has focused heavily on temperature and duration while leaving the role of löyly almost entirely unstudied. The steam that Finns have considered essential for centuries has barely registered in the research literature.
At Harvia Labs, we decided it was time to take löyly seriously — not just as a cultural tradition but as a physical phenomenon with measurable, meaningful effects on the human body. What we have found is surprising, even to us.
What We Actually Measured
In a large collaborative study conducted in spring 2025, Harvia, Toyota, and KIHU (the Research Institute for Olympic Sports) brought together 69 participants across four different sauna types: wood-burning, hydrogen, electric, and LPG gas. Each sauna was equipped with 26 sensors, and each participant was monitored across 11 biomarkers. It was the first study of its kind to simultaneously capture both the sauna room environment and the physiological response of participants in real time.
One of the most striking things the data revealed was just how differently people use a sauna when left to their own preferences. Across 96 ten-minute sessions — all at approximately 75 °C — the variation in löyly behavior was extraordinary. Some groups threw as little as 120 grams of water; others used nearly 3 kilograms. Some threw löyly once; others ten times in a single session. The result was an average relative humidity ranging from 6% to 59%, and a dew point temperature ranging from 24 °C to 57 °C. Same sauna. Same temperature setting. Completely different thermal environments.
This raises an obvious question: does that difference matter? It does — far more than most people realize.
The Fourth Mechanism of Heat Transfer
A sauna heats the body through three mechanisms that most people have heard of: convection (hot air circulating around the body), radiation (infrared heat from the heater and surfaces), and conduction (direct contact with hot benches and air). There is a fourth mechanism that receives almost no attention in either popular writing or scientific literature: condensation.
When löyly is thrown and the dew point temperature in the sauna rises above skin temperature — which in a sauna is approximately 41 °C — water vapor begins to condense directly onto the bather's skin. This is the same physics you see when a cold drink sweats on a warm summer day, but the energy flows in the opposite direction. Instead of heat leaving the glass, latent heat is released onto the skin as vapor transitions back to liquid. The process is efficient and rapid.
To quantify this effect, Harvia Labs developed a proof-of-concept measurement apparatus — a sealed ventilation pipe filled with 41 °C water to mimic skin temperature, suspended inside a sauna cabin and weighed second-by-second from a scale mounted on the roof. As löyly was thrown, the pipe gained mass as moisture condensed on its surface. That mass gain was converted to energy — and then to power per square meter of skin.
In a dry sauna at the same temperature, that entire layer of condensation heating simply does not occur. The experience may feel the same in temperature, but the thermal input to the body is substantially different.
0 W/m²
Peak condensation power at 80 °C: the strongest heat burst as löyly steam condenses on your skin.
0 W
Peak load on a person (1.5 m² skin): the most heat your whole body absorbs at the hottest moment of a steam wave.
0 W*
111-second average: the gentle extra warmth löyly steam adds for about two minutes, on top of the sauna’s normal heat. *additional to other heat sources
Two Saunas at the Same Temperature — Two Completely Different Experiences
The implications of this became very concrete in the human testing phase conducted in September 2025, where 11 participants from Harvia R&D and outside volunteers took part in controlled sessions in two identical cabins: one humid (with regular löyly bursts) and one dry, both at 60 °C and 80 °C. Each participant swallowed a core temperature capsule, wore skin temperature probes on the forehead, chest, and foot, and had heart rate monitored continuously.
The core temperature data showed clearly that even at identical air temperatures, humid löyly conditions produced meaningfully different thermal trajectories through the body — with core temperature continuing to rise after the sauna session ended, as the heat absorbed through condensation made its way inward. In a dry session, core temperature dropped more quickly once the participant left the sauna. Two people sitting in saunas at the same thermostat reading were having, in measurable physiological terms, different experiences.
What this means for how we understand and communicate sauna is significant. Temperature alone — the single number we have traditionally used to describe a sauna — does not tell the full story of what is happening inside it.
Why This Matters for Sauna Science
The focus on temperature in sauna research is understandable: it is easy to measure, easy to communicate, and has been the standard variable for decades. But temperature is only one input into the actual thermal load the body experiences. Humidity — and specifically the dew point, which determines whether condensation occurs and at what rate — is another input of comparable importance. Ignore it, and you have an incomplete picture.
This is part of what motivates the ongoing research program at Harvia Labs, conducted in collaboration with the University of Jyväskylä. The central research question we are now pursuing is: what happens physiologically when löyly increases humidity, and how does that affect the health and recovery outcomes that sauna is associated with?
We think the answer will matter — not just academically, but practically. For anyone who has wondered why the same temperature in one sauna feels completely different from another, or why the löyly thrown by experienced bathers produces something that a dial setting simply cannot replicate, the physics of condensation may be a significant part of the explanation.
Sauna is more than temperature. Löyly is not just tradition. It is heat transfer — and understanding it precisely is what we are here to do.
Humidity Has Been Overlooked in Sauna Research
A new study measures, for the first time, both the sauna environment and the human body simultaneously — and finds that humidity is an independent force
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MyHarvia Smart Sauna Sensor
If this article has made you curious about what is actually happening inside your own sauna, the MyHarvia Smart Sauna Sensor is a good place to start. Install the wireless sensor in any sauna — including wood-burning — and the free MyHarvia app tracks temperature and humidity in real time, session by session. The humidity graph gives you a tangible feel for how löyly changes the sauna environment — you can see the difference between a dry session and one with generous steam. It won't give you the precision of a research lab, but it does make the invisible visible, and it builds a picture of your sauna habits over time.
Learn more