Evaporation in a sauna

Evaporation in a sauna is the phase change by which liquid water becomes water vapour at hot stones, wet skin or damp room surfaces. The change requires energy; where that energy is drawn from the liquid or the contacted material, evaporation cools that source. Condensation is the reverse change and transfers latent energy at the surface on which liquid forms.

Within a sauna, those phase changes form part of a coupled mass-and-energy balance. Water applied to stones, sweat secreted onto skin and moisture retained by room materials have different sources and destinations. Their behaviour cannot be inferred from the amount poured, a single humidity reading or visible wetness alone.

Physical basis

Evaporation occurs at a liquid surface and can take place below the boiling temperature. Boiling also creates vapour within the liquid when local thermodynamic conditions permit. The enthalpy needed to heat and vaporise a given mass of water varies with pressure, temperature and state, so a single latent-heat value is an approximation tied to specified conditions.

Net evaporation from a wet surface is governed partly by the difference between the water-vapour pressure at that surface and in the adjacent air. Surface temperature, liquid composition, wetted area, air movement, boundary-layer resistance and the available supply of energy also matter. Saturation is an equilibrium between moist air and a condensed water phase at a stated temperature and pressure, not evidence that the gas is a vessel filled to a fixed water capacity.

Water vapour is normally invisible. A white plume is visible because it contains suspended liquid droplets formed by condensation, though splashing or incomplete vaporisation can also entrain droplets. Vapour, droplets and bulk liquid must therefore be distinguished in a water balance.

Relative humidity compares the actual water-vapour pressure with a saturation reference at the current temperature. It changes when temperature changes even if no vapour is added or removed, and does not by itself give either vapour mass or the local pressure difference driving evaporation. In the wider Sauna climate, temperature, pressure, location and time must accompany an RH result.

Water applied to stones

During löyly, liquid reaching sufficiently hot stones is warmed, and some may evaporate or boil. The energy for vaporisation comes from the contacted stone region and heater system; the phase change itself is not a source of dry-bulb temperature. A four-page model by Vesala describes the coupled sensible and latent transfers, but its university record classifies it as a non-peer-reviewed conference contribution and it does not establish a universal water dose or conversion fraction.

Applied water need not become well-mixed room vapour immediately. It can spread across hotter and cooler surfaces, remain temporarily in the bed, pass through it, splash or travel as droplets. Starting and surface temperatures, wetted area, dose and delivery rate, stone geometry, packing and heater operation all affect the transient. No retained normal-use experiment measured every route well enough to assign a general vaporised percentage.

The thermal mass of the heater and stones supplies stored energy, and the heat capacity of sauna stones is one contribution to that storage. Neither quantity fixes the rate of evaporation: contact conditions and conduction within the stones also govern how energy reaches the wet surface. A cooled surface may vaporise water more slowly even while hotter material remains deeper in the bed.

Once water has changed phase, its subsequent transport is a different process. Airflow and convection mix and carry vapour-rich air; they do not create the vapour. In mechanically exhausted VTT test rooms, ventilation arrangement and mixing affected the measured vertical temperature field and the distribution of humidity during water casting. Those mechanically ventilated configurations do not define one path or vent arrangement for every room. The complete plume, mixing and bather response are treated in Löyly physics.

One modified-room experiment further illustrates the limits of a humidity trace. Nore and colleagues applied one, two or three litres of water while recording air temperature, RH and spruce-surface temperature. The door was sealed, a ventilation duct closed, most of the heater covered with aluminium foil and the thermostat unstable. The measured responses show a coupled transient in that apparatus, not how much water normally vaporises or how another sauna should be operated.

Sweat evaporation

Sweating brings liquid to the skin; it is not identical to evaporative cooling. Only the portion that changes phase removes the corresponding latent energy at the skin. Sweat that pools, is wiped away or drips off is still body-water loss, but it has not supplied the same local evaporative heat removal.

As ambient vapour pressure increases, the gradient favouring evaporation from skin decreases. Sweat secretion can continue while evaporative efficiency falls, leaving a thicker film or more dripping liquid. Air movement may remove vapour from the boundary layer and increase evaporation when a favourable gradient remains. The same movement also changes convective exchange; in hotter-than-skin air it can increase sensible heat transfer towards the body. Fanning is therefore not inherently a net cooling action in this environment. Thermal radiation, contact and respiration contribute separately, and detailed whole-body accounting lies outside this phase-change article.

Skin wettedness, clothing or a towel, liquid-film thickness and exposed area can change resistance and the area available for evaporation. Their combined effect depends on moisture, temperature and movement; no sauna-specific protection factor follows from the general physiology. Visible wetness cannot reveal how much sweat was secreted or how much actually evaporated.

Condensation on skin

Condensation can occur when the vapour pressure beside a surface exceeds the saturation value at that surface temperature. Equivalently, the surface is below the local dew-point condition. During a humid pulse, skin may be cooler than the adjacent vapour-rich air, allowing water derived from the stones to condense and release latent energy there. Suppressed sweat evaporation and condensation can therefore change the heat exchange without requiring a comparable sustained rise in dry-bulb temperature.

Zech and colleagues tested the origin of liquid by labelling the water thrown onto the stones with stable isotopes. Their two protocols and two mass-balance calculations attributed mean proportions of approximately 30–54 per cent of the collected dripping liquid to condensate derived from that water. The result shows that dripping liquid need not be pure sweat. It is not the condensate fraction of all skin moisture, all secreted sweat, room vapour or body heat transfer, and cannot be assigned to every session.

A 2026 experiment analysed 50 healthy, recreationally active adults during four ten-minute sessions and monitored temperature and RH at six positions. In adjusted models, each environmental variable was independently associated with changes in heart rate and core temperature. A dew-point-minus-skin-temperature index had a weak association with heart-rate change and no statistically significant association with core-temperature change. Participants chose their own water applications, the rooms used four different heater types and humidity was not randomised. The findings are short-term associations, not proof of a causal increment, a condensation threshold or an exposure prescription.

Room surfaces and drying

Liquid can remain on benches, walls and floors, while hygroscopic materials take up and later release moisture. Drying combines drainage, desorption and evaporation. Residual heat supplies energy and air replacement can remove vapour, but supply-air state, distribution, surface temperature and wetness all affect the result. A greater ventilation rate alone cannot be converted into a universal drying or evaporation rate.

The Nore experiment found a larger measured transient at spruce surfaces than in room-air temperature and interpreted this in terms of moisture sorption by dry wood. Its altered test room and small number of trials limit transfer to ordinary use. In selected Finnish public and apartment-building saunas, VTT field work documented continuing condensation on relatively cool tiled seating and different drying behaviour at warmer wooden seating. Because material, temperature and installation varied together, this does not establish an inherent ranking of finishes or a standard post-heating time.

Measurement and interpretation

An evaporation study needs an explicit mass and energy boundary. Depending on its purpose, measurements may distinguish applied water, drainage, liquid retained on materials, sorption, exhaust vapour and condensate, alongside stone and surface temperatures, heater state, air state, flow and time. A local humidity sensor records an air condition, not an evaporation rate or the fate of every drop.

Short humidity pulses may be comparable to an instrument's response time, and droplets or condensation on a probe can produce misleading readings. National Physical Laboratory guidance therefore stresses response, robustness at hot and wet extremes, calibration and the intended measurement range. ISO 7726:2025 specifies general minimum characteristics and methods for instruments measuring physical quantities in thermal environments. Its public scope does not prescribe a sauna evaporation test or certify a consumer humidity sensor.

Neither phase-change physics nor one room study yields a universal water dose, ventilation rate or exposure time. ISO 7933:2023 models predicted heat strain for average healthy workers and explicitly does not predict an individual response; it requires expert interpretation and is not a personal sauna prescription.