Sauna Hat vs Wet Towel: What the Wet-Bulb Temperature Actually Says

By Fuentes, founder of Lanoviss. Published 1 September 2026.

A soaked towel on your head is colder than a dry wool hat. That is not a concession made reluctantly — it is physics, and any sauna hat seller who tells you otherwise is hoping you will not check. Evaporation pulls a wet cloth down toward the room's wet-bulb temperature, and in a 2023 study of eight Finnish saunas the air read 74 to 120 °C while the average wet-bulb read 35 to 46 °C.

The interesting question is not which is colder in the first two minutes. It is what happens over a twenty-minute session, and what happens in the thirty seconds after someone throws water on the stove. That is where the two approaches diverge, and where the wet towel has two specific failure modes worth knowing about before you decide you do not need a hat.

What is the wet-bulb temperature in a sauna, and why does it matter?

Wet-bulb temperature is the lowest temperature any evaporating wet surface can reach in a given environment. It is the floor for a damp towel, and it is also the floor for your own sweating skin.

The numbers come from a 2023 measurement study by Saunalogia, run with instrumentation from Vaisala, the Finnish measurement company. Eight saunas were measured using an Indigo80 handheld with HMP7 and GMP252 probes. Sensors sat on a floor stand positioned 30 cm below the ceiling, sessions ran 15 to 26 minutes, and at least two scoops of water were thrown on the stove during each.

The gap between those two columns — as much as 70 °C in the hottest room — is the entire mechanism a wet towel runs on. It is a large gap, and a wet towel genuinely exploits it. For context, Professor Tuomo Tuomola recommended a wet-bulb range of 45 to 53 °C for sauna bathing in Sauna magazine back in 1969; several of the modern saunas measured here exceeded the humidity limits that older Finnish guidance would have called hard bathing.

How does a wet towel actually cool your head?

By evaporation, not insulation. Those are different mechanisms, and confusing them is where most of the bad advice in this category comes from.

Water leaving the cloth as vapour carries away energy — the latent heat of vaporisation — which is why the towel stays well below air temperature while it is still wet. Your body uses the identical mechanism when it sweats. The towel is essentially an external, oversized sweat gland sitting on top of your scalp.

A dry wool hat does something entirely different. It never gets colder than the room. It slows the rate at which the room's heat arrives at your scalp, by trapping still air in the felt. We went through that mechanism in detail in do sauna hats actually work.

Does a wet cloth insulate better or worse than a dry one?

Considerably worse. Water is a good conductor of heat, and soaking a fabric replaces the still air trapped in it with that conductor.

So a wet towel is a cooling device that is simultaneously a poor insulator. While evaporation is running, the cooling wins. The moment it stops, you are wearing a thin sheet of heat-conductive material on the hottest part of the room.

Wool is unusual here in that it retains a meaningful amount of its insulating capacity when damp, because of how the fibre holds moisture within its structure rather than only in the spaces between fibres. Cotton does not — a wet cotton towel is, thermally, close to wearing water. That difference is one reason the traditional Finnish hat is wool and not terrycloth, which we cover in merino vs wool vs synthetic.

Why does a wet towel stop working partway through a session?

Two failure modes, and both arrive at inconvenient moments.

It dries out. In a room at 74 to 120 °C, a towel gives up its water quickly. When the water is gone, so is the cooling, and you are left with the poor insulator described above. In practice this means getting up to re-wet it, which in a shared sauna means opening the door and dropping the room temperature for everybody else.

Löyly closes the gap. This is the more interesting one. When someone throws water on the stove, humidity in the room spikes. Higher humidity means the air is closer to saturation, which means the wet-bulb temperature rises toward the air temperature — the gap between the two columns in the first table narrows. A wet towel cools by exploiting that gap. So the towel loses effectiveness precisely at the moment the sauna feels most intense.

A dry wool hat is indifferent to this. It is not running on evaporation, so nothing about a humidity spike removes its mechanism. Its performance at minute nineteen is the same as at minute one.

Sauna hat vs wet towel: which should you use?

Is a wet towel ever the better choice?

Yes, and we would rather say so than pretend otherwise. It costs nothing, you already own one, and for a short session in a moderate room it will keep your scalp cooler than any dry hat can. If you are undecided about whether you even have a head-heat problem, the towel answers that for free.

The case for a hat is narrower and more specific than most sauna hat marketing admits. It is for longer sessions, for regular löyly, and for the top bench — the situations covered in how much hotter it is at head height. If your sessions are short and the stove stays dry, buy nothing.

If they are not, our 100% merino wool sauna hat is 3 mm of pressed seamless felt, $59.99 CAD, shipping from Vancouver, with the full colour range in forest, navy, cream and pink.

"Very reliable and generous seller. Highly recommend."
— Kirill, verified Etsy buyer, August 2026

Frequently asked questions

Is a wet towel colder than a sauna hat?

Yes, while it is still wet. Evaporation pulls a soaked towel down toward the wet-bulb temperature of the room, not the air temperature. In a 2023 Saunalogia study of eight Finnish saunas measured with Vaisala instruments, air temperatures ran 74 to 120 °C while average wet-bulb temperatures were 35 to 46 °C. A saturated cloth in that room trends toward the lower figure. A dry wool hat does not get colder than the air — it slows the air's heat from reaching your scalp.

Why does a wet towel stop working partway through a sauna session?

Two reasons. First, it dries. Once the water has evaporated the cooling stops and you are left with a thin cloth that insulates poorly. Second, throwing water on the stove raises humidity sharply, and higher humidity narrows the gap between air temperature and wet-bulb temperature — exactly the gap the wet towel depends on. The towel is least effective at the moment the sauna is hottest.

Does a wet cloth insulate better than a dry one?

No. Water conducts heat roughly 15 times better than dry wool — about 0.6 W/(m·K) against roughly 0.04 — and still air is lower again at about 0.026. Soaking a fabric replaces its insulating trapped air with a good conductor. A wet cloth cools you by evaporating, not by insulating, and those are different mechanisms with different failure points.

What is the wet-bulb temperature in a sauna?

In the 2023 Saunalogia measurements across eight Finnish saunas, average wet-bulb temperatures ranged from 35 to 46 °C while air temperatures ranged from 74 to 120 °C. Sensors were mounted on a floor stand 30 cm below the ceiling, sessions ran 15 to 26 minutes, and at least two scoops of water were thrown on the stove in each.

Can I just use a wet hat instead of a dry one?

You can, and some bathers dip a wool hat in cold water before entering. It gives you the evaporative cooling of a wet towel plus more mass, so it stays wet longer. The trade-off is that while it is saturated the wool is conducting heat far better than it does dry, so you lose the insulation you bought it for. Wetting the hat is a legitimate technique for very hot rooms; it is not the default.

Which is better for a beginner, a hat or a wet towel?

A wet towel, because you already own one and it costs nothing to try. If you find that ceiling-level heat is what limits your session, and you are re-wetting the towel every few minutes to keep it working, that is the point at which a dry wool hat starts to earn its price.

Does a wet towel on your head make the sauna safer?

No head covering, wet or dry, makes an unsafe sauna safe. Cooling your scalp can mask the discomfort that would otherwise tell you to leave, which is a risk rather than a benefit. Watch total time in the room and how you feel, not how cool your head is.

What about a wet towel in a steam room or banya?

Humidity is already near saturation in a steam room, so the wet-bulb temperature sits close to the air temperature and evaporative cooling barely functions. That is the same reason löyly reduces a towel's effectiveness in a dry sauna, taken to its limit. We compare the room types in sauna vs banya vs steam room vs infrared.

Sources

Wet-bulb and air temperature figures are from a 2023 sauna measurement study by Saunalogia, conducted with Vaisala instrumentation and reported by Juhani Lehto, global product manager for dew point instruments at Vaisala, in December 2024. Eight saunas; Indigo80 handheld with HMP7 and GMP252 probes; sensors 30 cm below ceiling on a floor stand; 15–26 minute sessions; two or more scoops of water on the stove. The 1969 wet-bulb recommendation of 45–53 °C is attributed in that reporting to Professor Tuomo Tuomola, writing in Sauna magazine.

Thermal conductivity values are standard published figures for still air, dry wool and water at room temperature and are given to one or two significant figures as approximations. They are used here to show relative magnitude, not to model a specific hat.

We have not yet run our own paired-session measurement — see the outstanding item above. Nothing in this article is presented as our own test data.

Heat safety note: this article is about thermal comfort, not medical advice. Sauna bathing places real load on the cardiovascular system. If you are pregnant, have a heart condition, take medication affecting blood pressure or thermoregulation, or have been drinking alcohol, speak to a doctor before using a sauna and follow their guidance over anything written here. Leave the room if you feel unwell.

Read next: do sauna hats actually work and sauna etiquette for your first visit.

Spotted an error in the physics or the sourcing? Tell us and we will correct it.