How to choose a sauna hat - and why wool wins

Hot air rises. In an 85-degree sauna it can be 20–30 degrees hotter at the ceiling than at your feet. Your head sits highest and gets the hottest air, while your scalp and ears have thin skin and little protection.

That is why most people leave because their head gives up, not the rest of their body. A hat insulates against the radiant and rising heat. Your head warms more slowly and the round is more even.

In Finland and the Baltics the sauna hat is standard. In Sweden it has been rare, but it is spreading fast among frequent sauna users.

• Felted wool: best. Wool insulates against dry heat, tolerates high temperatures without melting, breathes and resists odour. Felting makes it durable and keeps its shape.
• Linen: light and cool, but insulates less. An option for lower-temperature saunas.
• Cotton: works, but gets heavy and wet with sweat and dries slowly.
• Synthetics (polyester, acrylic): avoid. They can soften or melt near the heater and start to smell quickly.

The hat should cover your ears. Ears overheat first and are the most common reason people stand up early. A hat that stops above the ears does half the job.

It should sit loosely. A tight hat presses against your head and gets hot. Air between hat and hair is part of the insulation.

Shape is taste: a classic round crown, a pointed gnome shape or a wide brim. A brim protects the face and neck a little more but is clumsy on a crowded bench.

Wood-fired sauna: this is where a hat helps most. The heat is often uneven, with hot radiation from the stove and plenty of löyly. Thick felted wool is the right choice.

Electric sauna: drier, more even heat, often 80–95 degrees. The hat mainly helps on the top bench and during longer rounds.

Infrared sauna: lower air temperature, 45–60 degrees, with heat from panels. A hat matters less here. A thinner linen or light wool hat is enough if you want one.

Steam room and hammam: near 100% humidity and a lower temperature. A wool hat soaks through quickly. Many skip it or use a thin cotton cloth.

Smoke sauna: the most traditional form, with soft but intense heat and soot in the air. Wear a hat you do not mind about. It will smell of smoke afterwards, which many see as part of the experience.

1. 100% wool, ideally felted. Check the label, not just the colour.
2. Covers your ears when worn loosely.
3. Thickness: dense enough that light does not shine through.
4. No metal details or plastic tags that heat up.
5. Can be rinsed by hand and dried flat.

Wool rarely needs washing. Hang the hat to dry after each session, ideally in fresh air. Rinse in lukewarm water when needed, squeeze gently and dry flat. No washing machine and no tumble dryer: heat and agitation felt the wool further and the hat shrinks.

Odour usually disappears if the hat airs for a day. Wool is naturally resistant to bacteria.

The sauna hat has been used in Finland, Estonia and Russia for generations. The traditional form is a felted wool hat, often handmade, with a round or pointed crown. Its job today is the same as then: to let your head keep up while the rest of your body enjoys the heat.

In Sweden the sauna has long meant a towel around the hips and nothing else. That is changing, not least through sauna clubs and winter swimming, where the hat has become part of the kit.

Wool is made of crimped fibres that trap air. Still air is a poor conductor of heat, so a thick wool hat slows the heat from the sauna air before it reaches your scalp.

Wool can also absorb moisture without feeling wet. As sweat evaporates from the fibre it cools slightly. That is one reason a wool hat stays comfortable even after several rounds.

Wool does not ignite easily and does not melt. Synthetic fibres can soften near a hot heater or hot wood. That is the most important safety difference between the materials.

A dry towel wrapped around the head works in a pinch. It insulates, but it slips, soaks up sweat and rarely covers the ears properly. After one round it usually ends up in your lap.

A hat stays put, covers the ears and dries faster between sessions. If you use the sauna often, the difference is clear within a week.

Sauna hats are usually one size because they are meant to sit loosely. Still, measure your head just above the ears if yours is large or small. A hat that is too small rides up over the ears; one that is too large falls into your eyes.

Long hair: tie it low at the neck so the hat sits evenly. Beards and glasses do not affect the hat, but take glasses off in the sauna. Metal frames get hot.

For winter swimming and lakeside saunas the hat works twice. In the sauna it keeps your head cool, and on the walk to the water and back it holds in warmth. Many people keep the hat on when they dip, so the head never gets ice cold.

A wool hat that gets wet keeps insulating. That is one of wool's big advantages over cotton, which turns cold and heavy when wet.

Remember that a wet hat left outside in sub-zero temperatures will freeze. Bring it in and let it dry at room temperature.

A good wool hat lasts for years. It softens and shapes itself to your head. The colour may fade slightly from heat and light, which is normal.

Small pills or loose fibres can be removed by hand. If the hat shrinks, it has usually been washed too hot or dried too fast. Soak it in lukewarm water, gently stretch it back into shape and dry flat.

A sauna hat is personal, much like a towel. It absorbs sweat and skin oils. Keep your own and let it air between sessions.

If a sauna has a shared hat, rinse it in lukewarm water before use. For sauna clubs and hotels it is simpler to ask guests to bring their own.

• It does not make the sauna less hot. It only delays your head from overheating.
• It is no protection against overheating if you sit too long. Listen to your body.
• It does not replace water. Dehydration causes dizziness even with a hat.

• Can I wet the hat? Yes, some people dip it in cold water between rounds. It works, but it gets heavier.
• Do I need one for short sessions? It helps most at 80 degrees and above and for rounds longer than ten minutes.
• Can I use a regular winter beanie? Better than nothing if it is wool. Knitted beanies are often tighter and contain synthetics.