The best underwear for a sauna is made from a breathable, naturally derived fabric like TENCEL that stays cool and dry against the skin, with a fit that lets air move instead of trapping heat. Cotton holds sweat and heat, and tight synthetic blends trap both, which is the opposite of what you want in a hot room.
Here's how to choose, and why the fabric matters more than anything else.
In a sauna, the fabric is the whole game
A sauna is a high-heat, high-sweat environment, and underwear has one job in it: move heat and moisture away from your skin instead of holding it there. That comes down almost entirely to the fabric.
Cotton feels soft and natural, but it soaks up sweat and holds it against you, staying damp and warm for the length of your session. Polyester and most performance synthetics wick moisture but trap heat and can hold odor. Naturally derived fibers like TENCEL Lyocell and modal sit in a different category. They're made from wood pulp, breathe well, pull moisture off the skin, and stay cooler and drier than either cotton or synthetics.
For everyday wear that's a nice-to-have. In a sauna it's the difference between underwear that works with the heat and underwear that fights it.
Fabric comparison
| Fabric | Breathability | Moisture handling | Heat retention | Feel |
|---|---|---|---|---|
| TENCEL (Lyocell or modal) | High | Wicks and releases moisture | Low, stays cool | Soft, smooth, cool to the touch |
| Cotton | Medium | Absorbs and holds moisture | High when damp | Soft, but heavy when wet |
| Polyester or synthetic blend | Medium | Wicks but can trap heat | Medium to high | Slick, can hold odor |
The part most men miss: the testicles have to stay cooler
Whole-body heat from a sauna is good for you, and the research on regular sauna use is strong. But the one area that needs to stay cooler than the rest of your body doesn't benefit from being heated. Sperm production runs best a few degrees below core temperature, and heat exposure from saunas and tight clothing can temporarily lower sperm production. The effect is generally reversible, but if you're using heat regularly or thinking about fertility, it's worth managing.
This is also why tight underwear can work against you. It holds that area close to the body and traps heat exactly where you don't want it. The move is a breathable fabric and a fit that gives the testicles room and airflow, so you get the benefits of the heat while keeping that one area as cool as possible. For more on using saunas without the tradeoff, see our piece on how frequent saunas influence sperm production.
It's not just the fiber, it's the dye
Here's a part most fabric labels won't tell you. A fabric can start clean and still pick up a real chemical load in how it's dyed and finished. With cotton especially, the fiber itself is usually fine, but conventional dyeing can leave residues you'd rather not have sitting against your skin for sixteen hours a day. Most brands don't talk about their dye process at all, because the fiber makes for a nicer story than the chemistry.
So when you're comparing options, don't stop at the fiber name. Ask whether the brand controls how the fabric is colored and finished, not just what it's made of.
What to look for when buying
- Fabric first. Naturally derived TENCEL Lyocell or modal over cotton or pure synthetic. That's most of the decision.
- Clean dyes and finishes. The dye process is where a lot of the chemical load lives. Look for brands that name their dye standard, not just a clean-sounding fiber.
- A fit built for airflow, not compression. A gusset, an extra panel, does that without the underwear feeling baggy.
- A way to stay cool during the session. Some setups add a reusable cooling element for sauna use.
Where Oakman fits
Oakman is built specifically for active men looking for scrotal cooling all day and want the option of a cold pack for cooling in the sauna or after heat exposure. The fabric is TENCEL MicroModal, naturally derived from wood pulp rather than plastic, blended with ROICA V550, a Japanese clean stretch fiber, rather than conventional spandex. The fit is athletic with a gusset, an extra panel that gives the testicles room and airflow. Dyes are bluesign approved, so what touches your skin is controlled all the way through the supply chain. It's designed to keep the testicles cooler while you get the benefits of the heat.
Oakman launches Fall 2026, and the Founding 100 group gets first access and founding pricing.
Frequently asked questions
Should you wear underwear in a sauna?
Many men prefer to, for hygiene on shared benches and for support. If you do, the fabric matters more than usual. Choose a breathable, naturally derived fabric that stays cool and dry rather than cotton, which holds heat and sweat.
Is cotton bad for the sauna?
Cotton isn't harmful, but it isn't ideal. It absorbs sweat and holds it against your skin, staying damp and warm through your session. A naturally derived fabric like TENCEL moves moisture away and stays cooler. It's also worth checking how a fabric is dyed, since that's where much of the chemical exposure comes from.
What underwear fabric stays the coolest?
Naturally derived fibers like TENCEL Lyocell and modal tend to stay coolest and driest, ahead of cotton and most synthetics, because they breathe well and release moisture instead of holding it.
Do you need a cooling pack for the sauna?
You don't need one, but some men use a reusable cooling element to keep the area that needs to stay cool from overheating during longer or more frequent sessions. Breathable fabric and a fit that allows airflow do most of the work.
This article is for general educational purposes and isn't medical advice. If you have specific concerns about fertility or your health, talk to a qualified clinician.
The practical playbook
Want the practical version? Our free guide, the Heat-Smart Guide for Men, covers how to get the benefits of sauna and heat without the hidden tradeoff. Grab it and join the Founding 100 for first access when Oakman launches in Fall 2026.
