How to Prevent Mold in a Sauna (and Clean It Safely)

Short answer

Mold needs moisture, so the best prevention is drying your sauna after every session: open the drying vent or leave the door open, take towels out and wipe up water. The EPA advises drying damp areas within 24 to 48 hours and scrubbing mold off hard surfaces with detergent and water. Mold in porous wood may not fully come out.

A sauna gets very hot while it runs, but between sessions it is a closed wooden room with sweat, damp towels and little air movement. That is exactly where mold gets a foothold. The good news: preventing it mostly comes down to a two-minute drying routine.

Table of Contents

Why does mold grow in a sauna?

Mold grows where moisture stays. In a sauna that usually means wet benches and floorboards after a session, towels or mats left inside, water from the bucket or stones, poor ventilation, or a leak in the wall or roof of an outdoor sauna. Wood is porous, and the EPA’s guide to mold and moisture warns that mold can fill the crevices of porous materials, which makes it hard to remove once it is established.

Not every dark mark is mold. Benches often darken where people sit and sweat. Harvia’s KIP owner’s manual says perspiration stains can be lightly sanded with fine sandpaper. Fuzzy growth, spots that spread, or a musty smell point to mold; see why your sauna smells for how to tell the difference.

How do you prevent mold in a sauna?

Dry it after every session. Harvia’s heater manual says to keep the drying vent closed while heating and bathing, then open it afterwards, or to leave the sauna door open after bathing.
Take everything out. Remove towels, mats, clothes and buckets. Harvia also says not to dry clothes in the sauna or use it as storage.
Wipe up water. Dry puddles on the floor and wipe benches, especially if you throw water on the stones.
Keep the ventilation working. Harvia specifies air in the sauna room changing six times an hour, with a mandatory exhaust vent near the floor. Do not block vents; read do saunas need to be vented?.
Fix leaks fast. The EPA says that if wet materials are dried within 24 to 48 hours after a leak or spill, in most cases mold will not grow.
Watch the room around the sauna. The EPA recommends indoor humidity below 60 percent, ideally 30 to 50 percent. A small humidity meter helps, especially in a basement.
Use towels on the benches. Sitting on a towel keeps most sweat off the wood; here is the right sauna towel size.

How to clean mold in a sauna

These steps follow the EPA’s guidance for small areas, up to about 10 square feet. For anything bigger, after water damage, or if you suspect mold inside the walls, get professional help.

Switch the heater off at the breaker and let the sauna cool completely.
Fix the moisture source first, for example a leak or a blocked vent, or the mold will come back.
Wear rubber gloves and goggles. The EPA says you may also want an N-95 respirator to limit what you breathe in.
Scrub mold off hard surfaces with detergent and water, then dry everything completely.
Do not hose down the heater. Harvia says to wipe the heater with a damp cloth and, if needed, a mild detergent, and not to use running water on it.
Leave the door or drying vent open until all surfaces are dry.
If mold has gone deep into soft or porous wood, the board may need replacing.

The EPA does not recommend bleach as a routine part of mold cleanup, because the mold still has to be removed. If you use any cleaner, never mix chlorine bleach with ammonia-based products. Avoid stains and sealers on the inside wood: Harvia does not recommend them because vapors can form when heated. For the full routine, see how to clean a sauna.

Is mold in a sauna dangerous?

According to the EPA, molds produce allergens and irritants. They can cause allergic reactions in sensitive people, trigger asthma attacks in people with asthma who are allergic to mold, and irritate the eyes, skin, nose, throat and lungs. If you notice symptoms when you use your sauna, stop using it until it is cleaned, and talk to your doctor if they continue.

Wood choice and mold

Harvia recommends kiln-dried softwood such as spruce, pine, aspen, alder or cedar for sauna paneling, with a moisture content of no more than 11 percent. No wood is mold-proof, but dry, well-chosen wood plus good ventilation makes prevention much easier. Our guide to sauna wood compares the options, and outdoor sauna insulation matters if condensation is the problem.

Frequently asked questions

Does sauna heat kill mold?

Do not rely on it. A sauna is hot only while it runs; afterwards it is a warm, damp room unless you dry it. The EPA also notes that dead mold can still cause allergic reactions, so it has to be removed, not just killed.

Should I use bleach to clean mold in a sauna?

The EPA does not recommend bleach or other biocides as a routine part of mold cleanup. Scrub hard surfaces with detergent and water and dry them completely. If you do use bleach, ventilate well and never mix it with ammonia-based cleaners, because toxic fumes can form.

Can moldy sauna wood be saved?

Surface marks can often be scrubbed off, and Harvia says sweat stains can be lightly sanded with fine sandpaper. But the EPA warns that mold can fill the crevices of porous materials, so it may be difficult or impossible to remove completely. Badly affected boards may need replacing.

What humidity prevents mold?

For the home in general, the EPA recommends keeping indoor humidity below 60 percent, ideally 30 to 50 percent. Use a humidity meter in the room around your sauna, especially in a basement, and dry the sauna itself after every session.

Should I leave the sauna door open after use?

Yes, unless you have a drying vent. Harvia’s manual says to keep the drying vent closed while heating and bathing, open it afterwards, or leave the sauna door open after bathing to dry the room.

When should I call a professional about sauna mold?

The EPA points to professional guidance when mold covers more than about 10 square feet or after a lot of water damage. A musty smell with no visible source can mean hidden mold behind the panels, which is also a job for a professional. Get any leak fixed before you clean.

Sources

This guide is general information. If you have asthma, allergies or another health condition and are worried about mold exposure, talk to your doctor.