Can Mold Grow in a Sauna? How to Keep It Dry and Clean

After each session: dry the room with the vent open

Run the heat with the vent open before you shut the room down. Thermory USA recommends leaving the heater on for at least half an hour with the air vent open and the door and windows closed Thermory USA guide, p. 4. Thermory’s June 2026 catalog gives the same instruction Thermory catalog, p. 39.

Each habit below keeps liquid water off the wood or clears it quickly. All five come from Thermory USA’s guide Thermory USA guide, p. 4:

• During the session, throw water only on the heater, never on walls or benches.
• Skip wet swimwear on the bench.
• Afterward, take seat covers and wet towels off the bench boards.
• Wipe the benches with a clean damp cloth, never running water, and leave no water pooled on the wood.
• Never dry laundry in the sauna. Thermory cites fire risk and says the excess moisture may damage the wood surfaces.

The short version also lives on the questions page and in the buyer’s guide.

Build an air path behind the paneling

The handbook’s sauna drawings and Thermory USA’s guide both call for at least 13/16 in of air space behind the panels: the handbook as a ventilated gap, Thermory as the minimum batten thickness. Both also cap batten spacing at 23-1/2 in ITWA handbook, p. 38. Thermory adds a minimum batten width of 1-13/16 inand says the battens must “ensure air movement behind the wall paneling across the entire extent of the paneling” Thermory USA guide, p. 2.

A 1x furring strip is 3/4 in thick, just under 13/16 in. Nominal 5/4 stock, about 1 in thick when dry, clears it USDA Wood Handbook, Table 6-6, p. 6-11.

Four details keep that path and the room working. The first, second, and fourth come from Thermory USA’s guide Thermory USA guide, pp. 2, 3:

• Leave an airing gap where the first and last rows of ceiling paneling meet the walls.
• Finish all wet work before the paneling goes up. Thermory says wet work done afterward makes the paneling absorb moisture and swell.
• Size fasteners so they do not pierce the air or moisture barrier behind the battens, as the handbook directs ITWA handbook, p. 44.
• Provide sufficient ventilation in the sauna room itself.

The full wall build is in the framing, battens, and air gap guide and the installation checklist. To lay out the heater, openings, and vents together, plan the room in SaunaDesignPro.

If spots appear: clean off surface mold

Wipe it off, and scrape if needed. The handbook describes surface mold on ThermoWood® as superficial and removable by wiping or scraping ITWA handbook, p. 29. That passage concerns rain-exposed wood outdoors, and the same wipe-or-scrape step is the place to start indoors. The steps below follow Thermory USA’s guide Thermory USA guide, p. 4:

• Wipe the spot with a cloth and a cleaner made for sauna wood. Read its label first and confirm it is safe for wooden surfaces.
• If a mark remains on a bench, sand it carefully with fine-grained sandpaper, then treat the sanded wood, as Thermory advises for stubborn stains.
• Let the surfaces dry with the vent open.
• Then find the water that fed it, using the checks in the next section.

Leave three things out of the routine. Disinfectants can bleach and damage the surface, per Thermory. Running water and pressure washers add the excess moisture Thermory says could permanently damage the wood’s structure and surface. Plain water on untreated thermally modified wood also strengthens its scent, Thermory adds Thermory USA guide, p. 4.

The USDA Wood Handbook notes that people with asthma or allergies to certain molds may react to exposure, and it points to U.S. EPA guidance on removing mold from surfaces USDA Wood Handbook, p. 14-2. For the stones, buckets, and heater, see Sauna Marketplace’s guide on how to clean a sauna.

Finishes: keep them breathable

Thermory’s catalog calls for a breathable wood protector, with sauna wax or another suitable coating on walls and floor grids Thermory catalog, p. 39. For Thermory thermo-aspen, follow Thermory and the finish maker’s label.

The handbook adds two points for ThermoWood® products. Linseed oil is unsuitable because it enables fungal growth, and a treatment that seals wood too effectively stops moisture moving between the wood and the room air ITWA handbook, pp. 30, 33. Oil and wax options are compared in the bench finishing guide.

If mold keeps coming back, look for the water

Recurring mold points to a moisture source. For decay repairs, the USDA Wood Handbook says every effort should go into correcting the moisture condition behind the damage USDA Wood Handbook, p. 14-8. Mold calls for the same search. Start with the habits in Thermory USA’s guide Thermory USA guide, p. 4, then check the build:

• Is the vent open during the post-session drying run?
• Do towels, mats, or seat covers stay on the benches between sessions?
• Is water landing on walls and benches instead of the heater?
• Does any board touch a floor that stays wet? The handbook does not recommend ThermoWood® in direct contact with water ITWA handbook, p. 29.
• Is there a clear air space behind the paneling, at least 13/16 in deep? ITWA handbook, p. 38
• Was wet work done after the paneling went up? Thermory USA guide, p. 2

Dark marks only around screws or nails may be fastener staining, not mold. The handbook says fasteners other than stainless steel react with ThermoWood® and stain the wood around them, and for ThermoWood® products it sets Type 304 (A2) stainless as the minimum for sauna cladding and benches ITWA handbook, p. 44. Thermory USA’s guide calls for stainless steel clips on its paneling Thermory USA guide, p. 2. The stainless fastener guide covers the choice.

Surface mold and decay fungi are different problems

Mold mainly discolors wood; decay fungi eat it. The USDA Wood Handbook describes mold as fuzzy or powdery growth, from light shades to black. On softwoods, it says, the fungus may reach deep, but the discoloring surface growth can often be brushed or planed off. Mold and stain fungi usually affect strength only slightly. Their greatest effect is on appearance and on toughness USDA Wood Handbook, p. 14-2.

Decay fungi feed on the wood’s structural components, cellulose and lignin. Humid air alone will not wet wood enough for significant decay, but it will let some molds grow. Serious decay needs liquid water reaching dried wood, such as rain, condensation, or wet ground. The USDA adds that wood kept at 20% moisture content or less has a reasonable margin of safety against fungal damage USDA Wood Handbook, p. 14-3.

In a sauna, that split sets the response. A mold spot on a bench is a cleaning job. Per the USDA, serious decay in a building almost always signals faulty design, construction, handling, or maintenance USDA Wood Handbook, p. 14-7. The fix starts with the water source.

Why thermally modified wood stays under the 20% line

Thermal modification lowers equilibrium moisture content, the moisture level wood settles at in air of a given humidity. In tests by the Finnish research center VTT, reported in the handbook, spruce modified at 428°F held about half the moisture of untreated spruce, and the gap widened as the air got more humid ITWA handbook, p. 26.

These are VTT results for spruce modified hotter than 414°F, the Thermo-D (durability class) temperature for pine and spruce ITWA handbook, p. 18. The handbook gives no aspen moisture curve. The Thermo-S and Thermo-D explainer covers the classes.

Thermory’s catalog says thermal modification improves the wood’s durability, dimensional stability, and resistance to moisture Thermory catalog, p. 6. The process page explains how heat and steam change the wood.