Questions and answers
46 short answers for anyone planning a sauna: how the wood is made, how it handles heat and steam, how to install it, and how to care for it. Each answer links the sources it comes from.
01 · Definition and process
Aspen is the species and thermal modification is the process. The kiln schedule, the class, and the maker’s documents decide what a given board can do.
Thermo-aspen is aspen wood that has been thermally modified with heat and steam. The International ThermoWood Association (ITWA) defines thermal modification as changing wood’s chemical properties with heat and steam at 320°F or more. It calls the resulting changes permanent. The brown color runs through the whole board instead of sitting on the surface.
No. Aspen is the species, and “thermo” names the treatment. In the ThermoWood® Handbook’s species table, aspen is Populus tremula, a soft hardwood from the Nordic and Baltic regions. Makers use different kilns, schedules, grades, and profiles, so check each product’s own documents.
No. ThermoWood® is a registered trademark of the International ThermoWood Association, and only its members may use it. The generic terms are thermally modified wood and thermally modified timber, and thermo-aspen is thermally modified aspen.
No. Heat, steam, and water do the modifying. The ThermoWood® Handbook states that no chemicals are used to produce ThermoWood® products. Thermory’s catalog says its thermally modified wood is modified using only heat and steam, without added chemicals. A wax, oil, or paint applied later is a separate surface treatment.
It depends on the maker and the process. Thermory’s catalog says its sauna materials are heated up to 356°F in a high-pressure chamber while pressure rises to about 130 psi. The ThermoWood® Handbook defines thermal modification as starting at 320°F. For ThermoWood® products, its Thermo-S class modifies hardwoods as a group, aspen among them, at 374°F (±5°F). Its Thermo-D table has no aspen row, so there is no aspen Thermo-D temperature to quote; pine and spruce reach 414°F.
Typically 36 to 72 hours, according to Thermory’s Hannes Tarn in a 2025 conference talk. He noted that species, cross-section, and conditions can make it shorter or much longer. The ThermoWood® Handbook’s example cycle for Nordic softwood in the Thermo-D class runs about 42 hours. It holds about 3 hours near 414°F, then cools with water spray and conditions with steam.
No. The result depends on the starting wood, species, cross-section, moisture, temperature, time, steam or pressure, and process control. The ThermoWood® Handbook says a separate process instruction is written for each species. In his 2025 talk, Thermory’s Hannes Tarn said it is much easier to turn timber brown than to reach the right properties.
No. Color shows that heat changed the wood, and it says little about whether the target properties were reached. Thermory’s Hannes Tarn described boards made brown enough to look treated that never gained the intended biological durability. Ask for the maker, the treatment level or intended use, current technical documents, and a physical sample.
Yes. The ThermoWood® Handbook says the color of ThermoWood® products runs through the board and stays after sawing or planing. Thermory’s Hannes Tarn made the same point about his company’s boards in his 2025 talk. Board-to-board variation still exists, so Thermory USA advises opening several packages to match panels.
02 · Sauna performance
Each answer says what was tested, on which wood, and what it means in a hot room. Where the handbook gives data only for pine or spruce, the answer says so.
Aspen is resin-free, smooth, and stays comfortably cool, according to Thermory’s 2026 catalog, which calls it a preferred sauna wood. The ThermoWood® Handbook lists aspen as a soft hardwood from the Nordic and Baltic regions. Thermory sells aspen both natural and thermally modified.
Every sauna bench heats up, and thermally modified wood conducts heat less readily than untreated wood. The ThermoWood® Handbook reports thermal conductivity 20 to 25% lower for thermally modified Nordic softwood than for untreated timber. It gives no aspen figure. Thermory’s catalog says its modified wood stays cooler to the touch on benches and walls. If a bench still feels too hot, Thermory advises a seat cover or a dry towel.
Less than untreated wood. ThermoWood® products leave the kiln at 4 to 7% moisture. Equilibrium moisture content is the level wood settles at for a given humidity. In tests on spruce modified at 428°F, the handbook reports it at about half that of untreated spruce. Less absorbed water means less swelling as the room swings from dry heat to steam. Thermory still advises checking and tightening bench screws at least once a year, because heat and humidity make wood swell and shrink.
For ThermoWood® products, the handbook recommends both of its classes for saunas. Thermo-S stands for stability, and Thermo-D for durability, meaning stability plus resistance to decay and weather. The handbook places saunas in use class 2 of the European standard EN 335: indoors, but potentially exposed to moisture and condensation. The handbook’s species table lists aspen in both classes. Thermory uses its own treatment levels and lists thermo-aspen as Medium, so use its catalog for those boards.
No. Thermal modification lowers how much water wood takes up, yet the ThermoWood® Handbook does not recommend ThermoWood® products for direct contact with soil or water. In a sauna, Thermory says to throw water only on the heater, not on walls or benches, and to keep it from pooling. After each session, dry the room with the heater on and the vent open.
Yes, on the surface, as on any material that gets wet. The ThermoWood® Handbook notes that mold can appear on ThermoWood® surfaces exposed to rain, stays superficial, and wipes or scrapes off. Decay fungi need wood moisture above 20%. The handbook says thermally modified wood stays well below that at any relative humidity. Dry and ventilate the room after use, and skip linseed oil, which the handbook says enables fungal growth.
Yes, but less than untreated pine, in the tests the ThermoWood® Handbook reports. The Technical Research Centre of Finland (VTT) measured ThermoWood® pine about two months after modification. Total VOC emissions were about 44% lower than unmodified pine after modification at 356°F, and about 84% lower after 446°F. The handbook concludes that ThermoWood® is a safe choice for indoor use in terms of emission levels.
It does not bleed sap, and planed boards are usually smooth. Thermory’s catalog describes aspen as resin-free. The ThermoWood® Handbook says ThermoWood® products contain no resin, and that planed ThermoWood® usually needs no sanding. Handle boards with care, since it also notes that ThermoWood® is more easily damaged mechanically.
No. The ThermoWood® Handbook says ThermoWood® is not strength graded and must not be used for load-bearing structures. In tests on spruce, pine, and birch, thermal modification cut splitting strength by 30 to 40%. Impact strength fell about 25% in a spruce test. Carry bench loads on framing or supports built for them, because Thermory USA says paneling must not carry bench weight.
Its fire rating comes from testing the exact product. The ThermoWood® Handbook reports a smaller fire load and lower smoke production than untreated timber, but says designers may not use that directly in fire design, and that makers of ThermoWood® products are responsible for testing each product and the fire class stated for it. If your project needs a fire rating, ask the board’s maker for the tested classification of that exact product. Stay within your heater maker’s maximum temperature, and never dry laundry in the sauna, as Thermory USA warns.
No. The ThermoWood® Handbook says studies to date do not prove ThermoWood® resistant to termites, and it recommends local tests because termite types vary by region. The handbook reports resistance to longhorn beetles, the common furniture beetle, and Lyctus beetles, plus good resistance to the old house borer. Where termites are a local risk, rely on the building’s termite protection, such as foundation membranes.
Only if its maker documents it for that exposure. For ThermoWood® products, the handbook recommends Thermo-D for exterior cladding and decking in prolonged wetting, and neither class for direct contact with soil or water. Unfinished thermally modified wood turns gray under UV, and the handbook recommends a surface treatment for anything exposed to weather.
03 · Storage and installation
Store boards dry and flat, finish wet work first, and fix stainless fasteners to battens that let air move behind the lining.
Indoors, dry, flat, and off the floor, in closed packages. Thermory USA says to store its paneling in the room where it will be installed, ideally at 59 to 72°F, on a flat surface raised on joists. Keep packages sealed and remove the shrink film just before installation. For ThermoWood® products, the handbook adds a heated space, enough support battens, and protection from dirt and UV. Handle packages and boards with care, as Thermory USA asks.
Yes, on the maker’s timing. Thermory USA calls a few weeks in the installation room best practice, so the boards reach the room’s temperature and humidity. The ThermoWood® Handbook says conditioning at room temperature can take several days, depending on cross-section. Finish tiling, plastering, and other wet work first, because Thermory says panels installed before wet work absorb moisture and swell.
Use fine-toothed blades and good dust extraction. The ThermoWood® Handbook says coarse saw teeth can chip the edges of ThermoWood® boards. Its dust is dry and fine, so the handbook calls for effective extraction and a respirator whenever dust is generated. Milling resembles working a hard, brittle hardwood, so plan cuts against the grain with care.
Stainless steel for every clip, staple, nail, and screw. Thermory USA recommends a staple gun with stainless steel clips for its paneling. For ThermoWood® products, the handbook’s fastener table sets A2 stainless (Type 304) as the minimum for sauna wall and ceiling cladding and for sauna benches, with Type 316 (A4) as the acid-resistant step up. The handbook gives the reason: ThermoWood® products measure about pH 4, against 4.5 to 5 for untreated wood, and other fasteners react with the wood and stain around the fastener.
For Thermory paneling, staple through the groove. Thermory USA’s guide fixes its panels to the battens with a staple gun from the groove side, and says finishing nails are not suitable. Set the depth so each clip neither stands proud nor digs into the face. For ThermoWood® cladding, the handbook also shows visible side fixing with Dyckert nails, heads set just over 1/32 in below the surface. It adds that nails and screws must never pierce an air or moisture barrier behind the battens.
No more than 23-1/2 in apart. That is the maximum in the ThermoWood® Handbook’s sauna drawings. Thermory USA’s guide sets the same maximum for its paneling and recommends about 15-3/4 in between battens. A 16 in on-center layout sits close to that recommendation; 24 in on center exceeds the maximum. Run battens across the panels: vertical battens for horizontal paneling, horizontal battens for vertical paneling.
Yes, at least 13/16 in deep. That is the minimum ventilated gap behind wall and ceiling panels in the ThermoWood® Handbook’s sauna drawings. Thermory USA’s guide sets the same figure as its minimum batten thickness, asks for battens at least 1-13/16 in wide, and says they must let air move behind the whole lining. A 1x furring strip is 3/4 in thick, just under 13/16 in; 5/4 stock, about 1 in thick when dry, clears it. Follow the current installation guide and local code for the vapor barrier and insulation behind the gap.
Yes, in the part made for each job. Thermory’s catalog lists thermo-aspen wall panels such as STS4 and separate SHP bench boards. Paneling is finish cladding. Thermory USA says to add bench supports behind the paneling, because the paneling must not carry the weight of the benches.
Order the whole room at once, then blend boards from several packages. Thermory USA notes slight shade differences between production batches and advises opening several packages to match panels. Check the boards when they arrive, before you cut: Thermory USA asks that any concern about quality or scent be raised before the wood is sawn, installed, or treated.
04 · Finish and care
Keep water on the stones, finish the wood before first use, dry the room after every session, and check it once a year.
Yes, Thermory recommends it. Its catalog says to treat all wooden surfaces before use with a breathable wood protector, and walls and floor grids with sauna wax or another suitable coating. Clean off dust first and let the surfaces dry. Choose a product made for sauna conditions and follow its maker’s instructions. Thermory also lists some aspen and alder panels and bench boards pre-waxed.
Use what your board’s maker specifies, and skip linseed oil. The ThermoWood® Handbook recommends mineral oil for interior walls, ceilings, and benches in saunas, and calls linseed oil unsuitable because it enables fungal growth. Thermory recommends oiling its unfinished PLATE bench board, available in thermo-aspen. The product notes for Harvia sauna paraffin oil recommend it particularly for heat-treated wood and alder. Test any oil under a bench first to see the shade change.
Yes, with a product suited to sauna heat. The ThermoWood® Handbook says ThermoWood® takes the same kinds of finish as standard timber, water- or solvent-based. Its acidity and low absorbency mean some water-based finishes adhere poorly. A surface sanded with P100 paper can hold a finish better than a planed one. Thermory’s catalog shows thermo-aspen benches oiled black and thermo-aspen wall panels waxed black. Its pre-waxed boards are listed in aspen and alder, in black, white, or translucent.
Wipe the benches and dry the room. Thermory says to wipe benches with a clean damp cloth, take out seat covers and wet towels, and keep water from pooling. Then leave the heater on for at least half an hour with the air vent open and the door and windows closed. Avoid sitting on the benches in wet swimwear.
A sauna cleaner or another product made for wooden sauna surfaces, used as its label directs. Thermory warns that unsuitable products, disinfectants for example, can bleach and damage the wood. For a stubborn mark on a bench, sand it out rather than reaching for a stronger cleaner.
No. Thermory USA says never to wash a sauna with a pressure washer or running water, because excess moisture can permanently damage the structure and surface of the wood. Clean benches with a damp cloth instead. Thermory USA also advises against washing untreated thermally modified wood with water, which strengthens its scent.
At least once a year, and more often for heavy use. Thermory’s catalog says to retreat the wood with a product made for saunas and to clean seats, walls, and ceiling with a soft brush or cloth. Tighten bench and support screws, since heat and humidity make wood swell and shrink. Check the heater stones too, since they can crumble over time.
Yes. Thermory’s catalog advises carefully sanding the entire length of the stained bench with fine sandpaper, then treating the sanded area with a wood preservative. The ThermoWood® Handbook notes that a surface sanded with P100 paper can hold a finish better than a planed one. Wear a respirator: the handbook says dust from ThermoWood® is dry and fine.
Yes, a smoky scent, most noticeable when the sauna heats. The ThermoWood® Handbook says it probably comes from compounds called furfurals, and that it seems stronger than untreated wood’s even though VOC emissions are far lower. Thermory USA says the scent weakens over time but never disappears completely. Long heating with airing speeds that up, and a suitable wax or maintenance product can reduce it.
Yes, gradually. Thermory USA says wooden sauna surfaces may darken over time, and faster where hot steam reaches them. A breathable protector applied before first use guards against moisture and dirt and makes cleaning easier, per Thermory’s catalog.
05 · Buying and design
Order from covering width, allow for waste, and look at a sample under your own lights before you commit a room.
Choose the face and shadow line you want, then check its coverage. Thermory’s catalog lists thermo-aspen in STS4, STEP, KYTE, and VIRE profiles, each with its own covering width. Compare samples in your room’s lighting before ordering.
Tongues, grooves, and reveals overlap, so a panel covers less wall than its full width. Thermory’s catalog gives thermo-aspen STS4 1×4 a covering width of about 3.15 in, and the 1×5 about 4.33 in. Nominal names also overstate size: for ThermoWood® products, the handbook says kiln shrinkage leaves its example board about 3% smaller than nominal. Calculate quantities from the covering width the maker prints, never from the nominal name.
Plan on 5 to 10% more, per Thermory USA’s guide, depending on panel length. Check that allowance against your actual layout. Order the full quantity at once, since shade can differ slightly between production batches.
Yes. A sample shows tone, texture, profile depth, and machining under your own lighting. Thermory USA notes slight shade differences between production batches, so treat the sample as a guide and check the delivered order before cutting.
Sauna Marketplace, which publishes this site, sells Thermory thermo-aspen wall profiles and bench boards. Its product pages carry current lengths, sample options, and live pricing.
Yes. SaunaDesignPro can open a room with STS4 1×4, STS4 1×5, STEP, KYTE, or VIRE already selected. Pick a profile from the launch links, then size the room and plan benches, heater, openings, and ventilation in the full workspace.
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Editorial method
Topics come from what people search for about thermo-aspen and sauna wood. Every answer lists its sources. Links to the handbook, the catalog, and the paneling guide open the PDF at the cited page, and video links start at the cited moment.
Figures from the ThermoWood® Handbook (International ThermoWood Association, 2026, Swedish edition) describe ThermoWood® products and name the species tested. Its data and page numbers match the 2025 English edition, apart from the production chart on p. 5, and the English edition supplies the terminology used here.
For Thermory boards, answers follow Thermory’s June 2026 sauna catalog and Thermory USA’s April 2026 paneling guide. A 2025 conference talk by Thermory’s Hannes Tarn adds process context.
Last reviewed September 26, 2026. Care instructions, specifications, stock, and pricing can change.