Thermo-S vs Thermo-D: What the ThermoWood® Classes Mean for Saunas

What do Thermo-S and Thermo-D mean?

They are the two product classes of ThermoWood®. S stands for stability. D stands for durability, which the handbook defines as stability plus resistance to decay and weather ITWA handbook, pp. 6, 18. The English terms follow the 2025 edition ITWA handbook (English), p. 18.

ThermoWood® is a registered trademark of the International ThermoWood Association (ITWA), and the classes belong to that system. ITWA uses them to decide which applications suit a product. Softwoods and hardwoods both come in both classes, and within a class the two count as separate products, because their properties and modification temperatures differ ITWA handbook, pp. 4, 18.

Both classes start from the same definition: thermal modification changes the wood’s chemical properties with heat and steam, at 320°F or higher, with permanent results. They also share a baseline. Compared with standard timber, ThermoWood® products swell and shrink less with moisture, have better dimensional stability and biological durability, are darker all the way through, contain no resin, and conduct heat less ITWA handbook, pp. 4, 6. The class tables then show how far each class moves stability, darkness, and weather resistance ITWA handbook, p. 18.

How hot is each class made?

Thermo-S uses one temperature, 374°F, for pine, spruce, and hardwoods as a group. Thermo-D is set species by species. Pine and spruce are made at 414°F, and the other rows run from 374°F for iroko to 446°F for radiata pine. Iroko’s Thermo-D figure equals the Thermo-S temperature, so a temperature alone does not name the class ITWA handbook, p. 18. The handbook writes separate process instructions for each species ITWA handbook, p. 9.

An example schedule

Temperature is one part of a schedule. The handbook’s example run, Nordic softwood in Thermo-D, takes about 42 hours. The kiln heats to 212°F fast, climbs to 414°F by about hour 24, holds, then cools. The wood is dried to 0% moisture on the way up, and water and steam raise it to 4 to 7% at the end ITWA handbook, p. 7.

Run length depends on the class, the species, and the timber’s starting moisture and size. A control system adjusts the temperature to keep the wood from cracking, with different settings per species and board size ITWA handbook, p. 7. The process page walks through each phase.

What does each class change in the wood?

Thermo-D goes further on nearly every property the class tables track. Table 2 marks Thermo-S pine and spruce as improved in weather resistance, dimensional stability, and darkness. Table 3 marks every Thermo-D row as significantly improved in all three, except iroko’s darkness ITWA handbook, p. 18.

The Thermo-S hardwoods row differs. Stability and darkness improve, but weather resistance stays the same as unmodified wood ITWA handbook, p. 18. For a hot room, Table 9 still recommends both classes ITWA handbook, p. 29. The column matters when the same wood goes outside.

Color

Thermo-D is the darker class. In the handbook’s photos of Nordic softwood products, Thermo-S is light and golden, and Thermo-D is a much darker brown ITWA handbook, p. 9. Those photos show pine or spruce, not aspen, so judge color on samples of the wood you plan to use. You can order a sample of Thermory thermo-aspen, and the questions page answers more on color and scent.

Weight

In pine and spruce, both classes are lighter than untreated wood, and Thermo-D slightly more so. Table 4 gives air-dried averages for both species ITWA handbook, p. 20.

Most of the drop happens by Thermo-S. The step from Thermo-S to Thermo-D is 0.6 lb/ft³ in both species. A VTT study of pine found density falls as modification temperature rises, with wide scatter from natural variation ITWA handbook, p. 20. For a sauna wall, the weight gap between classes is small.

Which class does a sauna need?

Either one. Table 9 lists sauna, with cladding and ceilings, in use class 2: indoor use that may see moisture and condensation. For that use class it recommends both Thermo-D and Thermo-S ITWA handbook, p. 29.

The handbook’s sauna drawings agree. Wall panels, ceiling panels, battens, and bench boards are each labeled “Thermo-S / Thermo-D” ITWA handbook, pp. 38, 39. For batten spacing and the air gap behind the boards, see framing, battens, and the air gap.

Table 9 separates the classes only under prolonged wetting. It recommends either class for dry interiors in use class 1, such as interior cladding and furniture, and for windows and doors in use class 3.1. For use class 3.2, such as exterior cladding and decking, it recommends Thermo-D only. The handbook also advises a surface finish against moisture, erosion, and UV on weather-exposed thermally modified wood ITWA handbook, p. 29.

• Hot-room walls, ceiling, and benches: either class, per Table 9 and the sauna drawings ITWA handbook, pp. 29, 39.
• Choosing between them indoors: Thermo-D rates higher on dimensional stability and darkness ITWA handbook, p. 18.
• Exterior cladding on an outdoor sauna: Table 9 recommends Thermo-D only, and the handbook advises a protective finish ITWA handbook, p. 29. For a specific board, follow its maker’s exterior guidance.
• Sills and bases: keep the wood out of direct contact with soil or water, as the handbook advises ITWA handbook, p. 29.
• Specs and quotes: name the maker’s own product and grade, then check it against the maker’s documents. The spec guide covers terminology and claim review.

Where does aspen fit?

Aspen is made in both classes. Table 1 lists aspen (Populus tremula) as a hardwood from the Nordic and Baltic regions, available as Thermo-D and Thermo-S. It is the only hardwood in that table rated soft ITWA handbook, p. 9.

The temperature data is thinner. As a hardwood, aspen falls under the Thermo-S hardwoods row at 374°F. Table 3 has no aspen row, and no birch row either, so the handbook gives no aspen Thermo-D temperature ITWA handbook, p. 18. Do not borrow the pine figure to fill that gap.

Test data is thinner still. Aspen appears in the handbook only in Table 1 and in a chart of typical raw timber sizes ITWA handbook, pp. 9, 10. The handbook’s measured properties, such as density, strength, and emissions, come from tests on other species. For how modified aspen differs from unmodified aspen, see thermo-aspen vs plain aspen. The buyer’s guide covers profiles and ordering.

How does Thermory describe its thermo-aspen?

In its own terms. Thermory’s June 2026 catalog shows aspen in two versions, Natural and Medium Thermo Thermory catalog, p. 4. Thermory USA’s sauna brochure calls its thermo-aspen “Medium Modified,” one of three modification levels it names Thermory USA brochure, p. 5.

On process, the catalog says Thermory uses a high-pressure chamber for sauna materials. There the wood is heated to as much as 356°F while pressure rises to about 130 psi. Thermory says this controlled environment lets the wood be modified without over-drying, reducing internal stresses and minimizing cracking or warping. The final color, it says, depends on the species and on treatment temperature and duration Thermory catalog, p. 6.

Makers publish different process details, so compare a board with its maker’s documents, not with the class temperatures above.

The profiles page compares Thermory thermo-aspen wall profiles, and the designer opens them in a full room plan. Current paneling and bench boards are listed in the sauna wood category at Sauna Marketplace. For the rest of the 2026 edition, see 12 numbers from the 2026 ThermoWood® Handbook.