Thermo-Aspen vs Plain Aspen: What Do Heat and Steam Change?

What the kiln does to aspen

Thermal modification runs far hotter than ordinary kiln drying. A typical hardwood drying schedule starts at 120°F and reaches about 180°F once the lumber is down to 15% moisture USDA Wood Handbook, p. 13-10. ITWA’s floor for thermal modification is 320°F ITWA handbook, p. 4. Kiln-dried aspen and thermo-aspen are two different products.

The ThermoWood® process has three phases. High-temperature drying takes the wood down to zero moisture. The kiln then holds a steady temperature while the modification happens, with steam protecting the timber. Finally, a water spray cools the load, and water and steam bring the moisture back up to 4 to 7% ITWA handbook, p. 7.

Every species gets its own schedule. The handbook says the process is always optimized for the species, and a separate process instruction is written for each one. A control system adjusts the temperature to keep the wood from cracking, with different settings for each species and board size ITWA handbook, pp. 7, 9.

The handbook draws one example schedule, for Nordic softwood in the Thermo-D class. It is not an aspen schedule, but it shows how far the modification phase climbs above drying temperatures.

Where aspen sits in the ITWA classes

Table 1 lists aspen (Populus tremula) as a soft hardwood from the Nordic and Baltic regions, made in both ThermoWood® classes ITWA handbook, p. 9. S stands for stability. D stands for durability, meaning stability plus resistance to decay and weather. Hardwoods in Thermo-S are modified at 374°F, within about 5°F, and Table 3 gives no Thermo-D temperature for aspen ITWA handbook, p. 18. The Thermo-S vs Thermo-D comparison covers both classes.

Thermory documents its own process for sauna materials. Its catalog describes a high-pressure chamber that heats the wood up to 356°F while pressure rises to about 130 psi Thermory catalog, p. 6. Makers publish different process details, so compare a board with its maker’s documents. The process page goes deeper on kiln control.

What changes inside the wood

Heat and steam mostly break down the hemicelluloses, the shorter sugar chains in the wood. Wood is roughly 40 to 50% cellulose and 25 to 35% hemicelluloses. Lignin makes up 20 to 25% of hardwoods, and extractives about 5%. After modification, the hemicellulose content is significantly lower ITWA handbook, p. 11.

That drop drives two results. Fewer water-absorbing hydroxyl groups remain, so the wood holds its dimensions better. Decay fungi also find less material they can break down. Hardwoods, the group aspen belongs to, contain more hemicelluloses than softwoods, so the breakdown happens more readily in them. Within each class, the handbook treats hardwood and softwood as separate products ITWA handbook, pp. 11, 18.

Color that runs through the board

The brown tone forms as heat changes the wood’s chemistry, and reactions in lignin during modification have a significant effect on it. The color runs through the full thickness, and the new properties, color included, stay the same when the board is sawn or planed ITWA handbook, pp. 6, 7, 12. A cut end on a bench or a planed edge shows the same tone as the face.

Extractives, acidity, and scent

In the kiln, softwoods release resin and other compounds, and hardwoods lose extractives, most of which evaporate easily ITWA handbook, pp. 7, 12. The process also lowers pH, to about 4 for ThermoWood® products against 4.5 to 5 for standard wood. That acidity may keep some finishes from adhering and may affect fastener corrosion, so the handbook calls for stainless or acid-resistant steel fasteners ITWA handbook, p. 12. The guide to stainless fasteners for sauna paneling covers the grades.

Thermally modified wood also has a smoky scent, which the handbook says probably comes from compounds called furfurals. It adds that thermally modified timber emits only a fraction of the volatile organic compounds that standard pine does ITWA handbook, p. 30. Thermory’s US guide says the scent weakens over time but never fully disappears. It fades faster when the sauna runs hot for a long time while the room is aired. A suitable wax reduces it, and washing untreated boards with water strengthens it Thermory USA guide, p. 4. The VOC test post covers the emissions data.

How much less it swells and shrinks

In VTT tests on spruce reported in the handbook, modification at 428°F halved the equilibrium moisture content, the moisture level wood settles at in air of a given humidity. The gap between modified and untreated spruce grows as humidity rises ITWA handbook, p. 26. The handbook has no equivalent curve for aspen.

Less absorbed water means less movement. VTT’s spruce data show radial and tangential swelling significantly reduced, and the handbook says the wood holds its dimensions well even without a finish ITWA handbook, p. 27. For hardwoods in Thermo-S, Table 2 marks dimensional stability and darkness as enhanced and weather resistance as unchanged ITWA handbook, p. 18.

The handbook says decay fungi become active above 20% moisture content, and that at any relative humidity the equilibrium moisture content of thermally modified wood stays well below that line ITWA handbook, p. 26. That figure describes moisture drawn from the air, so keep splashed and pooled water off the wood. Thermory’s US guide says to throw water only on the heater and wipe benches with a damp cloth, and after cleaning to run the heater at least half an hour with the vent open Thermory USA guide, p. 4. Those habits suit plain aspen equally. More in can mold grow in a sauna.

Boards finish smaller than nominal

Drying the wood that far shrinks it, so modified boards finish below their nominal size. In the handbook’s worked example, the board ends up about 3% smaller ITWA handbook, p. 15. Plan layouts and orders on the maker’s actual covering width. The profile comparison lists covering widths, and the quantity guide does the math.

What stays the same: it is still aspen

The species does not change. The handbook rates aspen soft, the only hardwood in Table 1 with that rating ITWA handbook, p. 9. It calls the change in Brinell hardness from modification small and ties hardness mostly to density, though its Table 8 shows Thermo-D pine about 14% and Thermo-D spruce about 9% softer than untreated wood ITWA handbook, pp. 24, 25. There is no aspen hardness figure, so treat thermo-aspen like any soft wood and protect benches and trim from sharp or heavy objects.

The grain and surface stay aspen’s too. USDA describes North American aspen as usually straight grained with a fine, uniform texture, easily worked, lightweight, and soft USDA Wood Handbook, p. 2-4. Thermory lists aspen as light-toned, smooth, and resin-free, sells it both natural and thermally modified, and calls it a preferred sauna wood that stays comfortably cool. Resin-free matters in a hot room: Thermory’s catalog says sauna woods must not release resin Thermory catalog, p. 4.

Plain aspen or thermo-aspen: choosing for your room

Both suit a sauna. The handbook places saunas in use class 2, indoor space potentially exposed to moisture and condensation, and recommends both ThermoWood® classes there ITWA handbook, p. 29. Thermory calls aspen a preferred sauna wood Thermory catalog, p. 4. Weigh these points for your room.

• Color. Plain aspen heartwood is grayish white to light grayish brown USDA Wood Handbook, p. 2-4. Thermally modified wood is brown through the board ITWA handbook, p. 6. Sauna wood of either kind may darken with use, fastest where hot steam reaches it Thermory USA guide, p. 4. To see the tone in your own light, order a sample.
• Movement. Thermally modified wood takes up less moisture and swells less, per the spruce data above ITWA handbook, pp. 26, 27. Acclimate either wood. Thermory’s US guide says to store paneling in the sauna room for a few weeks, ideally at 59 to 72°F Thermory USA guide, p. 2.
• Scent. USDA notes that well-dried aspen passes no odor or flavor to food USDA Wood Handbook, p. 2-4. Thermally modified wood carries the smoky scent described above, which Thermory’s US guide says weakens with long heating and airing Thermory USA guide, p. 4.
• Fasteners. Use stainless with either. Thermory’s US guide recommends stainless steel clips for its paneling Thermory USA guide, p. 2. For ThermoWood® sauna cladding and benches, the handbook’s minimum is Type 304 ITWA handbook, p. 44.
• Finish. Thermory’s US guide recommends a coating made for sauna use on all wooden surfaces Thermory USA guide, p. 4. For ThermoWood® sauna walls, ceilings, and benches, the handbook recommends mineral oil and rules out linseed oil, which enables fungal growth ITWA handbook, p. 33. See how to finish thermo-aspen benches.

If cedar is also on your list, the thermo-aspen vs cedar comparison sets the two side by side, and the buyer’s guide covers all three. To see a thermo-aspen profile in a full room, open it in the designer. The spec guide and the questions page cover specifying, care, and scent.

Sauna Marketplace sells both woods. Compare its plain aspen boards and trim with its thermo-aspen cladding and bench boards.