Why Thermo-Aspen Sauna Benches Stay Comfortable to Sit On

Wood feels milder to the touch than metal or stone

The USDA Wood Handbook gives the reason. Wood has low conductivity and moderate density and heat capacity, so its thermal diffusivity is much lower than that of metal, brick, or stone. For that reason, the USDA says, wood does not feel extremely hot or cold to the touch USDA Wood Handbook, p. 4-17.

Thermal diffusivity measures how quickly a material takes up heat from its surroundings. The USDA gives a typical value for wood of 0.00025 sq in per second. Stone is 0.002 and steel is 0.02, or 8 and 80 times higher USDA Wood Handbook, pp. 4-14, 4-17.

Conductivity tells the same story. Lower numbers mean slower heat flow through the material.

The sauna consequence is metal on the seat. In these values, steel conducts heat about 390 times faster than quaking aspen. The build section below covers how to keep screw heads off the sitting surface.

Lighter, drier wood conducts less heat

Wood’s conductivity rises as its density, moisture content, temperature, or extractive content rises USDA Wood Handbook, p. 4-13. Two of those levers come with the board you buy: how dense it is and how much water it holds.

The USDA formula scales conductivity with specific gravity, and moisture raises that slope. Aspen sits toward the light end. The USDA table lists quaking aspen at a specific gravity of 0.40, with a conductivity of 0.67 oven-dry and 0.80 at 12% moisture USDA Wood Handbook, pp. 4-13, 4-15. Moisture alone adds about a fifth.

Temperature moves every wood in the same direction. Conductivity rises about 1 to 2% per 10°F USDA Wood Handbook, p. 4-14, so a hot room raises all these values without reordering them.

One scope note. The USDA’s aspens are bigtooth and quaking aspen, cut mainly in the northeastern and Lake states, with some in the Rocky Mountain states, and the USDA says its specific gravities are not species averages USDA Wood Handbook, pp. 2-4, 4-16. The handbook’s aspen is European aspen (Populus tremula), a soft hardwood from the Nordic and Baltic regions ITWA handbook, p. 9.

What thermal modification changes, in ITWA testing

In International ThermoWood Association (ITWA) data, thermally modified Nordic softwood conducts heat 20 to 25% less than standard timber. The handbook names saunas among the structures this suits ITWA handbook, p. 28. It does not say which change causes the drop. Its own data show modification lowering density and moisture content, two of the USDA’s levers.

ITWA’s two product classes are Thermo-S (S for stability) and Thermo-D (D for durability). In the ThermoWood® process, pine and spruce are modified at 374°F for Thermo-S and 414°F for Thermo-D ITWA handbook, p. 18. The process page explains the kiln stages.

Density

Air-dried pine fell from 30.6 lb/ft³ to 26.8 lb/ft³ in Thermo-S and 26.2 lb/ft³ in Thermo-D. Spruce fell from 28.7 lb/ft³ to the same two values ITWA handbook, p. 20.

Moisture

Equilibrium moisture content is the level at which wood neither gains nor loses moisture USDA Wood Handbook, p. 4-3. Spruce modified at 428°F settled at about half the equilibrium moisture content of untreated spruce, and the gap widened at higher humidity. ThermoWood® leaves the kiln at 4 to 7% moisture ITWA handbook, pp. 7, 26.

Resin and extractives

During modification, softwoods secrete resin and hardwoods lose extractives. The handbook lists the absence of resin among the product’s traits ITWA handbook, pp. 6, 7.

Thermory describes its own thermally modified sauna wood as better insulating. Its catalog says the wood “stays cooler to the touch for improved comfort on benches and walls,” and that lower moisture content makes it lighter. It credits aspen with a resin-free composition, a smooth texture, and the ability to stay comfortably cool Thermory catalog, pp. 4, 6. For the limits of the process, see what thermal modification does not do.

Bench boards and what carries them

Bench boards sit on supports fixed behind the paneling, and the paneling itself must not carry the bench's weight Thermory USA guide, p. 4.

Thermory’s catalog lists thermo-aspen SHP bench boards 1.1 in thick in seven widths, from about 1.65 to 7.28 in, and 0.83 in thick in three widths Thermory catalog, p. 16. For Thermory benches, follow Thermory’s documents. The 1.1 by 3.5 in board is listed at Sauna Marketplace as Thermory 5/4 SHP thermo-aspen bench wood. The profiles page shows where SHP fits among the wall profiles.

Build details that affect comfort

• Put bench supports in the framing behind the paneling. Thermory USA’s guide says the paneling must not carry the weight of the benches Thermory USA guide, p. 4.
• Screw the top boards from below so no screw head reaches the seat. Sauna Marketplace’s bench design guide shows this method, along with bench heights, depths, and layouts.
• Choose the screw grade with care. For ThermoWood® products, the handbook’s Table 10 sets Type 304 stainless as the minimum for sauna benches, with Type 316 as the acid-resistant option ITWA handbook, p. 44. See stainless fasteners for sauna paneling.

Batten, fastening, and bench-support details for the whole room are in the spec guide.

Keep the seat dry, and sit on a dry towel if it feels hot

The maintenance page in Thermory’s catalog covers the in-use side Thermory catalog, p. 39:

• If the bench feels too hot, sit on a seat cover or a dry towel.
• Avoid sitting on the bench in wet swimwear.
• Throw water only on the heater, never on the walls or benches.
• Wipe the bench with a clean damp cloth, not running water, and do not let water pool.
• After cleaning, run the heater at least half an hour with the air vent open and the door and windows closed.

Moisture raises wood’s conductivity USDA Wood Handbook, p. 4-13, which is one more reason to keep the seat dry between sessions.

For finish, Thermory asks for a breathable wood protector before first use Thermory catalog, p. 39. For ThermoWood® products, the handbook recommends mineral oil for sauna walls, ceilings, and benches, and rules out linseed oil because it enables fungal growth ITWA handbook, p. 33. The steps are in how to oil thermo-aspen sauna benches, and more short answers are on the questions page.