How Thermo-Aspen Is Made: Heat, Steam, and Time

Thermory’s sauna process

How Thermory makes its sauna boards

For its sauna materials, Thermory uses a high-pressure chamber. Its June 2026 catalog says the wood is heated to as much as 356°F while the pressure rises to about 130 psi, using only heat and steam, without added chemicals. Thermory catalog, p. 6

The catalog says this controlled environment lets the wood be modified without over-drying, which reduces internal stresses and minimizes cracking or warping. Final color depends on the species and on treatment temperature and duration, and the modified wood stays cooler to the touch on benches and walls. Thermory catalog, p. 6

Thermory USA says it tailors a separate process to each species by controlling temperature and moisture. Thermory USA brochure, p. 3 For cycle length, a Thermory slide at the Sauna Innovation Conference 2025 describes a 36 to 72 hour treatment in a thermokiln, monitored throughout. The slide describes Thermory’s thermokiln process as a whole and does not single out sauna materials. SIC 2025 presentation, Thermory slide

Watch Hannes Tarn’s talk on Saunologia

The kiln hall at Thermory’s Loo plant in Estonia. Video: Sauna Marketplace, September 2026.

ITWA, Nordic softwood

The industry definition, and one documented example schedule

Makers publish different process details. The International ThermoWood Association (ITWA) publishes the industry definition and one example schedule. Test figures come from ITWA’s handbook, mostly on pine and spruce. For Thermory boards, use Thermory’s catalog.

What counts as thermal modification

The ThermoWood® Handbook (2026) defines thermal modification as a permanent change to the wood’s chemical properties, made with heat and steam at or above a minimum temperature. For ThermoWood® products, it says no chemicals are used: the process runs on heat, steam, and water, and the new properties hold when the board is sawn or planed. ITWA handbook, pp. 4, 6

320°F: Minimum modification temperature in the ITWA definition ITWA handbook, p. 4
Heat, steam, water: The inputs the handbook lists for ThermoWood® production
Permanent: The changes survive sawing and planing, color included

Three phases in the kiln

The ThermoWood® process runs in three phases: high-temperature drying, thermal modification, then cooling and moisture conditioning. The handbook charts one example, for Nordic softwood in the Thermo-D class. ITWA handbook, p. 7

1 High-temperature drying
2 Modification, held at 414°F
3 Cooling and conditioning
Thermo-D 414°F Thermo-S 374°F

One documented example, not a universal schedule: Nordic softwood, Thermo-D class, about 42 hours. Moisture falls from about 20% to 0% in phase 1 and is raised to 4 to 7% with water and steam in phase 3.Redrawn in °F from data in the ThermoWood® Handbook 2026, Figure 3, p. 7. Schedules differ by species, class, board size, and producer.

Phase 1

High-temperature drying

The kiln heats quickly to 212°F, then climbs gradually toward the target. The wood dries until its moisture content reaches zero.

Phase 2

Thermal modification

The kiln holds a steady temperature while the modification takes place. Steam protects the wood and takes part in the permanent changes. The example holds 414°F for about 3 hours.

Phase 3

Cooling and conditioning

A water spray brings the temperature down. Water and steam then raise the moisture content to 4 to 7%, which improves machining and stability.

Phase details as the handbook describes them ITWA handbook, p. 7

Two classes, set by temperature

ThermoWood® comes in two classes. Thermo-S stands for stability. Thermo-D stands for durability, meaning stability plus resistance to decay and weather. ITWA handbook, p. 18

Aspen is on the handbook’s species list in both classes, but the Thermo-D temperature table has no aspen row, so there is no aspen Thermo-D temperature to quote. ITWA handbook, pp. 9, 18

For a sauna, the handbook recommends either class. It lists cladding, ceilings, and saunas under use class 2: indoors, but possibly exposed to moisture and condensation. ITWA handbook, p. 29

One schedule per species

A separate process instruction is written for each species. A control system adjusts the temperature to prevent cracking, with different settings for each species and board size. Cycle length depends on the class, the species, and the starting moisture and dimensions of the wood. ITWA handbook, pp. 7, 9

ThermoWood® classes and modification temperatures

ThermoWood® classes and modification temperatures
Class	Letter stands for	Modification temperature	Aspen
Thermo-S	Stability	374°F (±5°F) for pine, spruce, and hardwoods as a group	Made in this class (Table 1)
Thermo-D	Durability: stability plus resistance to decay and weather	414°F (±5°F) for pine and spruce	Made in this class (Table 1). Table 3 gives no aspen temperature.

Tables 1 to 3 of the ThermoWood® Handbook 2026, converted to °F. Other Thermo-D species in Table 3 run from 374°F (iroko) to 446°F (radiata pine). ITWA handbook, pp. 9, 18

Moisture content of example boards, from sawmill to kiln exit

Material	Moisture content
Green lumber	30%
Dried lumber	20%
After thermal modificationFinished panels stay at 4 to 7%	4 to 7%

ITWA handbook, Figure 11, p. 15

Why boards measure under nominal size

That drop in moisture shrinks the board. In the handbook’s example, a thermally modified board comes out about 3% smaller than its nominal size. ITWA handbook, p. 15

In a sauna build, this shows up in coverage. Plan wall area from the covering width on the product drawing, not from the nominal name. The spec guide has the coverage math.

What changes in the wood

What the heat does inside the board

In the handbook’s account, heat breaks down hemicellulose in the cell walls, and after modification its content is significantly lower. That leaves fewer water-absorbing groups and less material for decay fungi to feed on. ITWA handbook, p. 11

Changes against unmodified wood, ITWA data

Changes against unmodified wood, ITWA data
Property	Change	Tested on	Source
Equilibrium moisture content	About half	Spruce modified at 428°F	p. 26
Swelling and shrinkage	Significantly lower	Spruce	p. 27
Thermal conductivity	20 to 25% lower	Nordic softwood	p. 28
Color	Darker, through the full thickness	ThermoWood® products	p. 6
Resin and extractives	No resin; most extractives evaporate	ThermoWood® products	pp. 6, 12
Acidity	About pH 4, against 4.5 to 5	ThermoWood® products	p. 12
Density	30.6 falls to 26.8 lb/ft³	Pine, untreated to Thermo-S	p. 20
Splitting strength	30 to 40% lower	Spruce, pine, and birch	p. 24
Hardness	Very small in the pine chart; in Table 8, Thermo-D pine about 14% and spruce about 9% softer	Pine and spruce	pp. 24, 25

Values from the ThermoWood® Handbook 2026, converted to US units. None of these rows is an aspen measurement; the handbook has no aspen-specific test data.

Less movement through steam cycles

A sauna swings between dry heat and bursts of humidity each time water hits the stones. In the handbook’s spruce tests, modified wood took up less water and swelled and shrank less. ITWA handbook, pp. 26, 27

Still leave the gaps and expansion room your board’s installation guide calls for.

Slower to pass heat to skin

Thermally modified Nordic softwood conducts heat 20 to 25% less than standard timber, and the handbook names saunas among the uses this suits. ITWA handbook, p. 28 A bench or backrest with lower conductivity passes heat into skin more slowly.

Treat the figure as softwood data. The handbook gives no aspen value.

Color that holds when you cut it

In ThermoWood® products the color runs through the board and holds when sawn or planed, so cut ends and chamfers match the face. ITWA handbook, p. 6 Surfaces can still darken in use. Thermory USA’s guide says wood surfaces may darken over time, and faster where hot steam reaches them. Thermory USA guide, p. 4

Approve a real sample in the room’s light before you order.

No resin bleed, a distinct scent

Most extractives evaporate during modification, and the handbook names the absence of resin as a property of ThermoWood® products. ITWA handbook, pp. 6, 12 Aspen starts out resin-free, as Thermory’s catalog notes. Thermory catalog, p. 4

In VTT tests on ThermoWood® pine, wood modified at 356°F gave off about 44% less total VOC than unmodified pine, and wood modified at 446°F about 84% less. ITWA handbook, p. 30

Thermory USA’s guide says the scent diminishes over time, and disperses faster when you heat the sauna for a long time and air it at the same time. Thermory USA guide, p. 4

Quality markers

What to check in a thermally modified board

Color is the first thing you notice. These checks tell you more about how a board was made. Read the species, treatment class or level, intended use, profile drawing, and installation guide for the exact product.

01

Color through the board

Look at a cut end or a planed edge. The handbook describes the color of ThermoWood® products as running through the board. ITWA handbook, p. 6 A dark face alone does not prove a good schedule.

02

A named schedule

In the handbook, class, species, starting moisture, and board size all set the cycle. ITWA handbook, p. 7 The product document should name the species and the class or treatment level.

03

Moisture response

In the handbook’s spruce tests, modification lowered equilibrium moisture content and reduced swelling and shrinkage. ITWA handbook, pp. 26, 27 For its sauna materials, Thermory’s catalog describes dimensional stability under temperature and humidity changes. Thermory catalog, p. 6 Whichever board you use, keep the gaps its installation guide calls for.

04

Clean machining

In the ThermoWood® process, conditioning back to 4 to 7% moisture is there to improve machining and stability. ITWA handbook, p. 7 Look for crisp profiles, tight end matches, and consistent widths across a bundle.

05

A declared sauna use

Match the document to the room. Thermory’s catalog lists its thermo-aspen as sauna wall panels and bench boards. Thermory catalog, pp. 8, 16 Thermory USA’s guide also covers paneling the ceiling. Thermory USA guide, p. 3

Correct use

What to know before you build

Settle these four at the design stage. Each card gives the rule and the step to take.

Keep it out of the structure

ThermoWood® is not sold as strength-graded timber, and the handbook says it must not be used in load-bearing structures. In tests on spruce, pine, and birch, splitting strength fell 30 to 40%. ITWA handbook, pp. 21, 24

Build studs, joists, and bench frames from structural lumber. Thermory USA’s guide says to add bench support behind the paneling, which must not carry the benches’ weight. Thermory USA guide, p. 4 Thermo-aspen bench boards, such as Thermory’s SHP, sit on that frame. Thermory catalog, p. 16

Use stainless fasteners

ThermoWood® products measure about pH 4, against 4.5 to 5 for standard timber. Because of that acidity, the handbook says fasteners that are not stainless can corrode and stain the wood around them. ITWA handbook, pp. 12, 44

Thermory USA’s guide recommends a staple gun and stainless steel clips. Thermory USA guide, p. 2 For ThermoWood® products, the handbook sets Type 304 (A2) stainless as the minimum for sauna cladding and benches, and names Type 316 (A4) as the acid-resistant option. ITWA handbook, p. 44

Choose the finish with care

Thermory USA’s guide recommends treating the wooden surfaces with sauna wax or another suitable coating. Thermory USA guide, p. 4

Skip linseed oil. The handbook says it enables fungal growth on ThermoWood® products. ITWA handbook, p. 33

Ask for the fire classification

The handbook says each maker of ThermoWood® products is responsible for testing its product and for the fire class it states. ITWA handbook, p. 24

If your project needs a fire rating, request the tested classification for the exact product from its maker.

More on what thermal modification does not do

Evidence register

Claims you can trace

Updated September 26, 2026. Handbook values are converted to US units, with minimums rounded up and maximums rounded down.

How thermo-aspen is made