01 · Definition
Use the right term before you judge the claim.
Thermo-aspen is aspen permanently changed by heat and steam. ITWA’s definition of thermal modification starts at 320°F. Time, peak temperature, and pressure vary by species and by producer.
A working definition built from the ThermoWood® Handbook of the International ThermoWood Association (ITWA), 2026, p. 4 and p. 18, and the Thermory Sauna Materials Product Catalogue, June 2026, p. 6.
Five terms get mixed up in product copy. Each one means something different.
Aspen
The tree and its raw wood. Modification changes the wood’s properties, and the species stays aspen. The handbook’s species table lists aspen as a soft hardwood. ITWA handbook, p. 9 Thermory’s catalog calls aspen a preferred sauna wood because it is resin-free, smooth, and stays comfortably cool. Thermory catalog, p. 4
Thermally modified wood
Wood whose chemistry has been permanently changed with heat and steam, at 320°F or above in ITWA’s definition. It is a generic term, separate from any trademark or product name. ITWA handbook, p. 4
Thermory thermo-aspen
Thermory’s catalog lists its thermally modified sauna aspen at the Medium level, in profiles such as STS4, STEP, KYTE, and VIRE. For its sauna materials, Thermory heats the wood up to 356°F in a high-pressure chamber while pressure rises to about 130 psi. Thermory catalog, pp. 6, 8, 9, 10
ThermoWood®
The International ThermoWood Association’s registered trademark. ITWA’s handbook, the source of the test figures on this page, sorts ThermoWood® products into two classes, Thermo-S (stability) and Thermo-D (durability), and lists aspen in both. ITWA handbook, pp. 4, 9, 18
Profile
The profile sets thickness, board width, shadow line, and installed cover. Read those from the exact product’s drawing. A nominal name like 1×4 does not give the covering width.
02 · Quality test
Ask how it was made, and get the documents.
Color alone cannot show how well a board was modified. Thermory notes that the final shade depends on the species and on treatment temperature and time. Thermory catalog, p. 6 In the ThermoWood® process, run time depends on the class, the species, and the raw wood’s moisture and size, and settings change by species and dimension to keep the wood from cracking. ITWA handbook, p. 7
- 01Identify the producer and its process.
“Thermo” alone does not say who modified the wood or how. Ask the maker for its process description and quality documents, and check each claim against them.
- 02Confirm the species and the intended use.
A sauna aspen panel should not borrow exterior, structural, or insect claims from another species or class. Match each claim to the product and the room.
- 03Ask for the current technical documents.
Get the product sheet, declaration, tested classification, profile drawing, and installation guide that match the exact item.
- 04Measure installed geometry.
Nominal 1×4 and 1×5 names do not give the covering width left after tongues and grooves overlap. Take it from the catalog or drawing, as in section 04.
- 05Approve a sample, then open several packages.
Shade can differ slightly between production batches. Thermory’s guide says to order enough material and to open several packages so the wall matches. Thermory USA guide, p. 2 Order a sample of STS4 1×4 to see the tone and face first.
What to know before you build
- 06Plan for a soft surface.
In the handbook’s pine chart, modification changes Brinell hardness very little. Its Table 8 shows Thermo-D pine about 14% and Thermo-D spruce about 9% softer than untreated wood. The handbook has no aspen hardness figure, and its species table classes aspen as soft. ITWA handbook, pp. 9, 24, 25 Handle boards with care and do not drag them across each other. Thermory USA guide, p. 4
- 07Keep loads off the paneling.
Splitting strength fell 30 to 40% in handbook tests on spruce, pine, and birch. ITWA handbook, p. 24 Thermory’s guide asks for bench supports behind the paneling, since the panels must not carry the weight of the benches. Thermory USA guide, p. 4
- 08Expect scent and tone to change.
Thermory notes a characteristic scent that weakens over time and fades faster with long heating and airing. A suitable wood wax reduces it. Surfaces may darken, fastest where hot steam reaches them. Thermory USA guide, p. 4
03 · Claim boundaries
What common language does and does not prove.
Common sauna-wood claims: what they mean, what to ask for, and where they stop
| Claim or label | Reasonable meaning | Evidence to request | What it does not prove |
|---|---|---|---|
| Thermally modified | Heat and steam permanently changed the wood, at 320°F or above in ITWA’s definition. ITWA handbook, p. 4 | The producer, its process description, the species, and the intended use. | One performance grade shared by every producer. |
| No added chemicals | The modification itself uses heat and steam, with no chemicals added. ITWA handbook, p. 6 Thermory catalog, p. 6 | The producer’s process statement, plus separate finish documents. | That a later wax, oil, paint, adhesive, or coating is absent. |
| Dimensionally stable | Lower equilibrium moisture content, so less swelling and shrinking. In handbook test data, spruce modified at 428°F held about half the moisture of untreated spruce. ITWA handbook, p. 26 | Species, class, test method, and product data. | Zero movement, waterproofing, or permission to skip the ventilated gap behind the panels. |
| Sauna suitable | The exact part is documented for a hot-room use such as wall cladding or benches. | Application listing, section drawing, and installation and care guide. | Structural capacity. The handbook says ThermoWood® is not strength graded and must not be used in load-bearing structures. ITWA handbook, p. 21 |
| Durable | A documented gain for a defined exposure. The handbook puts saunas in use class 2 (EN 335), indoor use potentially exposed to moisture and condensation, and recommends Thermo-S or Thermo-D there. ITWA handbook, p. 29 | Use class, species-specific tests, and the assembly and care conditions. | Protection from every organism in every exposure, or a room that needs no care. Studies to date do not prove ThermoWood® resists termites. ITWA handbook, p. 29 |
| Fire performance | The exact product or assembly earned a named, tested class. | A current test report for the exact product and installation, in a form your building official accepts. | That modification alone earns a rating. ITWA notes a lower fire load for ThermoWood® products and says that cannot be used directly in fire design. ITWA handbook, p. 24 |
| Brown through-color | Heat changed the color through the cross-section. The handbook calls it through-stained, and it stays after sawing and planing. ITWA handbook, p. 6 | A cut sample plus process and product documents. | Complete modification, even tone, or quality by darkness. |
04 · Open coverage data
Estimate from installed covering width.
Linear feet depend on installed cover, the width each board adds to the wall once its tongue and groove overlap. The table converts the dimensions in Thermory’s June 2026 catalog to inches for four thermo-aspen profiles in five sizes. Thermory catalog, pp. 8, 9, 10
Coverage calculator
Estimated STS4 1x4
885linear feet
- Walls
- 182 sq ft
- Ceiling
- 42 sq ft
- Net area
- 211 sq ft
- Rows per wall
- 27
211 sq ft × 12 ÷ 3.15 in cover = 804 ft, plus 10%.
A first-pass estimate for wall and ceiling paneling only. Benches, trim, framing, and bundle rounding are separate, and the manufacturer's drawings govern the installed cover.
For 100 sq ft of STS4 1×4 at 3.15 in cover with a 10% allowance: 100 × 12 ÷ 3.15 × 1.10 = 419.05, rounded up to 420 linear feet.
Installed coverage for Thermory thermo-aspen sauna profiles, in inches
| Profile | Board size (in) | Installed cover (in) | Linear ft per sq ft | Rows on an 8 ft wall | Linear ft for 100 sq ft + 10% | Catalog |
|---|---|---|---|---|---|---|
| STS4 1×4 | 0.59 × 3.54 | 3.15 | 3.81 | 31 | 420 | p. 8 |
| STS4 1×5 | 0.59 × 4.72 | 4.33 | 2.77 | 23 | 305 | p. 8 |
| STEP 5/4×3 | 1.06 × 2.52 | 2.20 | 5.44 | 44 | 599 | p. 9 |
| KYTE 1×4 | 0.59 × 3.43 | 3.15 | 3.81 | 31 | 420 | p. 9 |
| VIRE 1×4 | 0.59 × 3.50 | 3.11 | 3.86 | 31 | 425 | p. 10 |
The CSV gives the same rows in inches, with the source catalog page for each. It is free to reuse with credit under CC BY 4.0. Thermory’s catalog stays the source for the dimensions.
Treat the factor as a first pass before the cut list.
Subtract only openings that replace paneling, such as the door and glass. A final takeoff also settles board direction, stock lengths, joints, corners, color matching, ceiling direction, usable offcuts, and bundle rounding.
Thermory’s guide puts material loss at about 5 to 10%, depending on board length. Thermory USA guide, p. 2
“Rows on an 8 ft wall” is 96 in divided by installed cover, rounded up. It ignores top and bottom gaps. Factors and totals use the unrounded conversion, so hand math with the rounded cover can differ slightly.
05 · Field checklist
Frame, fasten, and finish to the documents.
Two documents give framing figures: the sauna drawings in ITWA’s handbook and, for Thermory panels, Thermory USA’s installation guide. Where both set the same limit, it appears once below.
Framing and board minimums
- Batten spacing
- 23-1/2 in
- Maximum. The handbook drawings and Thermory USA’s guide both call for it.
- Air gap and batten thickness
- 13/16 in
- Minimum. The handbook drawings call for a ventilated gap this deep, and Thermory USA’s guide for battens this thick.
- Panel thickness
- 9/16 in
- Minimum for sauna wall and ceiling panels in the handbook drawings for ThermoWood® products.
- Bench boards
- 1-1/16 in
- Minimum bench board thickness in the handbook’s bench drawing for ThermoWood® products.
Thermory USA’s guide also calls for battens at least 1-13/16 in wide and recommends about 15-3/4 in between them. Thermory USA guide, p. 2 Battens 16 in on center sit close to that. A 1x furring strip is 3/4 in thick, just under the 13/16 in minimum; 5/4 stock, about 1 in thick dry, clears it.
Thermory panels: steps from Thermory USA’s guide
Acclimate in the room
Store panels for a few weeks in the room where they will go, flat and above the floor. Keep them at the installation temperature, ideally 59 to 72°F, in sealed packages until you install. Thermory USA guide, p. 2
Finish wet work first
Complete all wet work before the panels go up. Wet work done afterward makes the panels absorb moisture and swell. Make sure the room has enough ventilation. Thermory USA guide, p. 2
Straight battens, open air path
Use straight, level battens, sized and spaced as in the framing figures above. Run them across the panel direction so air moves behind the whole lining. Thermory USA guide, p. 2
Carry benches on the structure
Add bench supports behind the paneling before it goes up. The panels must not carry the weight of the benches. Thermory USA guide, p. 4
Ceiling first, then top down
Panel the ceiling first, leaving an airing gap where its first and last rows meet the walls. Run horizontal wall panels from the top down, tongue up and groove down, one row at a time around the room so corners align. Thermory USA guide, pp. 2, 3
Leave the corner gap
Leave at least 7/16 in beside the adjacent corner or board at the first and last panel, and cover it later with corner molding. Check level at least every three rows, since early errors grow row by row. Thermory USA guide, p. 3
Stainless clips, set flush
Fix through the groove side with a staple gun and stainless steel clips. Set the depth so clips neither stand proud nor dig into the face. Finishing nails are not suitable. Thermory USA guide, pp. 2, 3
Treat with a sauna product
Clean and dry the wood, then apply a coating made for sauna use or a sauna wax. Any paint must suit sauna conditions and follow its maker’s instructions. Thermory USA guide, p. 4
Wipe, ventilate, dry
Wipe benches with a clean damp cloth, never running water, and remove wet towels. Leave the heater on for at least half an hour with the vent open and the door and windows closed. Thermory USA guide, p. 4
Clean, treat, tighten
Clean seat, wall, and ceiling surfaces and renew the treatment, more often with heavy use. Tighten bench fastenings, and sand stubborn stains with fine-grit paper before treating again. Thermory USA guide, p. 4
Condensed from Thermory USA’s sauna wall paneling installation and maintenance guide. Read the full guide before installing.
Fasteners, finish, and storage for ThermoWood® products
- For sauna wall and ceiling cladding and sauna benches, the handbook sets Type 304 (A2) stainless fasteners as the minimum. Type 316 (A4) acid-resistant steel also works. ITWA handbook, p. 44
- The reason is acidity. ThermoWood® products measure about pH 4, against 4.5 to 5 for standard wood, and other fasteners react with the wood and stain around them. ITWA handbook, pp. 12, 44
- The handbook recommends mineral oil for sauna walls, ceilings, and benches, and rules out linseed oil, which enables fungal growth. ITWA handbook, p. 33 For Thermory panels, use the sauna wax or coating in Thermory’s steps above.
- Store interior products in a heated indoor space, flat on enough battens and off the ground, with bundles closed until use. ITWA handbook, p. 34
06 · Source register
Every figure traces to a page.
Version 1.1 moves citations to the 2026 handbook and gives every figure in US units, with framing limits as fractions rounded to the safe side. It adds framing and board minimums, fastener grade, use class, and a coverage calculator, and its installation steps follow Thermory USA’s guide. Version 1.0 was published August 9, 2026.
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