Building a Sauna? 12 Numbers From the 2026 ThermoWood® Handbook

What is the ThermoWood® Handbook?

ITWA publishes it as a guide to the production, properties, and uses of ThermoWood® products, written for readers from architects and contractors to carpenters and schools ITWA handbook, p. 3. Both editions are PDFs on the association’s site: the 2026 Swedish edition, which this post cites, and the 2025 English edition, which has the same page numbers.

Twelve numbers for a sauna build

1. 320°F: where thermal modification starts

ITWA defines thermal modification as changing wood’s chemical properties with heat and steam at 320°F or higher, and the changes are permanent. A separate process instruction is written for each species, and ThermoWood® boards leave the kiln at 4 to 7% moisture content ITWA handbook, pp. 4, 7, 9.

The chart redraws the handbook’s example schedule in °F: Nordic softwood, Thermo-D class, about 42 hours. Real schedules change with species, class, moisture content, and board size ITWA handbook, p. 7. Read more: how thermo-aspen is made.

2. 374°F and 414°F: two classes, both recommended for saunas

Thermo-S (S for stability) is modified at 374°F (±5°F) for pine, spruce, and hardwoods as a group. Thermo-D (D for durability) runs at 414°F (±5°F) for pine and spruce. Aspen is made in both classes, but Table 3 lists no Thermo-D temperature for it ITWA handbook, pp. 9, 18. Table 9 puts saunas in use class 2, indoor use potentially exposed to moisture and condensation, and recommends both classes there ITWA handbook, p. 29. Read more: Thermo-S vs Thermo-D.

3. 20%: the moisture level decay fungi need

Decay fungi become active when wood moisture content passes 20%. Equilibrium moisture content is the level wood settles at in steady air, and the handbook says that of thermally modified wood stays well below 20% at any relative humidity. In tests by the Technical Research Centre of Finland (VTT), modifying spruce at 428°F halved it ITWA handbook, p. 26. Standing water is a separate matter: Thermory USA’s guide says not to let water pool on wooden surfaces Thermory USA guide, p. 4. Read more: can a sauna get moldy?

4. 20 to 25%: lower heat conductivity

The thermal conductivity of thermally modified Nordic softwood is 20 to 25% lower than that of standard timber, and the handbook names saunas among the uses this suits. It gives no figure for aspen ITWA handbook, p. 28. Read more: why sauna benches stay comfortable.

5. 44% and 84%: lower VOC emissions

VTT measured volatile organic compounds (VOCs) from ThermoWood® pine about two months after modification. Total VOCs were about 44% lower than unmodified pine after modification at 356°F, and about 84% lower at 446°F. The smoky smell probably comes from furfurals, and it can seem stronger than untreated wood’s despite the lower emissions ITWA handbook, p. 30. Read more: does thermo-aspen off-gas in a sauna?

6. pH 4 and Type 304: stainless fasteners

ThermoWood® products have a pH of about 4, against 4.5 to 5 for standard wood, and that acidity can affect fastener corrosion ITWA handbook, p. 12. Table 10 sets Type 304 stainless (class A2) as the minimum for sauna wall and ceiling cladding and for sauna benches, and the handbook also allows Type 316 (A4), the acid-resistant grade. Other fasteners react with the wood and stain around the fastener ITWA handbook, p. 44. For its paneling, Thermory USA recommends a staple gun with stainless steel clips Thermory USA guide, p. 2. Read more: stainless fasteners for sauna paneling.

7. 9/16 in: minimum panel thickness

The handbook’s sauna wall and ceiling drawings show panels at least 9/16 in thick ITWA handbook, p. 38.

8. 23-1/2 in: maximum batten spacing

The handbook drawings and Thermory USA’s guide both call for battens no more than 23-1/2 in apart, and Thermory USA recommends about 15-3/4 in ITWA handbook, p. 38 Thermory USA guide, p. 2. A 16 in on-center layout sits close to that recommendation; 24 in on center exceeds the maximum.

9. 13/16 in: minimum ventilated gap

Behind the panels, the handbook drawings show a ventilated gap of at least 13/16 in ITWA handbook, p. 38. Thermory USA’s guide calls for battens at least 13/16 in thick and 1-13/16 in wide Thermory USA guide, p. 2. A 1x furring strip is 3/4 in thick, just under 13/16 in; 5/4 stock, about 1 in thick dry, clears it. Read more: framing, battens, and the air gap.

10. 1-1/16 in: minimum bench board thickness

The handbook’s example bench drawing shows bench boards at least 1-1/16 in thick ITWA handbook, p. 39. Whatever the boards, Thermory USA says to add bench supports behind the paneling, because the paneling must not carry the benches’ weight Thermory USA guide, p. 4.

11. 30 to 40%: lower splitting strength

Thermal modification cut splitting strength by 30 to 40% in tests on spruce, pine, and birch. ThermoWood® is not available strength graded, and the handbook says it must not be used for load-bearing structures ITWA handbook, pp. 21, 24. Frame the room and the bench supports with structural lumber. Keep fasteners back from board ends or pre-drill them, and cut with fine-toothed blades, since coarse teeth can chip the edges. The dust is dry and fine, so use airtight dust extraction and a respirator whenever cutting makes dust ITWA handbook, pp. 31, 44. Read more: cutting, sanding, and storing thermo-aspen.

12. 3%: why you estimate from actual sizes

Modification shrinks boards as they dry. In the handbook’s example, a board comes out about 3% smaller than its nominal size ITWA handbook, p. 15. Work out coverage from each profile’s covering width, the face a board shows once installed; the profiles page lists it. Read more: how much thermo-aspen do I need?

Two rules without a number: finish and storage

Finish: mineral oil, not linseed oil

The handbook recommends mineral oil for sauna walls, ceilings, and benches, and calls linseed oil unsuitable because it enables fungal growth. Some water-based finishes adhere poorly because of the wood’s acidity and low water uptake, and a finish that seals too well stops moisture moving between the air and the wood ITWA handbook, pp. 30, 33. For Thermory paneling, Thermory USA recommends a sauna wax or a similar coating made for sauna use Thermory USA guide, p. 4. Read more: how to oil thermo-aspen sauna benches.

Storage: heated, flat, and bundled

Store interior products in a heated indoor space, flat on an even base with enough battens, off the ground, and shielded from dirt and UV light, with bundles closed until use. Check that the wood’s moisture matches the room before work starts, since conditioning can take several days ITWA handbook, pp. 31, 34. Thermory USA calls it best practice to keep its paneling in the installation room for a few weeks first, and says to finish any wet work before the paneling goes up Thermory USA guide, p. 2.

The sauna numbers in one table

What the handbook does not tell you

Much about aspen

Aspen appears in the species table as a soft hardwood made in both classes, and in a chart of typical raw timber sizes ITWA handbook, pp. 9, 10. The handbook reports no test data measured on aspen: its results come mostly from pine, spruce, or birch, and its density table has no aspen row ITWA handbook, pp. 20, 24, 26, 30.

Heater clearances and other woods

The handbook covers the wood, not the heater. Clearances and temperature limits come from the heater’s manual, and Thermory USA’s guide also says not to exceed the heater maker’s maximum temperature Thermory USA guide, p. 4. The handbook has no cedar data; for that comparison, see thermo-aspen or cedar for your sauna.

Your final design

The handbook calls its products and details examples only, says final design decisions belong to qualified professionals for each project, and says to install ThermoWood® products per the manufacturer’s instructions ITWA handbook, pp. 3, 44.

To choose a profile, start with the buyer’s guide, try it in a full room with the 3D designer, and order a sample. Thermory thermo-aspen boards are on the Sauna Marketplace thermo-aspen page, and more short answers are on the questions page.