Building With Thermo-Aspen: 7 Things to Plan For

Is thermally modified wood worth it for a sauna?

For sauna walls and ceilings, the handbook’s own table points to yes. It places a sauna in use class 2, indoor use potentially exposed to moisture and condensation, and recommends both ThermoWood® classes there ITWA handbook, p. 29. Thermo-S stands for stability, and Thermo-D for durability, meaning stability plus resistance to decay and weather. The handbook lists aspen in both classes ITWA handbook, pp. 9, 18. Thermo-S vs Thermo-D explains the difference.

Two properties explain the fit. Equilibrium moisture content is the moisture level wood settles at in steady air. In the handbook’s data, thermally modified wood stays well below 20% at any relative humidity, and decay fungi become active above 20% ITWA handbook, p. 26. Thermal conductivity of thermally modified Nordic softwood is 20 to 25% lower than standard timber, and the handbook names saunas as an ideal use ITWA handbook, p. 28.

Thermory gives its own reasons in its catalog. It says its thermally modified wood stays cooler to the touch on benches and walls, and keeps its shape through extreme temperature and humidity changes Thermory catalog, p. 6.

The points below are about correct use. None of them rules out a thermo-aspen sauna. Each one changes how you build it. How thermo-aspen is made covers the kiln side of the story.

Structure, strength, and handling: points 1 to 3

1. Frame with graded structural lumber

ThermoWood® products are not sold as strength-graded timber and must not be used for load-bearing structures ITWA handbook, p. 21. The USDA Wood Handbook says the same of heat-treated wood in general USDA Wood Handbook, p. 19-9.

In your sauna, that means studs, joists, rafters, and floor framing are ordinary structural lumber sized to local code. Thermo-aspen is the finish that covers them. Framing, battens, and the air gap covers the layer in between.

Benches are a normal use. The handbook’s sauna bench drawing shows Thermo-S or Thermo-D beams under the bench boards ITWA handbook, p. 39. The load still has to reach the wall framing. Thermory USA’s guide says to add bench support behind the paneling, because the paneling must not carry the bench weight Thermory USA guide, p. 4.

2. Pre-drill and fasten with care, since it splits more easily

Thermal modification lowers some strength properties and leaves others unchanged or higher. In ITWA test data, splitting strength fell 30 to 40% across spruce, pine, and birch, with bigger drops at higher temperatures. Impact strength fell about 25% in spruce modified at 428°F for three hours ITWA handbook, p. 24.

Hardness changes less. The handbook calls the change small in its pine chart, and its Table 8 puts Thermo-D pine about 14% and Thermo-D spruce about 9% below untreated wood ITWA handbook, pp. 24, 25. Compression strength along the grain is not reduced. In those compression tests, modified pieces broke into smaller sections instead of buckling, because the wood is less elastic ITWA handbook, p. 23. The USDA handbook describes heat-treated wood as more brittle, with a tendency to crack and split USDA Wood Handbook, p. 19-9.

What to do: pre-drill screw holes, especially close to board ends, so fasteners do not crack the wood. In the handbook’s screw tests, lower-density wood held better with smaller pre-drilled holes ITWA handbook, pp. 23, 44. Cut with fine-toothed blades, since large teeth chip the edges ITWA handbook, p. 31. Fasten sauna cladding and benches with stainless steel. For ThermoWood® products, the handbook sets Type 304 as the minimum ITWA handbook, p. 44. The stainless fastener guide explains the choice.

3. Store boards flat and lift long ones with care

The handbook says ThermoWood® takes a little more care to handle than unmodified wood, because it is more prone to mechanical damage ITWA handbook, p. 31. Its storage and handling rules are specific ITWA handbook, p. 34:

• Store interior boards in a dry, ventilated, heated indoor space.
• Lay them flat on an even base, off the ground, on enough battens.
• Protect them from dirt and UV light, and keep bundles closed until use.
• Lift long boards with their reduced bending strength in mind.
• Handle tongue-and-groove boards carefully, long ones most of all.

Thermory USA’s guide adds rules for its panels: check them for transport damage on arrival and never install a defective one, handle packages with care, and never drag boards across each other Thermory USA guide, pp. 1, 4. Cutting, sanding, and storing thermo-aspen covers the workshop side.

Fire ratings: point 4

4. Ask for fire test data on the exact product

Compared with standard timber, ThermoWood® has a smaller fire load, meaning less fuel, and produces less smoke. The handbook credits lower density and fewer extractives, and it says these advantages cannot be used directly in fire design. It makes the maker of each ThermoWood® product responsible for testing it and for the fire class it states ITWA handbook, p. 24.

With a fire-retardant treatment, the handbook says ThermoWood® products can reach European class C or B ITWA handbook, p. 33. Those are European classes. Most US model building codes use a different scale: interior finish is classed A, B, or C by flame spread index in the ASTM E84 tunnel test USDA Wood Handbook, p. 18-5. The letters overlap, so never read a European class as a US one.

Termites, ground contact, and sun: points 5 to 7

5. Plan termite protection for your region

Studies to date do not prove ThermoWood® resistant to termites. The handbook recommends local tests, because termite types vary by region, and notes that termites come up from the soil and attack both wood and concrete-based materials ITWA handbook, p. 29.

The beetle results are stronger. Studies showed resistance to longhorn beetles, the common furniture beetle, and Lyctus brunneus, and good resistance to the old house borer ITWA handbook, p. 29.

For a sauna in termite country, plan protection as you would for any wood building. The handbook mentions polyethylene membranes in foundations and bituminous coatings that seal entry routes ITWA handbook, p. 29. Follow local code and local pest-control practice.

6. Keep it off soil and standing water

The handbook does not recommend ThermoWood® for structures in direct contact with soil or water, and its use-class table gives no recommendation for the classes that cover soil, fresh water, or salt water ITWA handbook, p. 29. The USDA handbook also advises against ground contact for heat-treated wood USDA Wood Handbook, p. 19-9.

For an outdoor sauna, set the building on a foundation or base made from materials rated for ground contact. The handbook’s exterior cladding drawing keeps Thermo-D boards at least 12 in above the ground, with ventilation behind them ITWA handbook, p. 40.

Inside, avoid standing water on wood. Thermory USA’s guide says to throw water only on the heater, keep water from pooling on wood, and never wash a sauna with running water or a pressure washer Thermory USA guide, p. 4.

7. Finish exterior boards against UV, or let them silver

Thermal modification does not stop UV light from graying unfinished wood, and rain can shift the color slightly. For weather-exposed wood, the handbook recommends a finish that protects against moisture, erosion, and UV, especially in warm, humid climates ITWA handbook, p. 29.

Both outcomes are valid choices. Thermory USA’s sauna brochure says the exterior of its DIY sauna kits, one model line excepted, ages to silver in sun and rain. It suggests a water-based UV protection to keep the original color Thermory USA brochure, p. 16.

Match the board to the exposure, too. For exterior cladding with prolonged wetting, the handbook recommends Thermo-D only ITWA handbook, p. 29. A board sold for sauna interiors is not automatically an exterior board, so use what its manufacturer documents for outdoor use. Inside the room, wood may darken over time, faster where hot steam hits it Thermory USA guide, p. 4.

A seven-point check before you build

Run this list against your plans and the product documents. Each line maps to one point above.

• Framing: studs, joists, and rafters are graded structural lumber, and thermo-aspen is the finish.
• Benches: supports carry the load into the wall framing, not through the paneling.
• Fastening: screw holes pre-drilled, with stainless steel fasteners (Type 304 or better for ThermoWood® products).
• Handling: boards stored flat, dry, and bundled until use, and long tongue-and-groove boards lifted with care.
• Fire: US test data for the exact product, local code checked, and heater clearances followed.
• Termites and ground: a base rated for soil contact, wood kept off the ground, and local termite measures in place.
• Exterior: a UV finish applied, or silvering accepted, on a board documented for outdoor use.

To compare boards, browse the thermo-aspen range at Sauna Marketplace and check each product’s documents against this list. The profile comparison shows exact dimensions and coverage, and the spec guide collects the claims and install data in one place.

Short answers on fire, termites, and outdoor use are on the questions page. To test a profile in a room, open it in SaunaDesignPro. To see the wood itself, order a sample.