ThermoWood Output Hit a Record 132 Million Board Feet in 2025. Almost None of It Was Aspen
Research, standards and the sauna industry
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International ThermoWood Association members made 9.5% more thermally modified wood in 2025, 84% of it pine and spruce. Aspen rounded to zero, so the thermo-aspen in US saunas comes from producers outside the association.
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Members of the International ThermoWood Association produced 132.3 million board feet of thermally modified wood in 2025, a record. That was up 9.5% from 2024 and 42% from 2019, according to the association’s annual production statistics, published in March (ITWA Production Statistics 2025). Pine and spruce made up 84% of it. Aspen, the classic sauna bench wood, rounded to zero.
The record is pine and spruce
The association’s 2025 breakdown by species:
- Pine: 56%.
- Spruce: 28%.
- Birch and aspen: 0% each. In 2001 they were 11% and 9%.
- Other species: the remaining 15%, about 20.1 million board feet. Ayous, the African hardwood sold in sauna shops as abachi, was 34% of that and ash 25%. Frake, iroko, radiata pine and hemlock made up most of the rest.
Most of the output is the more heavily treated grade. Thermo-D, the class sold for exterior use and humid rooms, was 94% of 2025 production. Thermo-S, the lighter interior class, fell to 5% from 14% in 2019. Lunawood, an association member, lists its Thermo-D boards for spas and saunas (covered in our story on its Georgia mill).
Exports outside Europe nearly doubled since 2019
Of 2025 output, 6% went to Finland, 74% to the rest of the EU and 20% outside it. That outside share works out to about 26.5 million board feet, up from about 14 million in 2019, when it was 15% of a smaller total. The association doesn’t break out North America.
Producing members include:
- Finland: Lunawood and Stora Enso.
- Turkey: Novawood.
- North America: Arbor Wood Co. in Minnesota and Thermal Wood Canada.
Thermory, the Estonian producer whose thermo-aspen and thermo-alder sauna wood is sold across the US, isn’t a member. It uses its own process and brand, so none of its volume is in these figures. Members made essentially no aspen, so nearly all thermo-aspen comes from producers outside the association. Our look at Estonia’s sauna manufacturing cluster covers where much of that wood is made.
US import data points the same way. US imports of Estonia’s profiled hardwood, the tariff category that includes thermo-aspen and alder paneling, rose 12% to $8.0 million for January through July 2026. Finland’s profiled softwood fell 30% over the same months (UN Comtrade).
“ThermoWood” names a process, not a species
- Ask for the species and the treatment class. ThermoWood is the association’s registered trademark for its process. A thermally modified sauna board can be pine, spruce, aspen, alder or ayous, and Thermo-S and Thermo-D are treated at different temperatures.
- Ask who made it. Thermally modified wood from non-members, including nearly all thermo-aspen, is made to each producer’s own process and specifications.
