Your cedar sauna has a secret. It's not rotting. It's not cracking. But it's slowly changing shape every single day — and you might not notice until year three, when the door doesn't close quite right anymore.

Cedar is beautiful. It smells incredible. But here's the honest truth that most sauna companies skip: cedar moves. In Canada, where temperatures swing from -30°C to +35°C over the course of a year, cedar expands and contracts constantly. Multiply that by 3-5 years, and you get warped panels, gaps in the joints, and a sauna that leaks heat [8].

There's a better way to build. And it doesn't involve chemicals, sealants, or annual maintenance weekends.

What Is Thermally Modified Wood?

Thermally modified wood — often called ThermoWood — is natural wood that's been heated to 180-240°C in a controlled environment with only heat and steam. No chemicals. No additives. Just physics [1].

The heat permanently changes the wood at the cellular level. The cell walls become less able to absorb water. The natural sugars that attract fungi and decay break down. What's left is a material that's dimensionally stable, rot-resistant, and remarkably durable — without a single chemical treatment [2].

Research from the USDA Forest Service confirms that thermally modified wood performs significantly better than untreated wood in severe outdoor conditions, making it suitable for above-ground exterior applications [3].

Six Reasons Thermally Modified Wood Wins for Canadian Saunas

1. It Doesn't Warp When the Weather Changes

This is the big one. A peer-reviewed review in the Journal of Materials Science found that thermal modification improves wood's dimensional stability by 30-90%, depending on the species and treatment temperature [1]. That means your sauna stays square through BC rain, Alberta cold snaps, and Ontario humidity — year after year.

Cedar, by comparison, can shrink by up to 5% in dry winter air and swell back in summer. Over time, those micro-movements add up to visible gaps and warping [8].

2. It Resists Moisture Without Chemicals

Thermally modified wood absorbs 40-60% less moisture than untreated wood [1]. This matters for outdoor saunas exposed to rain, snow, and morning dew. Less absorbed water means less freezing-and-thawing damage in winter — the cycle that eventually destroys most outdoor wooden structures.

3. Lower Maintenance Means More Time Enjoying

Cedar saunas need annual oiling, tightening, and sometimes sanding to stay in good shape outdoors. Thermally modified wood requires far less — typically just a clear UV-protective oil every few years to maintain its rich colour tone.

4. It's Naturally Rot-Resistant

The heat treatment breaks down hemicellulose — the part of wood that fungi and bacteria feed on. Research published in MDPI's Journal of Materials Science confirms that this process creates wood that naturally resists decay without preservatives [2].

5. Interior Safety — No Harmful Off-Gassing

Because no chemicals are used in the thermal modification process, ThermoWood is safe for interior sauna environments. The only thing that heats up is clean wood — no formaldehyde, no VOCs, no mystery chemicals [1].

6. A Bonus — Better Heat Retention

Because the thermal treatment makes the wood denser and less porous, it reflects heat back into the sauna more efficiently than untreated wood. You reach your target temperature faster, and the sauna holds heat longer — using less energy overall. This is one reason ThermoWood has become so popular for outdoor barrel saunas in Canada.

The Science Behind the Heat

The health benefits of regular sauna use are well-documented. The landmark Laukkanen study published in JAMA Internal Medicine followed 2,315 Finnish men for over 20 years and found that those who used a sauna 4-7 times per week had a 40% lower risk of all-cause mortality compared to once-weekly users [4].

A follow-up study in BMC Medicine extended these findings to women and confirmed that regular sauna use reduces cardiovascular mortality risk across both sexes [5].

What does this have to do with wood? Simple: your sauna only delivers these health benefits if you use it consistently. And you'll use it consistently if it works reliably — without warped doors, drafty gaps, or seasonal maintenance headaches.

The Bottom Line

Cedar is a good wood. For indoor saunas or mild climates, it performs well. But for Canadian backyards — where saunas face rain, snow, heatwaves, and freeze-thaw cycles in the same calendar year — thermally modified wood is the smarter long-term investment.

At Inland Sauna, we build with thermally modified wood for one reason: our saunas are designed to last. Not five years. Not ten. Generations.

Ready to explore what a thermo-wood sauna could look like in your backyard? Browse our sauna collection → or contact us for a custom consultation →

References

1. Hill, C., Altgen, M., & Rautkari, L. (2021). Thermal modification of wood — a review: chemical changes and hygroscopicity. Journal of Materials Science, 56, 6581–6614. https://link.springer.com/article/10.1007/s10853-020-05722-z
2. MDPI (2025). Thermal Modification of Wood — A Review. Journal of Materials Science and Chemical Engineering, 6(3), 19. https://www.mdpi.com/2673-4079/6/3/19
3. USDA Forest Service. Field Performance of Thermally Modified Wood in a Severe Test Site. https://research.fs.usda.gov/treesearch/66525
4. Laukkanen, T., Khan, H., Zaccardi, F., & Laukkanen, J.A. (2015). Sauna bathing is inversely associated with dementia and Alzheimer's disease. JAMA Internal Medicine, 175(4), 542-548. https://jamanetwork.com/journals/jamainternalmedicine/fullarticle/2130724
5. Laukkanen, T., Kunutsor, S.K., Kauhanen, J., & Laukkanen, J.A. (2018). Sauna bathing is associated with reduced cardiovascular mortality and improves risk prediction in men and women. BMC Medicine, 16, 219. https://link.springer.com/article/10.1186/s12916-018-1198-0
6. InHouse Wellness (2025). Sauna Wood Species: Durability, Off-Gassing & 10-Year Maintenance. https://inhousewellness.com/blogs/saunas/sauna-wood-species-wet-heat-durability-off-gassing-maintenance