What Is Thermowood, and Does It Outlast Cedar or Spruce?

Key Takeaways

• Thermowood is pine or spruce modified with heat and steam, no added chemicals (International ThermoWood Association).

• It outlasts untreated spruce outdoors. Against cedar it's a draw on rot resistance and a win on supply.

• The treatment cuts the wood's water uptake, so boards move less when steam meets frost.

• Fired three times a week, a sauna runs roughly 150 wet-dry cycles a year.

• Our cube exteriors are charred thermowood. Our barrels, 2 to 6 m, ship in Northern European pine or thermowood.

• Thermowood isn't worth the extra cost indoors, on a weak build, or on sheltered trim.

Timber is the one sauna decision you can't correct after delivery, and it decides who pays for winter. We build saunas from thermowood every working day in our 2,000 m² workshop in Klaipėda. By the end you'll know what the treatment changes, and the three cases where thermowood isn't worth your money.

What is thermowood, and does it outlast cedar or spruce?

Thermowood is softwood, usually pine or spruce, heat-treated in a kiln with steam and no added chemicals until it permanently takes up less water. Against untreated spruce the answer is simple: it outlasts it, because bare spruce soaks, swells and works its own joints open. Against cedar it's a draw on rot resistance and a win on supply, because boards are milled from European-grown softwood rather than shipped across an ocean. We build with it every day, and there are still saunas where I'd tell you to keep the cheaper board.

The advice you'll find online rarely admits that last part. Here's what the comparison turns on.

What do buyers get wrong when they compare sauna timbers?

The comparisons you'll read rank boards by price per square metre and by colour in the showroom. Both numbers are real. Neither predicts what the wall looks like once frost has been through it.

There's a second trap. Search any sauna timber question in English and the answers come from retailers writing for the American market. Cedar is their house default. That advice holds where it was written, and it breaks on a coastal European winter, where thaw and frost trade places for months.

And the third trap is the word "treated". Pressure-treated timber is wood plus chemicals. Thermowood is wood minus what feeds decay. Only one of them belongs anywhere near a heat cabin, and quotes confuse them all the time.

Now and then a new partner walks our workshop floor in Klaipėda and asks whether cheaper untreated boards would do. The answer I give is the same every time: "We build by hand from thermowood, which resists moisture and movement and lasts."

That sentence carries the whole argument.

Moisture. Movement. Time. A sauna wall takes all three at once, from both sides, and the board either holds its shape through them or it doesn't.

How does heat treatment actually change the wood?

The process is plainer than the marketing around it. The International ThermoWood Association defines it as modification of wood "at elevated temperature in the presence of steam under atmospheric pressure, without added chemicals". That definition is from thermowood.fi, accessed July 2026. The kiln drives out the compounds that bind water, and the change runs through the whole board, not just its face.

What comes out of the kiln holds less water at rest and takes up less when wet. The association grades the result in two classes. Thermo-S is graded for dimensional stability, Thermo-D for biological durability and weather resistance. On an exterior wall, the durability grade is the one doing the work.

Now the sauna part. If your customer fires the sauna three times a week, that's roughly 150 cycles a year of hot steam pushing out from inside the wall. Through winter, the outside face freezes and thaws on its own schedule at the same time.

Ten years of that is around 1,500 cycles, and every one is a small argument between water and wood.

Untreated spruce loses the argument by soaking, swelling and opening its own joints. Cedar defends itself with the oils it grew, and defends itself well. Thermowood defends itself with what the kiln removed. Less water in, less movement, fewer open joints.

Here's the comparison the way we'd lay it out for a partner.

So, can it outlast cedar or spruce? Spruce, clearly, by the mechanism above. Cedar it matches on decay resistance and beats on supply. Our timber is sourced from within roughly 200 km of the factory, while cedar crosses an ocean before reaching a European yard. Local to source, consistent to grade, simple to reorder.

When is thermowood not worth the extra cost?

Thermowood is our default board, so treat this section as the part a brochure leaves out. There are three cases where I'd keep your money.

1. An indoor sauna in a dry building. Rain, frost and the freeze-thaw cycle never reach a cabin standing inside a heated house. Check the installation site before the timber spec. If weather can't touch it, plain spruce has served Finnish saunas for generations, and the extra cost buys nothing your customer will notice.

2. A treated board on a weak build. Thermowood can't rescue a shell whose joints pump water into every junction. Check how the unit is built and checked before you ask what it's clad in. A sound build in Northern European pine beats a poor one in thermowood, which is exactly why our barrel line offers both timbers.

3. Sheltered, replaceable trim. Battens that sit under an overhang and never take driving rain won't earn the treatment back. Check which lines of the quote face weather at all. Pay for thermowood where water lands, and let the protected parts stay plain.

Those are the three cases I'll defend. Wherever the weather truly works on the wood, I stand by the treated board.

What should you ask a supplier about the timber?

Five questions, ready to paste into your next supplier email.

1. Which species is the exterior board, and is it thermally modified or surface-treated?

2. Does the colour run through the whole board? Send a photo of a fresh cross-cut.

3. Which treatment class is the exterior board, the stability grade or the durability grade?

4. Where was the timber sourced, and how far did it travel to your factory?

5. Which parts of the sauna are not thermowood, and what protects those?

A manufacturer who builds with the material answers all five in one email. Question four takes us one sentence, because our timber arrives from within roughly 200 km. The EU's deforestation regulation is turning that same question from curiosity into paperwork across the timber trade, and it deserves its own article rather than a line here.

You don't need to become a timber engineer this week. Do one thing: when the next quote arrives, ask for a cross-cut offcut of the exterior board. Treated timber shows its colour through the full section, and a stained board shows you its surface and nothing else. It's your name on the warranty, so make the wood show its insides before you sign.

Is thermowood the same as thermally modified timber?

Thermally modified timber is the general category: wood changed by heat and steam rather than by added chemicals. ThermoWood is the International ThermoWood Association's name for timber produced under its defined process and classes. Pressure-treated and stained boards belong to neither group, because they add substances to the wood instead of changing its structure.

Does thermowood need oiling or staining outdoors?

Not for survival. Left bare, the surface weathers to silver-grey, which is a colour change rather than decay. Oil it if your customer wants to keep the brown tone, and plan to repeat the coat as with any exterior timber. On our cube line we char the exterior instead, which is a separate subject.

Is thermowood strong enough for the structure of a sauna?

It machines and fixes well, but the treatment trades away some toughness, and the board is more brittle than untreated pine. That's why it earns its keep as cladding, doors and frames rather than as a load-bearing skeleton. In our cube line the wall build-up is 120 mm, and the treated timber does the weather work on the outside.

How can you tell real thermowood from stained pine?

Cut it. Heat treatment colours the entire cross-section an even brown, while stain sits in the surface of the board. Treated wood is also lighter in the hand, because it holds less water, and freshly cut it smells faintly of burnt sugar. A supplier who won't show you a cross-cut has answered the question.

Which parts of an outdoor sauna benefit most from thermowood?

The parts where water lands and where movement breaks seals: exterior cladding, doors and their frames, and any board that meets steam on one side and frost on the other. Sheltered trim and interior panelling gain the least. We put the treated board on the weather side, and we build barrels from 2 to 6 m in pine or thermowood.