What Is Thermowood? The Sauna Timber Explained
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Thermowood turns up on almost every quality outdoor sauna, usually as a paid upgrade over spruce, and it is rarely explained properly. This guide covers what the treatment actually does to timber, why it matters more in a British garden than a Finnish one, and whether the upgrade is worth paying for.
What is thermowood?
Thermowood is timber that has been heat-treated at 180–220°C in a low-oxygen chamber. That is the whole process: heat, steam and the deliberate absence of air, over roughly 30 to 40 hours.
There is no chemical preservative involved. Nothing is injected, painted on or pressure-forced into the wood. What changes is the wood itself.
What the heat actually does
Three things happen inside the timber at those temperatures:
- The hemicellulose breaks down. Hemicellulose is the component of wood that most readily absorbs water. Degrade it and the timber loses much of its appetite for moisture — typically taking on 40–50% less water than untreated softwood.
- The cell structure closes. Less open capillary structure means less water travelling into and through the wood.
- The sugars and resins are driven off. Those are what fungi and insects feed on. Remove them and the timber becomes a far less appealing food source, which is why thermowood resists rot without chemical help.
The oxygen-free part matters: without it, timber at 200°C simply burns. The chamber is flooded with steam to displace air, which is why the process is sometimes called steam-thermal modification.
Why it matters in a British garden
The enemy of outdoor timber in the UK is not really cold. It is repeated wetting and drying.
Untreated softwood swells when it takes on moisture and shrinks as it dries. Do that a few hundred times over a few British winters and the result is cupping, twisting, splitting and lifted joints — a sauna door that no longer seals, staves that have moved apart, a roof line that has gone wavy.
Because thermowood absorbs so much less moisture, it moves far less. That is the practical benefit, and on a sauna it shows up in a specific place: the door and the panel joints. A sauna that leaks heat around a warped door is a sauna that costs more to run and never quite gets there.
The other side: it is more brittle
Heat treatment costs the timber some strength — typically 10–30% in bending. That is why thermowood is used for cladding, benches, decking and sauna shells, and not for structural beams. On a sauna it is irrelevant: the loads are trivial. But it is a real trade-off and worth knowing about.
How to recognise it
Thermowood arrives a rich, even honey-brown to chocolate colour — and crucially, that colour goes all the way through. It is not a stain. Cut a piece and the inside matches the outside.
Left outdoors and untreated, it weathers to a stable silver-grey, much like cedar. Oiled or treated, it holds the brown. Neither affects durability; it is purely appearance.
Thermowood vs the alternatives
| Nordic spruce | Thermowood | Western red cedar | |
| Cost | Lowest | Mid | Highest |
| Moisture uptake | High | Very low | Low |
| Movement | Noticeable | Minimal | Low |
| Rot resistance | Needs treating | Inherent | Inherent (natural oils) |
| Appearance | Pale, bright | Honey-brown | Reddish-brown |
| Chemicals | Usually treated | None | None |
Cedar is excellent and priced accordingly. Spruce is the value option and perfectly serviceable if you maintain it. Thermowood sits between them, and on a sauna it has one advantage over both: it is dimensionally stable at high temperature and humidity, which is exactly the condition a sauna creates on the inside face of every panel.
Is the upgrade worth it?
On our range the thermowood option adds roughly £450–£900 depending on cabin size. Whether that is worth paying comes down to three things:
- How exposed is the site? A sauna in the open, taking wind and rain from every side, works its timber far harder than one tucked against a wall. Exposed site — take the thermowood.
- How much timber is there? On a small cabin the difference is modest. On a four-metre barrel or a large cube there is a great deal more surface area to move, and proportionally more to gain.
- Will you actually maintain it? Spruce is fine if you treat it and re-treat it every couple of years. If you know yourself well enough to doubt that, thermowood is the more forgiving choice.
Our honest position: on a small, sheltered cabin, spruce well looked after is perfectly good and the money is better spent on a better heater. On anything large, exposed, or that you would rather not think about again — thermowood earns its money.
Thermowood and sauna benches
Benches are a slightly different question. Bench timber needs to be low-resin and low-conductivity — it has to be comfortable to sit on bare skin at 90°C. Thermo-aspen is the usual answer: aspen is naturally low-resin, and heat treatment removes what little there is.
Alder is the traditional alternative and works well. Pine is wrong for benches at any price — it weeps resin when hot.
Common questions
Does thermowood need treating?
Not for durability — the rot resistance is built in. But UV still greys the surface, so if you want to keep the brown colour, an exterior oil once a year or two will hold it. That is cosmetic maintenance rather than protective.
How long does thermowood last outdoors?
Properly detailed and kept off the ground, thermowood cladding is generally rated for 25–30 years in exterior use. The detailing matters more than the timber: standing water and ground contact will shorten the life of anything.
Is thermowood the same as pressure-treated?
No, and it is close to the opposite. Pressure treatment forces chemical preservatives into the wood. Thermal modification uses only heat and steam, and adds nothing. If you would rather not have a chemically treated building in the garden — particularly one you sit inside at 90°C — that distinction is worth having.
Does thermowood smell?
It has a faint smoky, toasted note when new, which fades over a few months. Some people like it. It is nothing like the resinous smell of fresh pine.
Is thermowood good for sauna interiors?
Yes, and it is arguably where it makes most sense. The inside face of a sauna panel goes through a brutal cycle — 90°C and a burst of steam, then cold and dry, several times a week. Timber that barely moves under those conditions is exactly what you want.
Where to see it
- Nordic Saunas — our Estonian-built range, thermowood throughout or as an option.
- Glass Saunas — thermowood shells with full-height glazing.
- Garden Saunas — the full range, spruce and thermowood.
- Traditional Saunas — stove-and-stones saunas in both timbers.
Also worth reading: sauna timbers compared and what is a Finnish sauna?
