What You Need to Know

Thermal modification has changed what's possible with sauna timber. By treating wood with controlled heat instead of chemicals, thermowood achieves a level of stability, moisture resistance, and longevity that untreated species simply can't match — which is why it's become the standard for premium sauna construction worldwide.

Here's an honest look at what thermowood does well, where it falls short, and how to know if it's the right choice for your project.

The Main Advantages of Thermowood

Superior Weather and Moisture Resistance

This is thermowood's defining advantage, particularly for outdoor and high-humidity installations. The thermal modification process permanently alters the wood's cellular structure, dramatically reducing its capacity to absorb moisture. The result is a timber that resists swelling, warping, and deformation through repeated wet-dry cycles in ways untreated wood cannot.

The same process delivers significant decay resistance. Rot and fungal growth need moisture to establish themselves in wood fiber, and thermowood's modified structure simply doesn't hold onto that moisture the way untreated species do. Properly specified thermowood installations routinely outlast untreated wood builds by a wide margin, particularly in coastal, humid, or high-rainfall climates.

Minimal Maintenance Requirements

Untreated sauna wood typically demands ongoing care — periodic treatments, vigilance against resin buildup, and refinishing to maintain appearance. Thermowood largely eliminates that burden.

Left untreated, thermowood will gradually weather to an attractive silver-grey patina over time — a purely cosmetic change that has no effect on performance. If you prefer to maintain the rich brown tone, an occasional application of UV-protective oil is all that's required. There's no chemical treatment regimen and no ongoing fight against resin or rot.

Enhanced Comfort During Use

Thermowood's reduced thermal conductivity isn't just a spec sheet number — it changes how a sauna actually feels to use. Untreated wood can become uncomfortably hot against bare skin during high-heat sessions, often requiring towels on benches or careful positioning to avoid hot spots. Thermowood stays touchable at temperatures where standard species would not.

It's a small detail until you're using the sauna regularly — at which point it becomes one of the most noticeable differences between a thermowood bench and a standard one.

Environmental Considerations

The thermal modification process uses no chemicals — only heat and steam. There are no preservatives to leach into soil or groundwater, and nothing that off-gasses when the wood is heated during sauna use.

Thermal modification also improves the wood's carbon storage capacity, and because thermowood lasts significantly longer than untreated alternatives, replacement cycles are less frequent — a meaningful sustainability advantage over the life of an installation. Sourcing from responsibly managed, certified forests further strengthens that profile.

Long-Term Durability and Value

Thermowood costs more upfront than comparable untreated species — typically a meaningful premium. But the lifespan difference is substantial: a well-built untreated wood sauna may need significant maintenance or replacement well within a couple of decades, while a properly constructed thermowood installation can perform reliably for decades with minimal upkeep.

Calculated over the full lifetime of the installation, thermowood frequently proves to be the more economical choice — not just the more durable one.

The Realistic Downsides of Thermowood

We believe strongly in thermowood's performance, but it isn't the right answer for every project. Here's where it falls short.

Higher initial cost.
There's no way around it — thermal modification adds real cost to raw material, and that premium shows up in the final price. For budget-constrained projects, this is a genuine barrier, even when the long-term value case is strong.

Limited species options.
Thermal modification works best with certain species — generally the lighter, faster-growing woods. This narrows the available aesthetic range compared to the full diversity of untreated species, so if you're after an unusual grain pattern or species-specific character, options are more limited.

Reduced natural "character."
Some sauna builders specifically want the knots, irregular grain, and natural imperfections of untreated wood. The selection and processing standards for quality thermowood tend toward a cleaner, more uniform appearance — which some find too refined for a rustic aesthetic.
Slightly increased brittleness. Thermal modification reduces wood density somewhat, which can make thermowood more prone to impact damage during handling and installation than untreated wood. This rarely matters once a sauna is built and in use, but it does call for a bit more care during construction.

Less necessary for fully indoor saunas.
In a controlled indoor environment, thermowood's moisture and decay resistance become less critical — the conditions that justify the investment outdoors simply aren't as severe indoors. For indoor builds, the case for thermowood comes down more to its comfort and clean aesthetic than strict necessity.

Advantages

Superior weather and moisture resistance
Minimal maintenance required
Significantly extended service life
No resin bleeding or sticky surfaces
Comfortable surface temperature, low conductivity
Chemical-free, more sustainable lifecycle
Strong natural rot and decay resistance
Excellent dimensional stability through seasons

Tradeoffs

Higher initial cost than untreated wood
Narrower range of available species
Slightly more brittle, requires careful handling
Less rustic, more uniform appearance
Less critical for fully indoor installations
Specialized processing required
Weathers to grey unless periodically oiled

Thermowood Maintenance and Care

Low maintenance is one of thermowood's strongest selling points — here's what it actually looks like in practice.

Natural weathering vs. maintaining color

Left untreated, thermowood gradually weathers to a silver-grey patina over time. This is purely cosmetic and has no impact on performance — many clients prefer the weathered look. To preserve the original brown tone, apply a UV-protective clear oil periodically; the process is quick and only needs to be repeated every couple of years.

Cleaning and care. Exterior surfaces benefit from an occasional rinse with water and a soft brush to clear dirt and pollen. For interior surfaces, regular sauna heat naturally helps keep the wood clean, and a light wipe-down with a damp cloth is generally all that's needed. An annual check of joinery and drainage is good practice but rarely takes more than a few minutes.

The contrast with the upkeep demands of untreated wood is significant — and it's a large part of why thermowood has become the default choice for low-maintenance, long-life sauna construction.

Is Thermowood Right for Your Sauna?

Thermowood makes sense if you:

• Want an outdoor or high-humidity sauna built to last for decades with minimal upkeep
• Are working in a climate with significant moisture or seasonal temperature swings
• Would rather use your sauna than maintain it
• Value long-term cost-effectiveness over the lowest possible upfront price
• Prefer a clean, refined, consistent aesthetic
• Prioritize chemical-free, sustainably sourced materials

Thermowood may be more than you need if you:

• Are building a fully indoor sauna in a stable, controlled environment
• Are working within a very tight initial budget
• Specifically want the rustic character of knotted, irregular untreated wood
• Don't expect to keep the installation long-term
• Are specifically after the classic cedar aroma experience

Our Recommendation

For outdoor and high-moisture sauna installations, thermowood is, in our view, worth the investment for the overwhelming majority of projects. The climate-resilience and low-maintenance profile consistently pay off over the life of the sauna.

If budget is a constraint, we'd generally recommend a smaller thermowood installation over a larger build in untreated wood — the performance gap is significant enough to be worth prioritizing.

For indoor saunas, thermowood remains an excellent choice, though the advantage over high-quality species like Cedar or Hemlock is less dramatic. In that context, aesthetic preference and budget carry more weight in the decision.

Frequently Asked Questions

What is thermowood made from?

Thermowood is conventional wood — commonly softer, faster-growing species — that has been heat-treated at high temperatures in a controlled, oxygen-free environment. This thermal process alters the wood's cellular structure without introducing any chemicals, breaking down moisture-absorbing components and increasing long-term stability.

Is thermowood worth the extra cost?

For outdoor and high-humidity saunas, generally yes. Thermowood typically carries a meaningful premium over untreated wood but delivers a substantially longer service life and far lower maintenance needs, which usually justifies the cost over the installation's lifetime. For fully indoor saunas, the advantage is real but less pronounced.

How long does thermowood last outdoors?

With proper construction, thermowood saunas can perform reliably outdoors for decades with minimal maintenance, well beyond the typical lifespan of comparable untreated wood installations in the same conditions. Actual longevity depends on climate, construction quality, and care.

Does thermowood need to be treated or sealed?

No — thermowood performs well left completely untreated and will weather naturally to an attractive silver-grey patina. If you'd prefer to preserve the original brown tone, a periodic application of UV-protective clear oil is sufficient. Synthetic sealers and chemical treatments should be avoided, as they can trap moisture rather than release it.

Can thermowood be used for sauna benches?

Yes — thermowood is an excellent choice for benches. Its lower thermal conductivity keeps the surface more comfortable against bare skin than untreated wood, its smooth finish is splinter-free, and its dimensional stability resists the warping that affects lower-grade materials over time.

Why does thermowood turn grey?

UV exposure causes surface oxidation, producing a silver-grey patina over time. This change is entirely cosmetic and has no effect on the wood's structure, durability, or performance — many people prefer the weathered look. A UV-protective oil will maintain the original brown tone if preferred.

Is thermowood environmentally friendly?

Yes. Thermal modification relies entirely on heat and steam, with no chemical inputs. The process also improves the wood's long-term carbon storage, and its extended service life reduces how often material needs to be replaced — a meaningful sustainability advantage when sourced from responsibly managed, certified forests.

What's the difference between thermowood and pressure-treated wood?

Thermowood is modified using heat alone, with no chemical additives. Pressure-treated wood, by contrast, is infused with chemical preservatives that can off-gas dangerously when heated — making it unsafe for sauna use. Thermowood carries no such risk and is specifically suited to high-heat environments.

Does thermowood smell different than untreated wood?

Thermowood has a mild, neutral woody scent — noticeably less aromatic than species like cedar or fresh pine, since the thermal process neutralizes most of the volatile compounds responsible for strong wood odors. Some clients prefer this neutral profile; others miss a more pronounced natural aroma. The scent becomes even milder with repeated heating over time.