If you're shopping for a sauna, the wood question comes up fast. Cedar has been the default in North America for decades, and for good reason — it's aromatic, beautiful, and naturally rot-resistant. But thermally modified wood (often called Thermowood) has been quietly overtaking cedar across Scandinavia, the Baltics, and increasingly in the U.S. and Canadian markets.
The two materials solve the same problem — surviving repeated cycles of extreme heat, steam, and moisture — but they get there through very different paths. Cedar relies on natural oils and tannins it developed over centuries of evolution. Thermally modified wood relies on a controlled industrial process that fundamentally restructures the wood at a cellular level. That difference in approach creates real, measurable differences in performance, maintenance, longevity, and cost.
This guide breaks down every factor that matters when choosing between the two, so you can pick the right wood for your sauna with confidence.
What Is Thermally Modified Wood?
Thermal modification is a process that heats wood to extreme temperatures — typically between 356°F and 482°F (180°C–250°C) — inside a specialized kiln with virtually no oxygen present. The absence of oxygen prevents combustion. Instead of burning, the wood undergoes a controlled chemical transformation: the heat breaks down hemicellulose (the wood sugars that fungi, mold, and insects feed on), drives out moisture, and permanently restructures the cell walls.
The result is wood that absorbs dramatically less moisture, resists biological decay without any chemical treatment, and holds its shape far more consistently than its untreated counterpart. For sauna applications, manufacturers typically use the more intensive end of the temperature range (above 410°F / 210°C) because saunas demand maximum moisture resistance and dimensional stability.
The technique isn't new. Nordic civilizations were heat-treating timber for shipbuilding long before the Viking Age. What's changed is precision — modern thermal kilns allow manufacturers like Thermory to achieve consistent results across large production runs, batch after batch. That consistency is what's made thermally modified wood viable as a mainstream building material rather than a craft technique.
Common species used in thermal modification for saunas include Nordic spruce, Scots pine, radiata pine, aspen, alder, and magnolia. Each species brings its own grain pattern, color profile, and density to the table, but the modification process elevates all of them to a performance tier they could never reach in their natural state.
What Is Cedar Wood?
When sauna builders say "cedar," they almost always mean Western Red Cedar (Thuja plicata), a softwood native to the Pacific Northwest that has been the go-to sauna material in North America for generations. Eastern White Cedar (Thuja occidentalis) also appears in some budget-friendly models, though it's a somewhat different species with different properties.
Western Red Cedar earns its reputation honestly. The wood produces natural oils and tannins — thujaplicins and other tropolone compounds — that make it inherently resistant to rot, fungal decay, and insect damage without any treatment. It's lightweight, has excellent natural insulating properties thanks to tiny air pockets within its cell structure, and it stays relatively cool to the touch even in a hot sauna. Its warm reddish-brown color and distinctive aromatic scent are iconic in the sauna world.
Cedar's position as the default North American sauna wood, however, is as much about historical availability as inherent superiority. For most of the 20th century, old-growth Western Red Cedar was abundant and affordable. That's changed significantly. As old-growth forests have been depleted, the clear, tight-grained cedar that once defined the species has become harder to source and considerably more expensive. Much of what's available today is second- or third-growth timber with wider grain spacing, more knots, and less consistent quality than what builders relied on a generation ago.
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Durability and Decay Resistance
This is where the comparison gets most interesting for sauna buyers, because both woods resist decay — but through fundamentally different mechanisms.
Cedar's rot resistance comes from its natural extractives: the oils and tannins embedded in the heartwood. These compounds are genuinely effective antimicrobial and antifungal agents. However, they're concentrated in the heartwood (the darker center of the tree), not the sapwood (the lighter outer rings). As old-growth cedar becomes scarcer, a higher percentage of commercially available boards contain more sapwood, which has significantly less protection. The extractives also diminish over time with repeated exposure to heat and moisture — exactly the conditions inside a sauna.
Thermally modified wood takes a completely different approach. Rather than relying on chemical compounds that can leach or degrade, the modification process permanently removes the food source that decay organisms need. By breaking down hemicellulose and wood sugars, the treatment makes the wood fundamentally inhospitable to fungi, mold, and insects. This protection doesn't diminish with age because it's structural, not chemical — the sugars are gone and they don't come back.
In standardized durability testing, quality thermally modified wood achieves Class 1 or Class 2 durability ratings (the highest tiers) from the International ThermoWood Association. This puts it in the same performance category as tropical hardwoods like teak and ipe — species that cost considerably more and carry a much heavier environmental footprint.
Dimensional Stability: Warping, Swelling, and Shrinking
A sauna is one of the most punishing environments any wood will face. Temperatures swing from ambient to 180°F–220°F and back, humidity goes from dry to near-saturation during steam sessions, and these cycles repeat hundreds of times per year. Wood that can't handle this will warp, cup, crack, or pull away from fasteners.
Cedar performs reasonably well here compared to most untreated softwoods. Vertical-grain Western Red Cedar shrinks roughly 2% as it dries, and flat-grain cedar can swell up to 5% as it absorbs moisture. That's decent — but in a sauna environment with extreme and rapid moisture cycling, even this modest movement adds up over the years. Cedar benches can develop gaps, wall boards can cup, and joints can loosen.
Thermally modified wood has a decisive advantage in this category. The modification process reduces the wood's equilibrium moisture content to roughly 4–6% (compared to 10–12% for kiln-dried cedar), which means it has far less capacity to absorb moisture in the first place. Less absorption means less expansion. Less expansion means less contraction when it dries out. The result is wood that holds its shape through thousands of heat-and-steam cycles with minimal movement. Thermally modified vertical-grain hemlock, for instance, shows roughly 1.5% shrinkage — about 25% less movement than comparable cedar.
For barrel saunas, dimensional stability is especially critical. The staves need to maintain tight joints to keep the structure weather-tight. Any significant swelling or shrinking can compromise the seal between boards, which is one reason European barrel sauna manufacturers have overwhelmingly shifted to thermally modified spruce and pine.
Moisture Resistance
Closely related to dimensional stability, but worth addressing separately: how much water can each wood absorb, and what happens when it does?
Untreated cedar has a natural moisture content that typically stabilizes around 10–12% in normal conditions, and it can absorb substantially more in a sauna's humid environment. Cedar's natural oils provide some water repellency at the surface, but they don't prevent moisture penetration into the wood's interior over time. In outdoor sauna applications, this moisture absorption creates vulnerability to freeze-thaw damage in cold climates and can lead to surface checking, green algae growth, and accelerated weathering if not treated with a sealant.
Thermally modified wood absorbs dramatically less water. When placed in a 100% humidity environment, thermally modified wood will absorb a maximum of roughly 12% moisture — and under normal sauna conditions, actual absorption is much lower than that. This reduced hygroscopicity (the technical term for a material's tendency to absorb moisture from its environment) is permanent and doesn't degrade with age or UV exposure.
In practical terms, this means Thermowood saunas dry out faster between sessions, are less prone to mold or mildew development, and maintain structural integrity longer — especially in outdoor installations exposed to rain, snow, and humidity year-round.
Thermal Conductivity and Comfort
Here's a factor that directly affects your day-to-day sauna experience: how hot does the wood feel when you sit on it or lean against it?
Cedar has long been praised for its low thermal conductivity. Those tiny air pockets within its cell structure act as natural insulation, meaning cedar surfaces stay relatively comfortable against bare skin even when the air temperature in the sauna is above 180°F. This is one of cedar's most tangible advantages and a genuine reason it became a popular sauna wood.
Thermally modified wood also performs well in this regard, though the specifics vary by species. The modification process reduces wood density slightly (because moisture and volatile compounds are driven off), which generally helps keep thermal conductivity low. Lunawood, one of the major Thermowood producers, reports a thermal conductivity of just 0.09 W/(mK) for their thermally modified products — which is very low and translates to comfortable surface temperatures during sauna use. Thermo-aspen, in particular, is prized for bench and backrest applications because it stays especially cool to the touch at high temperatures.
Both materials perform well here. Cedar may hold a slight edge in insulating properties at the species level, but the difference is subtle enough that it's unlikely to change your sauna experience in any meaningful way. You'll be comfortable on either surface.
Aesthetics: Color, Grain, and Aging
Appearance is subjective, but understanding how each wood looks — and how it changes over time — helps you plan for the long term.
Fresh Western Red Cedar has a warm, reddish-brown color with a straight, even grain and subtle variation between boards. It's a beautiful wood. The aroma is distinctive and widely loved — a woodsy, slightly sweet scent that intensifies with heat. Over time, however, cedar's color fades. Without regular applications of UV-protective finish or oil, the reddish tones bleach out to a silvery gray. Indoors (inside a sauna room), the fading is slower than outdoors, but it still happens. Maintaining that iconic cedar color requires ongoing effort.
Thermally modified wood has a deep, rich brown color — ranging from warm honey to chocolate brown depending on the species and the intensity of the modification process. This color is created by the caramelization of glucose within the wood during treatment, and it penetrates throughout the board (not just the surface). The grain patterns are accentuated by the process, creating a visually striking, sophisticated look. Like cedar, untreated Thermowood will eventually weather to silver-gray if left unfinished outdoors. But the starting color is naturally darker and more uniform, which many people find more contemporary and refined.
One aesthetic advantage of thermally modified wood: because the process is standardized, board-to-board color consistency tends to be higher than cedar. With cedar — especially in today's market where clear, consistent grades are harder to source — you're more likely to encounter variation in tone, occasional sapwood streaks, and visible knots.
The Aroma Question
Cedar's scent is one of its most iconic selling points. The warm, aromatic fragrance is part of the sauna ritual for many people, and it's a legitimate reason to choose cedar. The aroma comes from the same volatile oils that provide rot resistance, and it's most pronounced when the wood is new and heated for the first time. Over months and years of use, the scent gradually diminishes as the volatile compounds evaporate — though it never disappears entirely.
Thermally modified wood has a different scent profile. Freshly modified wood has a mild, toasty, slightly sweet aroma (sometimes compared to caramel or baked wood) that's pleasant but considerably subtler than cedar. This fades relatively quickly, and most Thermowood saunas become essentially scent-neutral within the first several weeks of use.
If aroma is a high priority for your sauna experience, cedar has a genuine advantage here. If you're sensitive to strong scents or simply prefer a neutral environment, thermally modified wood is the better fit. It's also worth noting that a small percentage of people are sensitive or mildly allergic to cedar's volatile oils, which can cause respiratory irritation or skin reactions. Thermally modified wood, because the volatile compounds are driven off during processing, is effectively hypoallergenic — an important consideration for commercial or shared-use saunas.
Maintenance Requirements
Long-term maintenance is where the day-to-day ownership experience diverges most sharply between the two woods.
Cedar maintenance (outdoor saunas): To preserve cedar's color and maximize its lifespan in outdoor applications, annual sealing with a UV-blocking, water-repellent finish is recommended. Without it, cedar weathers to gray and becomes more susceptible to surface checking and moisture damage. Periodic light sanding (every 2–3 years) refreshes the surface and removes superficial damage. In wet climates, you'll want to ensure good drainage and ventilation around the base to prevent moisture from sitting against the wood. Indoor cedar saunas require less maintenance, but occasional oiling helps preserve the color and condition.
Thermally modified wood maintenance (outdoor saunas): Thermowood is often marketed as low-maintenance, and that claim is largely accurate. The wood's structural resistance to moisture and decay doesn't depend on surface treatments, so skipping a year of maintenance doesn't compromise the wood's integrity the way it can with cedar. That said, if you want to preserve the original brown color outdoors, a UV-protective oil applied every 1–2 years will prevent the natural weathering to gray. Inside the sauna, a periodic application of paraffin oil maintains hygiene and appearance but isn't structurally necessary.
The practical difference: if you forget (or choose not) to maintain your cedar sauna exterior for a couple of seasons, you may be looking at color loss, surface checking, and potentially the early stages of moisture damage. If you do the same with a Thermowood sauna, you'll get cosmetic graying but the wood itself will be structurally unaffected. That margin of forgiveness is meaningful for busy homeowners.
Lifespan
How long each wood lasts depends on the application (indoor vs. outdoor), climate, and maintenance habits — but some general ranges are well established.
Quality Western Red Cedar, properly maintained in an outdoor sauna application, typically lasts 20–30 years. In milder, drier climates and with diligent annual maintenance, the upper end of that range is achievable. In wet, humid climates with inconsistent maintenance, cedar's lifespan shortens. Indoor cedar sauna interiors last longer because they're not exposed to weather, though the relentless heat cycling still takes a toll over decades.
Thermally modified wood, properly maintained, commonly achieves a 25–35+ year lifespan in outdoor applications. The Building Research Establishment (BRE) classifies quality Thermowood as Class 2 durability with a service life around 30 years. Some manufacturers, including Thermory, offer 20-year limited warranties against rot and decay on their thermally modified products — a warranty level that's rare or nonexistent for cedar.
The lifespan advantage compounds with the lower maintenance requirements: Thermowood lasts longer and demands less effort to get there.
Strength and Hardness
Cedar is a softwood — a very soft softwood. Western Red Cedar scores just 350 lbf on the Janka hardness scale, making it one of the softer commercial lumbers available. This means cedar surfaces dent and scratch relatively easily, which matters for high-traffic sauna benches and floors.
Thermally modified wood varies by species, but in general, the modification process maintains or slightly enhances hardness. Thermally modified hemlock, for example, scores approximately 622 lbf on the Janka scale — nearly twice the hardness of Western Red Cedar. In Modulus of Rupture testing (which measures how much force it takes to break the wood), thermally modified hemlock is roughly twice as strong as cedar. Even softer species like spruce and pine see meaningful improvements in surface hardness after thermal modification.
For sauna benches, floors, and exterior surfaces that take foot traffic and the occasional dropped bucket, the higher hardness of Thermowood translates to fewer dents, scratches, and wear marks over the life of the sauna.
Sustainability and Environmental Impact
Both woods have legitimate environmental credentials, but the details differ.
Western Red Cedar grows naturally in sustainably managed forests of the Pacific Northwest and British Columbia, and responsible sourcing certifications (FSC, PEFC) are available. Cedar requires no chemical treatment for sauna use, which is a significant environmental plus. However, the declining availability of old-growth and premium-grade cedar raises supply chain concerns, and long-distance shipping (from the Pacific Northwest to the rest of the continent) adds to the carbon footprint.
Thermally modified wood scores well on several sustainability metrics. The modification process uses only heat and steam — no chemicals, preservatives, or toxic compounds. The species used (spruce, pine, aspen, alder) are fast-growing and widely available from sustainably managed Northern European and North American forests, which reduces supply pressure compared to slower-growing cedar. And because thermal modification extends the lifespan of otherwise less-durable species, it effectively reduces the total demand for virgin timber over the long term. Leading producers like Lunawood report that their Thermowood products absorb five times more carbon during their growing life than is released during their production — a strongly carbon-negative profile.
The one environmental cost to note: thermal modification requires significant energy input (those kilns run at extreme temperatures for extended periods). However, many major producers power their kilns with biomass or renewable energy, and the lifecycle benefit of the extended lifespan generally outweighs the production energy cost.
Cost Comparison
Price is always context-dependent and fluctuates with market conditions, but the general pattern is fairly consistent.
Clear-grade Western Red Cedar has become increasingly expensive as old-growth supply has diminished. Premium clear cedar for sauna construction has seen sharp price increases over the past decade and can be difficult to source in consistent quality and quantity. Lead times for high-grade cedar can stretch to weeks or months depending on the supplier and region.
Thermally modified wood is generally priced competitively with — and in many cases below — clear Western Red Cedar, while offering superior performance characteristics. The species used as base material (spruce, pine, aspen) are more abundant and less expensive to source than premium cedar, and the thermal modification process, while adding cost, doesn't push the final price above cedar in most cases. Availability tends to be more consistent because the supply isn't constrained by old-growth forest depletion.
When you factor in the total cost of ownership — including reduced maintenance expenses, longer lifespan, and fewer repairs or replacements due to warping or decay — thermally modified wood typically delivers better value per year of service. A Thermowood sauna may cost roughly the same (or less) upfront than a comparable cedar sauna, with meaningfully lower total ownership costs over a 20–30 year period.
Resin and Sap: A Practical Concern
If you've ever used a sauna built with untreated spruce or pine, you may have encountered sticky resin seeping out of the wood as it heats up. It's messy, it can stain clothing and skin, and it's unpleasant to sit on.
Cedar largely avoids this issue — its resin content is naturally low, and resin seepage is uncommon in cedar saunas. This is one of the practical reasons cedar became popular for saunas in the first place.
Thermally modified wood eliminates resin entirely. The extreme temperatures of the modification process drive off all resin and sap, permanently. This is one of the key advantages of the modification process for sauna applications: it makes species like spruce and pine — which would be problematic untreated — completely suitable for sauna use. No sticky surfaces, no staining, no resin beads forming on benches or walls during heat sessions.
Indoor vs. Outdoor Sauna Considerations
The choice between cedar and Thermowood can depend on where your sauna will live.
For indoor saunas: Both woods perform well. Cedar's natural insulating properties and pleasant aroma are especially enjoyable in an enclosed indoor space. Thermally modified wood's superior moisture resistance and stability still offer long-term advantages, but the difference is less pronounced indoors where the wood isn't exposed to weather, UV, or freeze-thaw cycles. If the cedar aroma is important to your experience, an indoor sauna is where you'll notice it most.
For outdoor saunas: Thermally modified wood's advantages become significantly more meaningful. Rain, snow, UV radiation, and temperature extremes test every weakness in a wood's defenses. Cedar handles outdoor exposure reasonably well with diligent maintenance, but Thermowood's lower moisture absorption, superior dimensional stability, and structural (rather than chemical) decay resistance give it a clear performance advantage in exposed environments. This is why premium European outdoor sauna brands — Thermory, SaunaLife, and Auroom — have standardized on thermally modified wood for their outdoor models.
Climate Considerations
Your geographic location matters more than most sauna wood guides acknowledge.
Wet, humid climates (Pacific Northwest, Southeast, coastal areas): Thermally modified wood's low moisture absorption gives it a significant advantage. Cedar's natural oils provide some protection, but constant humidity exposure accelerates the depletion of those protective compounds. In these environments, the maintenance gap between the two materials is widest — Thermowood needs less attention and lasts longer.
Cold climates with freeze-thaw cycles (Midwest, Northeast, Mountain states, Canada): Moisture inside wood expands when it freezes, which can crack and damage the cell structure. The less moisture wood absorbs, the less freeze-thaw damage it suffers. Thermally modified wood's dramatically lower moisture uptake makes it the superior choice in cold climates.
Dry, mild climates (Southwest, Southern California): Both woods perform well. UV degradation (color fading) is the primary concern in sunny, arid environments. Both materials benefit from UV-protective finishes, and neither has a decisive advantage here.
Types of Thermally Modified Wood for Saunas
Not all Thermowood is the same. The base species matters, and different species work best for different parts of the sauna. Here's what you'll find in today's premium saunas:
Thermo-Spruce (Nordic Spruce): The workhorse of the Thermowood sauna world. Spruce is the most common species used for exterior cladding, barrel staves, and wall paneling. It has a knotty, rustic appearance that many find charming, excellent insulating properties, and strong moisture resistance after modification. European barrel sauna manufacturers rely heavily on thermally modified spruce. Browse Thermowood saunas built with modified spruce.
Thermo-Aspen: A favorite for sauna interiors, especially benches, backrests, and surfaces that contact bare skin. Aspen is naturally light in color (even after modification it remains relatively pale compared to other species), has a smooth texture, and has very low thermal conductivity — meaning it stays comfortable to sit on at high temperatures. It's hypoallergenic and splinter-resistant. Premium sauna brands like Thermory and Auroom use thermo-aspen extensively for their interior bench systems.
Thermo-Alder: Features a distinctive warm, reddish-brown hue and fine grain. Thermo-alder is prized for its visual richness and works beautifully for interior wall cladding, bench surfaces, and decorative trim. It offers a great balance of aesthetics and performance.
Thermo-Radiata Pine: Strong, durable, and attractive with a warm color and pleasant grain pattern. Used for both structural and decorative sauna applications. After modification, radiata pine develops a deep brown color that lends a luxurious feel to sauna interiors.
Thermo-Magnolia: A newer addition to the thermally modified lineup, magnolia offers exceptional durability and can handle the extreme conditions inside a sauna without issue. Its unique grain and warm tones make it a standout choice for builders wanting something different.
Types of Cedar Used in Saunas
For a fair comparison, it's important to distinguish between the cedar varieties you'll encounter:
Western Red Cedar: The premium choice. Superior rot resistance, the iconic cedar aroma, and the richest color. Most high-end cedar saunas use this species, but quality and availability are declining while prices are rising.
Eastern White Cedar: Less expensive and more readily available than Western Red. Lighter in color (pale straw to light brown), somewhat less aromatic, and slightly less rot-resistant. Common in budget-friendly Canadian-made barrel saunas. Still a solid wood, but not the same performance tier as Western Red.
When comparing cedar vs. Thermowood, the type of cedar matters significantly. A premium Western Red Cedar sauna is a closer competitor to Thermowood than an Eastern White Cedar model. Make sure you know which cedar species you're being quoted on.
Which Should You Choose?
After all the technical details, the decision often comes down to what you value most.
Thermally modified wood is the stronger choice if you prioritize: maximum durability and lifespan, low maintenance, outdoor installation in any climate, dimensional stability, hypoallergenic properties, value over the total ownership period, or a modern and consistent aesthetic. This is the direction the global sauna industry is clearly moving — and it's what the leading European manufacturers have already standardized on.
Cedar remains a solid choice if you prioritize: the classic cedar aroma as part of your sauna ritual, a traditional North American sauna aesthetic, indoor installation where weather exposure isn't a factor, or a preference for the lighter, warmer reddish tones that are unique to cedar.
There's no wrong answer here — both are real, high-quality natural woods that will give you years of excellent sauna sessions. But if you're investing in a sauna you want to last decades with minimal fuss, especially outdoors, thermally modified wood has earned its growing reputation as the superior performer.
Explore Both Options at Haven Of Heat
We carry saunas built with both thermally modified wood and cedar, from the industry's most respected manufacturers. Whether you're drawn to the enhanced performance of a Thermowood sauna from Thermory or SaunaLife, or the classic warmth of a cedar sauna from Dundalk or Golden Designs, we can help you find the right fit for your space, climate, and budget. We also carry a full selection of thermally modified sauna wood and cedar sauna lumber for custom builds and DIY projects.
Have questions about which wood is right for your project? Reach out to our team — we talk sauna wood every day and we're happy to help you sort through the details.
Frequently Asked Questions
Is thermally modified wood safe for saunas?
Yes. The thermal modification process uses only heat and steam — no chemicals, preservatives, or toxic compounds. The finished wood is completely safe for high-temperature use and direct skin contact, and it's hypoallergenic because the volatile organic compounds present in untreated wood are driven off during processing.
Does thermally modified wood smell?
Freshly modified wood has a subtle, pleasant toasty scent that fades within the first few weeks of sauna use. After that, it's essentially scent-neutral. If you want the signature cedar fragrance in your sauna, cedar is the way to go — but if you prefer a neutral environment or are sensitive to strong scents, Thermowood is the better option.
Can I mix cedar and Thermowood in the same sauna?
Absolutely. Many custom sauna builders use thermally modified wood for the structural shell and exterior cladding (where moisture resistance and durability matter most) while incorporating cedar elements inside for the aroma. You could also use thermo-aspen benches paired with cedar walls, or vice versa. There's no compatibility issue between the two materials.
How much more does Thermowood cost than cedar?
In today's market, thermally modified wood is often priced at or below premium clear Western Red Cedar, which has seen significant price increases due to declining supply. When you factor in lower maintenance costs and longer lifespan, Thermowood typically offers better total value. Pricing varies by species, profile, and source — contact us for current pricing on specific materials.
Which wood is better for a barrel sauna?
Both work, but thermally modified wood has become the preferred material for barrel saunas, especially for outdoor use. The stave joints in a barrel sauna need to stay tight to maintain structural integrity and weather resistance, and Thermowood's superior dimensional stability keeps those joints tighter over more years and more heat cycles than cedar typically can.
Will my Thermowood sauna turn gray?
If left untreated outdoors, yes — thermally modified wood weathers to a silver-gray over time, just like cedar does. If you prefer to maintain the original brown color, a UV-protective oil applied every one to two years will preserve it. Interior surfaces that aren't exposed to UV light retain their color indefinitely.
Is cedar still a good sauna wood?
Cedar is still a very good sauna wood — it wouldn't have dominated the market for decades if it weren't. It has genuine strengths: natural insulation, pleasant aroma, attractive appearance, and proven rot resistance. What's changed is that thermally modified wood now offers comparable or superior performance across most categories while often costing the same or less. Cedar hasn't gotten worse; the alternatives have gotten better.
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