The wood inside a sauna shapes far more than its appearance.
It influences how the room holds heat, how comfortable the benches feel, how the interior responds to repeated heating and cooling, and how the space ages through years of use. In outdoor saunas, the exterior materials must also manage rain, snow, sunlight and seasonal changes.
This is why thermally modified wood has become an important material in contemporary sauna design.
Its deeper colour and refined grain give it a warm, architectural appearance, while the modification process can improve dimensional stability and reduce the wood’s tendency to absorb moisture. These qualities make it particularly useful in environments where temperature and humidity regularly change.
At Theraluxe, material selection is never based on appearance alone. Each species and application is considered according to performance, tactile comfort, design direction and the conditions the finished sauna will experience.
This guide explains what thermally modified wood is, why it works well in sauna design, how it compares with cedar and what homeowners should consider before choosing it.
What Is Thermally Modified Wood?
Thermally modified wood is natural timber that has been exposed to high temperatures under carefully controlled conditions.
Depending on the process and manufacturer, treatment temperatures commonly fall between approximately 160°C and 220°C. Steam or another controlled atmosphere is often used to regulate the process and protect the wood while its physical and chemical properties change.
The treatment reduces the wood’s equilibrium moisture content and alters its response to humidity. It also deepens the colour throughout the material rather than simply colouring the surface.
The International ThermoWood Association’s guide to ThermoWood identifies several common characteristics of properly modified material, including improved dimensional stability, reduced swelling and shrinkage, lower thermal conductivity and improved resistance to moisture and decay.
No conventional chemical preservatives are added during the ThermoWood process. However, this does not necessarily mean every finished board, sauna component or exterior cladding product is completely free of finishes, adhesives or coatings. Those details should always be confirmed with the supplier or builder.
It is also helpful to understand the terminology:
- Thermally modified wood is the general material category.
- ThermoWood® is a registered name associated with material produced by members of the International ThermoWood Association according to its standards.
Why Wood Selection Matters in a Sauna
Sauna wood experiences demanding conditions.
During a session, the room heats quickly and the surfaces become warm. Water may be introduced onto the heater stones, briefly increasing humidity. After use, the room cools and dries before the cycle begins again.
Over time, this repeated movement can affect wood that has not been selected, dried or installed correctly.
The ideal sauna wood should support:
- Repeated heating and cooling
- Changes in humidity
- Comfortable skin contact
- Clean detailing
- Appropriate heat transfer
- Stable joints and profiles
- Long-term visual quality
- The intended interior or exterior application
Theraluxe’s guide to choosing sauna wood explains why species, density, grain, resin content and structural behaviour should be considered alongside colour.
Thermal modification can improve some properties, but it does not make every wood species suitable for every part of a sauna. Species selection and construction detailing remain essential.
Improved Dimensional Stability
One of the main reasons thermally modified wood is used in saunas is its improved dimensional stability.
Untreated wood naturally absorbs and releases moisture as surrounding conditions change. This movement can contribute to swelling, shrinking, cupping or small gaps between boards.
Thermal modification reduces the wood’s ability to absorb moisture, helping it remain more stable through changing environmental conditions.
A study published by BioResources on thermally modified hardwoods and dimensional stability found that thermal modification reduced water absorption and improved dimensional stability compared with untreated samples.
In a sauna, this can support:
- More consistent wall and ceiling panel alignment
- Reduced seasonal movement
- Cleaner joints
- Greater stability through repeated heat cycles
- Better performance in changing humidity
Dimensional stability does not eliminate all wood movement. Timber remains a natural material, and its behaviour will still depend on species, grade, profile, installation and the conditions of the sauna.
A Richer and More Consistent Colour
Thermal modification changes the wood’s tone throughout the board.
Depending on the species and treatment level, pale timber may become honey-coloured, caramel, medium brown or deeply toasted. The grain often appears more pronounced, creating visual depth without relying solely on a surface stain.
This makes thermally modified wood particularly effective in modern sauna interiors.
It pairs naturally with:
- Black metal details
- Brushed or darkened steel
- Large-format glass
- Stone flooring
- Warm integrated lighting
- Minimalist bench profiles
- Dark exterior cladding
- Indoor-outdoor wellness architecture
The colour can give a new sauna the warmth and richness normally associated with naturally aged timber.
However, exterior thermally modified wood will still react to ultraviolet exposure. Without an appropriate UV-protective finish, the surface can gradually weather toward a grey tone. Some homeowners welcome that natural appearance, while others prefer to maintain the original brown colour through regular finishing.
Lower Moisture Uptake and Improved Durability
By reducing the wood’s moisture uptake, thermal modification can make conditions less favourable for some forms of biological decay.
This can improve durability, particularly when the wood is used correctly and kept within its intended exposure class.
That does not mean the material is immune to:
- Surface mould
- Dirt or body oils
- Poor ventilation
- Standing water
- Incorrect installation
- UV exposure
- Weathering
- Neglected maintenance
A sauna still needs proper ventilation, drainage, drying time and regular cleaning. Exterior wood also needs thoughtful flashing, airflow and protection from trapped moisture.
Thermal modification supports good sauna construction. It does not replace it.
For high-traffic facilities, material performance is especially important. Theraluxe’s commercial sauna design considers wood selection alongside guest capacity, cleaning, ventilation, heat balance and repeated daily use.
Comfortable Heat and Tactile Quality
Thermally modified wood can have lower thermal conductivity than the same wood before treatment, meaning it may transfer heat more slowly.
This can be valuable in sauna interiors where benches, backrests and wall surfaces come into contact with the body. However, comfort also depends heavily on the species and density of the wood.
A dense hardwood may still feel warmer than a lower-density sauna wood. Surface finish, board thickness and bench design also affect the tactile experience.
For high-contact areas, look for wood that is:
- Smoothly machined
- Low in resin
- Free from sharp knots
- Properly sanded
- Suitable for sauna temperatures
- Comfortable against bare skin
- Selected for the specific application
Thermally modified aspen is often used for benches and backrests because of its smooth texture and restrained scent. Thermally modified spruce and pine may be better suited to selected wall, ceiling or exterior applications depending on grade and design.
Thermal Wood Is Not Automatically Antibacterial
Thermally modified wood is sometimes described as naturally antibacterial or completely hygienic.
That claim should be approached carefully.
The modification process changes the wood’s moisture response and reduces some of the compounds available to decay organisms. These changes may contribute to improved biological durability, but they do not make sauna surfaces self-cleaning or eliminate the need for proper hygiene.
Sauna benches still collect:
- Sweat
- Skin oils
- Water
- Dust
- Product residue
Users should sit on clean towels, allow the room to dry after use and follow the builder’s care instructions.
In commercial environments, regular cleaning and appropriate operational procedures remain essential regardless of the wood species.
Thermally Modified Wood vs Cedar
Cedar and thermally modified wood can both work beautifully in sauna design. They simply offer different qualities.
Cedar remains popular because it is naturally durable, relatively lightweight and recognizably aromatic. Some homeowners consider the scent an essential part of the sauna experience.
Thermally modified wood may be a stronger fit for someone who wants:
- A deeper and more consistent colour
- A quieter wood scent
- Contemporary architectural detailing
- Reduced swelling and shrinkage
- A wider choice of modified species
- A strong visual connection between interior and exterior materials
The decision does not need to be presented as one material defeating the other. The best choice depends on the architecture, intended experience, budget and application.
Trade-Offs to Consider
Thermal modification improves several properties, but it also introduces trade-offs.
A 2025 scientific review of thermal wood modification identified benefits such as improved dimensional stability and decay resistance while also noting possible reductions in certain mechanical properties.
Depending on the species and treatment intensity, thermally modified wood may become more brittle or experience reduced bending and impact strength.
This means it should be:
- Specified for the correct application
- Machined with suitable tools
- Fastened carefully
- Pre-drilled where recommended
- Protected from unnecessary impact
- Used cautiously in structural applications
- Installed according to supplier guidance
It is especially important to distinguish between wood used as interior panelling and wood expected to carry significant structural loads.
Thermally modified cladding or bench material should not automatically be treated as a structural framing product.
Where Thermally Modified Wood Can Be Used
Thermally modified wood can appear throughout a sauna project, but the exact species and grade should be selected for each application.
Interior Walls and Ceilings
Thermally modified boards can create a warm and visually consistent interior envelope. Their deeper colour works particularly well with soft LED lighting and modern glass.
Benches and Backrests
Smooth, low-resin species such as thermally modified aspen may be used in areas where the body touches the wood. Bench design must also consider ergonomics, ventilation and safe surface temperatures.
Exterior Cladding
Suitable thermally modified products can be used to create a continuous architectural exterior. Correct rainscreen detailing, fasteners and surface protection remain important.
Flooring and Duckboards
Removable wooden floor sections can provide warmth and comfort underfoot. They should be designed to dry properly and allow cleaning beneath them.
Feature Walls and Design Details
A contrasting grain or tone may be used behind the heater, on a ceiling or around an entry to create a focal point. All details near the heater must follow the required safety clearances.
Shelving and Storage
Matching wood can continue into towel shelving, robe storage or the surrounding recovery space, helping the full wellness environment feel cohesive.
Thermally Modified Woods Used by Theraluxe
Different wood species respond differently to thermal modification. Their original density, resin level, grain and colour still influence the finished result.
Thermally Modified Spruce
Spruce has a clear softwood character and may include visible knots. Thermal modification deepens its tone and improves dimensional stability.
It works well for homeowners who want a Scandinavian-inspired interior with natural variation rather than a completely uniform surface.
Often suited to: wall and ceiling panelling, selected exterior details and warm Nordic interiors.
Thermally Modified Hemlock
Hemlock has a subtle grain and a refined appearance. Its warmer post-treatment colour can create an understated, contemporary interior.
Often suited to: panelling, backrests and design schemes where a calm, consistent grain is preferred.
Thermally Modified Pine
Pine brings stronger grain and knot character. Thermal modification creates a darker, more rustic expression while improving its dimensional response.
Because pine naturally contains resin, grade and treatment quality should be confirmed before it is selected for high-contact or high-temperature surfaces.
Often suited to: feature surfaces, selected cladding and interiors with more natural character.
Thermally Modified Aspen
Aspen is valued in sauna interiors for its low resin content, restrained scent and smooth tactile quality. Thermal modification changes its pale colour into a richer caramel or brown tone.
Often suited to: benches, backrests, interior panelling and high-contact applications.
Thermally Modified Magnolia
Magnolia offers a distinctive grain and a design-forward finish. At Theraluxe, it may be used where the project calls for a more individual material expression.
Often suited to: feature walls, selected interior panels and bespoke design details.
The species should always be selected according to the project rather than chosen from colour alone.
Performance in Canadian Climates
Canadian outdoor saunas experience significant seasonal variation.
Depending on the region, exterior materials may face:
- Heavy rain
- Snow and ice
- Dry summer heat
- Coastal moisture
- Strong sunlight
- Rapid temperature changes
- Long winter heating cycles
Thermally modified wood’s reduced moisture uptake and improved dimensional stability can help it manage changing conditions more consistently than the same species in untreated form.
However, the material still needs a complete exterior strategy.
That includes:
- Proper wall assemblies
- Ventilated cladding
- Correct flashing
- Exterior-rated fasteners
- Ground clearance
- Controlled drainage
- UV protection where colour retention is desired
- Routine inspection and maintenance
A well-designed envelope protects the sauna more effectively than any material claim on its own.
Theraluxe’s guide to Shou Sugi Ban in modern sauna design explores another wood-finishing approach used to create visual depth and weather-aware exterior surfaces.
Sustainability Depends on the Full Material Story
Thermal modification is often positioned as a sustainable process because it can improve wood performance without conventional biocidal preservatives.
That is one meaningful advantage, but a complete sustainability assessment should also consider:
- Where the timber was grown
- How the forest was managed
- Transportation distance
- Energy used during modification
- Expected service life
- Maintenance requirements
- Finishes and adhesives
- Whether the material can be repaired or replaced
- End-of-life options
Longer-lasting material may reduce the frequency of replacement, but thermal modification alone does not verify responsible forestry.
Homeowners who prioritize sourcing can ask about FSC-certified wood. The Forest Stewardship Council’s wood certification information explains how its system supports material from responsibly managed forest sources.
A clear material specification should distinguish between:
- The wood species
- The treatment process
- The manufacturer
- The chain-of-custody certification
- The finished product used in the sauna
What to Ask Before Choosing Thermally Modified Wood
Before selecting thermally modified wood, ask the sauna builder or supplier:
- What wood species is being used?
- Which thermal modification process was used?
- Is the product certified or independently graded?
- Is it intended for interior, exterior or both?
- Is it suitable for benches and direct skin contact?
- What grade and knot profile should I expect?
- Does the material require an exterior finish?
- How will sunlight affect the colour?
- Which fasteners and installation methods are recommended?
- Is the wood structural or decorative?
- What cleaning and maintenance does it require?
- Is responsible-sourcing documentation available?
These questions help separate a genuine performance specification from a general “thermal wood” label.
A Commercial Example: Kolm Kontrast
Material selection becomes especially important in a commercial sauna where the room may experience several guided sessions and many guests each day.
For the sauna at Kolm Kontrast in Vancouver, Theraluxe selected thermally modified aspen for the interior.
The material supported the project through:
- A smooth tactile finish
- Restrained scent
- A warm caramel tone
- A contemporary appearance
- Suitability for repeated guest contact
- Visual consistency beneath ambient lighting
The wood was one part of a wider commercial design system that also considered bench levels, heater placement, ventilation, guest movement, lighting and cleaning.
This illustrates why sauna materials should be chosen as part of the complete room rather than as an isolated finish.
Why Theraluxe Uses Thermally Modified Wood
Thermally modified wood gives Theraluxe greater control over the relationship between performance and design.
It allows us to create sauna interiors that feel rich and warm while supporting dimensional stability through repeated use. The range of species and tones also makes it possible to respond to different architectural styles—from restrained Scandinavian interiors to deeper, more expressive contemporary spaces.
We use it selectively rather than automatically.
The final specification depends on:
- Sauna location
- Interior or exterior application
- Expected use
- Climate exposure
- Desired colour and grain
- Tactile requirements
- Maintenance expectations
- The architecture surrounding the sauna
Thermal modification does not turn every board into the perfect sauna material. Its value comes from pairing the right species, grade, treatment and construction method.
Explore Theraluxe’s bespoke custom saunas to see how wood, glass, heaters, lighting and spatial planning can be developed as one complete sauna environment.
Materials That Support the Ritual
A sauna is remembered through feeling.
The warmth of the bench, the grain beneath the hand, the scent of the room and the glow of the wood all become part of the ritual. These details influence whether the space feels temporary or enduring, ordinary or deeply considered.
Thermally modified wood offers a distinctive balance of stability, warmth and visual depth. When it is specified honestly and installed properly, it can support both the technical performance and architectural character of a sauna.
Its strongest qualities are practical:
- Reduced moisture uptake
- Improved dimensional stability
- A rich, consistent tone
- Wider design possibilities
- Suitability for selected interior and exterior applications
The best sauna design still comes from the full system: correct species, sound construction, balanced ventilation, proper maintenance and craftsmanship that respects the material.
At Theraluxe, those decisions are what transform wood into a wellness space designed to be used, felt and returned to for years.
FAQ: Thermally Modified Wood in Sauna Design
Thermally modified wood is timber that has been treated at high temperatures under controlled conditions. The process changes how the wood interacts with moisture and generally improves its dimensional stability.
What is thermally modified wood?
It also darkens the wood throughout the material.
Is thermal wood the same as ThermoWood?
Not exactly. Thermally modified wood is the general material category.
ThermoWood® is a registered product and certification name connected to the International ThermoWood Association and its approved production standards.
Is thermally modified wood good for saunas?
Yes, when the correct species, grade and product are selected. Its reduced moisture uptake, improved stability and warm appearance make it useful for many sauna interiors and exteriors.
The treatment does not make every species appropriate for benches, structures or direct heater exposure.
Is thermal wood better than cedar?
Neither material is universally better.
Cedar offers natural aroma, warmth and familiar traditional character. Thermally modified wood offers deeper colour, reduced moisture movement and a range of contemporary species and finishes.
The best option depends on the intended design and application.
Is thermally modified wood chemical-free?
Many thermal modification processes use heat and steam without adding conventional chemical wood preservatives.
The finished sauna component may still include coatings, adhesives or other products, so the full specification should be confirmed with the supplier.
Is thermal wood antibacterial?
Thermal modification may improve decay resistance by changing the wood’s moisture response and composition, but it should not be treated as self-cleaning or universally antibacterial.
Sauna surfaces still require towels, ventilation, drying and regular cleaning.
Does thermal wood require maintenance?
Yes. Interior surfaces need appropriate cleaning and drying. Exterior surfaces require inspection and may need UV-protective finishes if the original brown colour is to be maintained.
Maintenance depends on exposure, species, finish and installation.
Can thermal wood be used outdoors?
Suitable thermally modified products can be used outdoors, but performance still depends on correct cladding details, ventilation, drainage, fasteners and surface protection.
The material should be specified for its intended exposure.
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