Last updated 2026-07-11

What is thermally modified wood, exactly?

Thermally modified wood is softwood or hardwood baked in a sealed kiln at 160°C to 230°C under steam or low-oxygen conditions ^[1]. The heat drives out most of the free water and, more to the point, restructures the wood's hemicellulose, the carbohydrate fraction that decay fungi and wood-boring insects eat. No preservatives get injected. The wood itself becomes the preservative.

The process was commercialized in Finland in the 1990s under the ThermoWood name and independently standardized by the International ThermoWood Association ^[1]. Other brands work on related chemistry: Kebony uses furfurylation, Accoya uses acetylation, and a handful of heat-treatment labels compete on price. When people say "thermally modified" for sauna exteriors, they usually mean the dry-heat or steam process, because it's widely available, cheaper than acetylation, and leaves no residual chemicals.

The change is permanent. You cut, mill, and fasten the boards like any lumber. Unlike pressure-treated stock, the cut ends don't expose an untreated core that rots from the inside out.

Why does a sauna exterior need anything special?

A sauna exterior takes a beating that eats ordinary wood fast. Every session, extreme interior heat and steam push moisture outward through the wall assembly. Rain, snow, and morning dew hit the outside face. The wood swings from wet to dry and from near-freezing nights to 70°C+ interior air, cycling through expansion and contraction hundreds of times a year. Standard kiln-dried pine, cedar, or spruce checks, cups, and starts rotting at the end grain within five to ten years in most climates unless you seal and maintain it hard ^[2].

That maintenance load is real on an outdoor sauna. Clear sealers want reapplication every one to three years depending on sun and rain. Most homeowners skip a cycle, and the wood pays for it. Thermally modified wood absorbs roughly 50% less moisture than unmodified wood of the same species ^[1], which slows the expansion-contraction cycle and keeps the surface stable long enough that finish coats last longer between applications.

Then there's looks. Left bare, thermally modified wood weathers to a silver-gray patina over two to three years, similar to teak or ipe. Plenty of people building modern home saunas want exactly that weathered Scandinavian look without the environmental cost of tropical hardwoods.

How long does thermally modified wood last on a sauna exterior?

Field data and standardized testing put thermally modified wood at durability class 1 to 2 under European standard EN 335, which reads as "very durable" to "durable" and a service life of 25 years or more in above-ground exterior use ^[3]. The ITWA reports that ThermoWood D, the higher grade processed at 190°C and up, reaches durability class 1 in above-ground tests, so it should outlast untreated pine by three to five times in similar conditions ^[1].

Compare that to the alternatives. Untreated Scots pine in above-ground cladding is durability class 4 and typically lasts 10 to 15 years with paint or stain upkeep. Western red cedar, one of the most popular sauna cladding species in North America, sits at class 2 to 3 naturally and lasts 20 to 30 years when maintained ^[4]. Thermally modified pine can match or beat cedar's durability at a lower material cost than clear-grade cedar.

One honest caveat. Most long-term field studies come from northern Europe: Finland, Sweden, Germany. Build in humid subtropical Florida or coastal Louisiana and you're using this data outside its home range. The wood should still crush untreated softwoods, but nobody has 25-year-old thermowood sauna cladding in Miami to point to. Expect the chemistry to hold; plan for more frequent inspection of end grain and fastener holes.

Keep the bottom course of cladding at least 200mm off grade and make sure the wall assembly drains and dries. No wood, however well-modified, survives standing in water.

How does thermally modified wood compare to other sauna exterior materials?

Here's a practical comparison of the common choices.

Cost ranges are estimates from 2024 US lumber market reporting and swing a lot by region and board profile ^[5]. Get local quotes before you budget.

The takeaway is simple. Thermally modified wood sits in a sweet spot: cheaper than Accoya or clear cedar, more sustainable than tropical hardwoods, and far more durable than untreated softwood. For most outdoor sauna builds in temperate or cold climates, it's the rational pick.

Which species of thermally modified wood work best for sauna exteriors?

Species matters, but the modification grade often matters more than the species.

Thermally modified pine (usually Scots pine or radiata pine) is the most available and affordable. It takes the process well and reaches durability class 1 to 2 at the higher heat setting ^[1]. The color goes a warm brown, closer to walnut than pine's usual blonde, which most people like. It's the standard budget choice.

Thermally modified ash is harder and denser, so it resists surface dents on cladding that gets bumped or leaned against. It takes the brown color deeper and more evenly than softer species. Emerald ash borer has hammered ash across North America, which makes sustainably sourced thermally modified ash a decent way to use trees that would otherwise become waste.

Thermally modified spruce is common in Europe. It's lighter than pine and very workable, but slightly less durable at the highest grade, because spruce's lower starting density limits how much the modification improves it relative to pine.

Thermally modified radiata pine from New Zealand (sold under several brand names) is big in Australia and increasingly available in the US. Radiata takes modification evenly thanks to its uniform grain, and it comes in long, knot-free lengths that suit horizontal cladding.

Inside the sauna, the calculation flips. Thermally modified wood is fine indoors, but Nordic spruce, aspen, and alder are lower density, heat up more slowly to the touch, and don't leach resin when hot. Your exterior cladding choice is separate from what you line the inside with. If you're still sorting the full build, the sauna overview covers interior wood in depth.

Is thermally modified wood safe? Does it off-gas or leach chemicals?

Yes, it's safe. The process uses only heat plus steam or nitrogen. No chromium, no arsenic, no petroleum biocides ^[1]. There's nothing to leach into soil or groundwater and nothing to volatilize when the sauna gets hot. European Environmental Product Declarations for major thermowood makers classify it as non-hazardous waste ^[6].

One honest nuance. Thermally modified wood, like all wood, gives off volatile organic compounds when freshly cut and as a new structure first warms. Those are the same terpenes and aldehydes any fresh lumber produces, not a byproduct of the modification chemistry, and they clear quickly with ventilation.

For an outdoor sauna exterior, the safety question is really about ground contact and drainage. Even the best thermally modified wood isn't rated for direct ground contact, which is EN 335 durability use class 4, while thermowood generally hits class 1 to 2 for above-ground use ^[3]. Keep the base structure on concrete piers or pressure-treated sill plates, and keep the cladding boards clear of soil and standing water.

Pressure-treated lumber (CCA or newer copper formulations) still belongs on any structural framing that touches ground or concrete, even when you clad the outside face in thermowood. The two materials solve different parts of the same problem.

Does thermally modified wood need to be finished or stained on a sauna exterior?

It doesn't need finishing to survive, which is the big practical win over untreated wood. Left bare, thermally modified wood weathers to silver-gray in 12 to 24 months depending on UV exposure ^[7]. The modification protects the wood from biological decay no matter the surface color. Like the weathered look? Install it and walk away.

If you want to keep the warm brown, an initial coat of UV-stable exterior oil (a hardwax oil or a modified oil formulated for thermowood) slows the graying. The ITWA recommends a thin coat shortly after installation and re-oiling every five to eight years in most climates, against every one to three years for untreated wood ^[1]. That reduced schedule is a genuine quality-of-life gain.

A few things not to do. Skip film-forming paints and solid stains on thermally modified cladding. The wood's low moisture absorption means paint films can't grip as hard as they do on regular wood, so they flake instead of fade. Penetrating oils and semitransparent stains are the right category. Also avoid oil finishes with heavy-metal driers (some older formulas carry cobalt or manganese salts) near soil, since those concentrate over years of runoff.

For a sauna facing hard rain and UV on an exposed site, SweatDecks carries outdoor sauna packages with cladding already specified for the climate, which saves the back-and-forth of sourcing wood and finish separately.

How is thermally modified wood installed on a sauna exterior?

Installation is basically the same as any horizontal or vertical board cladding, with a few differences that matter.

Fasteners: use stainless steel screws or ring-shank nails. Thermally modified wood is more brittle than unmodified wood, because the process cuts its impact toughness by roughly 25 to 40% depending on grade and species ^[10]. Standard hot-dipped galvanized fasteners can react with tannins released during early weathering and leave rust streaks. Stainless is the right call.

Pre-drilling: always pre-drill near board ends. The brittleness means boards split at fastener holes if you drive screws without a pilot hole. A 2.5 to 3mm pilot for a 4mm screw shank works for most profiles.

Gap spacing: thermally modified wood moves less, typically 50 to 70% less expansion and contraction in the radial and tangential directions ^[1], so you need slightly less gap than standard cladding guidelines suggest. Follow the board manufacturer's install guide, though, since species and grades vary.

Ventilated rainscreen: build a rainscreen cavity behind the cladding, even with thermowood. A 25 to 40mm air gap between the cladding and the weather-resistive barrier lets the backside of the boards dry, which extends the life of the whole wall assembly and the cladding itself. This matters double on a sauna, where warm moist air actively pushes outward through the wall every session.

Horizontal versus vertical: both work. Vertical installation (board-and-batten or channel rustic) sheds water faster, a small edge in very wet climates. Horizontal lap or shiplap is more common and gives slightly more overlap coverage per linear foot of material.

What does thermally modified wood cost for a sauna exterior project?

Thermally modified cladding boards in the US run roughly $3.50 to $7.00 per linear foot for standard profiles (shiplap, channel rustic, tongue-and-groove) in pine or ash, or $12 to $22 per square foot of wall area once you account for coverage ratios and waste ^[5]. That's a real premium over #2 pine clapboard at $1.50 to $2.50 per linear foot, but it lands below clear western red cedar, which runs $5 to $12 per linear foot for the grades worth using on an exterior sauna.

Take a typical freestanding sauna box, say 8x10 feet by 8 feet tall. The clad exterior wall area is roughly 275 to 320 square feet after you subtract roof overhang and glass. At $14 to $22 per square foot installed (materials plus labor), you're looking at $3,850 to $7,040 for the exterior cladding alone. Doing the install yourself drops the material cost to $962 to $2,240, which is the range most homeowners actually work with.

Cost of ownership favors thermally modified wood if you plan to keep the sauna past ten years. Untreated pine at half the upfront material cost needs stripping, sanding, and restaining every two to three years plus board replacement around fastener holes and end grain. Over 20 years, that maintenance usually costs more than the thermowood premium.

The cheapest mistake here is buying cheap cladding and skipping the rainscreen cavity. That one decision accelerates rot no matter what wood you chose.

Are there any downsides to thermally modified wood on sauna exteriors?

A few, and they're worth knowing before you order.

Brittleness: the same heat treatment that cuts moisture absorption also lowers the wood's modulus of rupture and impact toughness ^[10]. For cladding this barely registers, since the boards aren't load-bearing. But use thermally modified wood for structural pieces like purlins or rafter tails and you need to size them conservatively and consult an engineer. Stick to approved structural lumber for structural members.

Cost and availability: thermally modified cladding isn't stocked at most big-box stores yet. You'll usually order from a specialty lumber dealer or online supplier, which means freight on a heavy product and longer lead times. Plan on 2 to 6 weeks depending on region.

Color variation: the process produces some board-to-board color variation within a batch, because heat penetrates more evenly in some parts of the kiln than others. Good producers minimize it, but request a sample batch before ordering the full quantity if visual consistency matters to you.

Not a substitute for good design: thermally modified wood is excellent, but no cladding compensates for poor drainage, missing flashing at window and door heads, or skimpy roof overhangs. Those details are what actually kill outdoor sauna structures, whatever the walls are clad in.

And don't confuse thermally modified wood with plain "heat-treated" lumber, which is a phytosanitary insect treatment for pallet wood that needs only 56°C for 30 minutes ^[12]. The temperatures are worlds apart and so are the effects. Thermally modified exterior cladding requires sustained treatment at 160 to 230°C ^[1].

What certifications and standards should I look for when buying thermally modified wood?

Two things to check on any product or data sheet.

First, modification temperature and grade. The International ThermoWood Association splits product into ThermoWood S (mild, around 190°C) for interior use and ThermoWood D (durable, around 212°C) for exterior above-ground use ^[1]. The European durability system EN 335 and testing standard EN 350 give you a cross-manufacturer benchmark ^[3][11]. Ask your supplier which EN 350 durability class the specific product hits. Class 1 or 2 is what you want for a sauna exterior.

Second, chain of custody for the source timber. Thermally modified wood is only as sustainable as the forest it came from. Look for FSC (Forest Stewardship Council) or PEFC certification on the raw material ^[9]. Major producers in Finland and Sweden usually carry FSC because their source forests are managed under national forest law, but North American suppliers vary.

You don't need a building permit for the cladding material itself, but the overall sauna structure may require one depending on your jurisdiction, its size, and whether it has electrical service. Check with your local building department before you build. Requirements vary widely, and a freestanding outdoor sauna with an electric heater can trip permit thresholds in plenty of US counties.

For the bigger picture on an outdoor build, the outdoor sauna guide covers permits, setbacks, and site prep alongside material choices.

Where can I buy thermally modified wood in the US?

Specialty lumber yards are your first call. In most metro areas, a hardwood dealer or architectural millwork supplier stocks at least one thermally modified product. Names to search: Arbor Wood (US producer), Thermory (Estonian brand, widely distributed in North America), Cambia by Bingaman & Son Lumber (a North American producer using pine), and Shou Sugi Ban House (which carries thermowood alongside charred products).

Online suppliers ship nationwide and often price volume orders better. Factor freight carefully. Thermally modified cladding isn't unusually heavy for hardwood, but shipping 500 board feet across the country adds up. Get a freight quote before the final comparison.

Home Depot and Lowe's don't generally stock thermally modified cladding as of 2024, though some locations carry Kebony or Accoya in limited profiles.

When ordering online, ask for the production lot's moisture content test results. Thermally modified wood should ship at 4 to 8% equilibrium moisture content, lower than standard kiln-dried lumber at 12 to 19% ^[1]. Boards that re-absorbed moisture in transit or storage give back some of the benefit. Reputable suppliers ship in sealed packaging.

Still early in planning and want to see complete units with materials already specified? SweatDecks has a range of outdoor sauna kits where the exterior cladding is pre-selected for durability and looks, which simplifies sourcing a lot.

Frequently asked questions

Can I use thermally modified wood for both the exterior cladding and the interior sauna lining?

You can, and some builders do for a consistent look. Thermally modified wood won't off-gas harmful chemicals when hot. But traditional interior woods like Nordic spruce, aspen, and alder are preferred inside because they're lower density, heat up to touch temperature more slowly, and absorb the steam cycle better. Save thermowood for the exterior, where its moisture resistance earns its keep.

Does thermally modified wood work on a barrel sauna exterior?

Yes. Barrel sauna staves are typically tongue-and-groove softwood, and thermally modified pine or spruce in matching profiles is an excellent choice. The curved assembly sheds water naturally, which suits thermowood. Standard install advice applies: pre-drill fastener holes, use stainless hardware, and keep the bottom staves clear of soil and standing water. Most thermowood suppliers can cut custom curved or tapered profiles on request.

Will thermally modified wood hold screws as well as regular lumber?

Slightly less well, because the process lowers density and fiber strength somewhat. Withdrawal resistance (how hard it is to pull a screw straight out) drops by roughly 10 to 20% depending on species and grade. For cladding, that isn't structurally meaningful. Always pre-drill near ends to prevent splitting. Use ring-shank nails or coarse-thread screws for the best hold, and stick with stainless to avoid tannin staining.

How does thermally modified wood handle snow and freeze-thaw cycles?

Very well. The core advantage is low moisture uptake, roughly 50% lower than unmodified wood, so there's much less liquid water inside the board to freeze and expand during cold cycles. That's a big reason thermally modified wood originated in Scandinavia, where freeze-thaw cycling is brutal. Sealing the end grain with a penetrating oil at installation adds protection at the most vulnerable points.

Is thermally modified wood the same as pressure-treated lumber?

No, and the difference matters. Pressure-treated lumber is injected with chemical preservatives, usually copper-based, under pressure to stop rot. Thermally modified wood gets its rot resistance from heat alone, with no added chemicals. Pressure-treated belongs on structural members in ground or concrete contact. Thermally modified wood is the right choice for above-ground cladding where you want durability without chemical exposure near soil or water.

Can thermally modified wood be used on a sauna near a pool or hot tub?

Yes, and it's a good choice for that spot. Splashed water, high ambient humidity, and UV exposure near a pool are hard on ordinary wood. Thermowood's low moisture uptake resists the repeated wetting and drying, and its chemical-free profile means nothing leaches near pool water. Apply a UV-stable penetrating oil at installation and re-oil every five to eight years for best looks. Keep pool chemical splash off any wood surface.

How long does thermally modified wood take to weather to gray outdoors?

Graying starts within three to six months in direct sun and full weather exposure, with most of the color change done by 12 to 24 months. The rate depends on UV intensity, rainfall, and whether the boards are oiled. A UV-protective penetrating oil at installation slows graying noticeably. Want the silver-gray look from day one? Install unfinished boards and let them weather naturally.

What is the difference between ThermoWood, Kebony, and Accoya?

All three improve durability and dimensional stability, but by different chemistry. ThermoWood uses dry heat at 160 to 230°C in a low-oxygen kiln. Kebony uses furfurylation, impregnating softwood with a bio-based furfuryl alcohol that polymerizes in the cell walls. Accoya uses acetylation, reacting the wood's hydroxyl groups with acetic anhydride. Accoya has the best dimensional stability data. ThermoWood is the most available and least expensive. Kebony sits between them.

Does thermally modified wood require special tools to cut and mill?

No special tools. Standard carbide-tipped saw blades and router bits work fine. The wood is slightly harder to hand-plane than unmodified softwood because of the brittleness, so power tools are preferred for profiling. Wear a dust mask when cutting: the dust carries the same general cautions as any fine wood dust, which OSHA treats as a nuisance particulate with permissible exposure limits. Good shop ventilation is enough.

Is thermally modified wood more sustainable than cedar or tropical hardwood for a sauna exterior?

Generally yes. Thermally modified pine or ash can come from FSC-certified northern European or North American managed forests and uses no chemical inputs beyond heat. Tropical hardwoods like ipe involve high-impact harvesting even when nominally certified. Western red cedar beats tropical hardwoods but old-growth cedar raises supply and pricing concerns. For a low-impact sauna exterior, thermowood from an FSC-certified source is a strong pick.

Do I need to seal the cut ends of thermally modified boards before installation?

Yes, and it's one of the most important install steps. End grain absorbs moisture faster than face or edge grain even in thermally modified wood, because the process protects the cell walls but can't fully close the open end-grain tubes. Apply two coats of a penetrating end-grain sealer or the same exterior oil you're using on the face, and let it absorb fully before fastening. This step extends the life of the whole installation.

Can I paint thermally modified wood on a sauna exterior?

Technically yes, but film-forming paints and solid stains perform poorly on thermowood and most manufacturers advise against them. The low moisture absorption that makes thermowood durable also means paint films can't grip strongly, so they flake earlier than on conventional wood. Stick with penetrating oils, semitransparent stains, or water repellents that soak into the surface instead of forming a film. Those last longer and are easier to maintain.

Sources

1. International ThermoWood Association, ThermoWood Handbook: ThermoWood process temperatures (160 to 230°C), durability classifications, moisture uptake reduction (~50%), dimensional stability improvement, and installation guidance for ThermoWood S and ThermoWood D grades
2. USDA Forest Products Laboratory, Wood Handbook: Wood as an Engineering Material (General Technical Report FPL-GTR-282): Natural decay resistance of common softwood species including pine and spruce in above-ground exterior applications
3. European Committee for Standardization, EN 335: Durability of wood and wood-based products: EN 335 durability use class definitions for above-ground exterior wood applications and service life expectations for each class
4. USDA Forest Products Laboratory, Wood Handbook Chapter 4: Moisture Relations and Physical Properties of Wood: Natural durability class of western red cedar (Thuja plicata) rated as durable to very durable in above-ground exterior use
5. USDA Economic Research Service, lumber and wood products market data: 2024 US lumber market pricing context used to estimate installed cost ranges for cladding materials
6. Thermory AS, Environmental Product Declaration (EPD) for ThermoWood Products: Thermally modified wood classified as non-hazardous waste under EU regulations; no chemical preservatives used in production
7. Welzbacher, C.R. and Rapp, A.O., Journal of Wood Science: Weathering and colour changes in thermally modified wood: Thermally modified wood weathers to silver-gray within 12 to 24 months of exterior UV exposure; UV-stabilizing oils slow this color change
8. Forest Stewardship Council, FSC Chain of Custody Certification: FSC certification covers chain-of-custody for sustainably sourced timber used in thermally modified wood production
9. Esteves, B.M. and Pereira, H.M., BioResources Journal: Wood modification by heat treatment, A review: Thermal modification reduces modulus of rupture and impact toughness by 25 to 40% depending on species and treatment temperature; hemicellulose degradation is primary mechanism of improved decay resistance
10. European Committee for Standardization, EN 350: Durability of wood and wood-based products, Testing and classification: EN 350 provides standardized testing and classification framework for natural and modified wood durability used to assign EN 335 use classes
11. OSHA, Occupational Safety and Health Standards: Wood Dust Permissible Exposure Limits: OSHA classifies wood dust as a nuisance particulate with permissible exposure limits; applies to thermally modified wood cutting operations