Thermory Sauna Wood

Thermory is an Estonian manufacturer that produces some of the highest-quality thermally modified wood available for sauna construction. Thermal modification is a process where raw lumber is heated to 400°F+ in an oxygen-free kiln for hours. This permanently alters the wood's cell structure — reducing moisture content to near zero, eliminating the sugars and resins that attract mold and insects, and dramatically improving dimensional stability. The result is wood that resists rot, warping, and decay without any chemical treatment. No sealants, no preservatives, no toxic finishes — just heat-modified timber that performs better than untreated wood in the extreme environment of a sauna.

Why Thermory?

There are several thermowood manufacturers, but Thermory has become the preferred brand among sauna builders for a few reasons. Their thermal modification process is precisely controlled and consistent across production batches — you get the same color, density, and performance characteristics from one order to the next. They offer a wider range of species than most competitors (aspen, spruce, alder, and radiata pine, each with different properties for different sauna applications). And their lumber is milled to tight tolerances with clean tongue-and-groove profiles that fit together without excessive gapping or forcing. Thermory has been manufacturing thermally modified wood since 1997 in Pärnu, Estonia, and supplies sauna builders and manufacturers across Europe and North America.

Available Species

Thermo-Aspen is the most popular Thermory species for sauna interiors. Aspen has a naturally low density (soft to the touch even when hot), minimal resin content, and a light, uniform grain. Thermal modification deepens the color to a warm honey-brown and eliminates the remaining moisture that would cause warping in a sauna's heat-humidity cycling. Thermo-aspen is the top choice for benches, backrests, and any surface your skin contacts directly — it stays cooler to the touch than denser species at the same air temperature.

Thermo-Spruce is a denser, more structural option. Spruce takes thermal modification well, developing a rich chocolate-brown color that darkens beautifully over time. It's commonly used for wall paneling, ceilings, and exterior cladding on outdoor saunas. Thermo-spruce is harder than thermo-aspen and handles heavier wear — a good choice for high-traffic commercial builds or exterior applications exposed to weather. It's also one of the most cost-effective thermowood options.

Thermo-Alder sits between aspen and spruce in density and is traditionally favored in Scandinavian and Baltic sauna culture. Alder has a tight, even grain with a distinctive reddish-brown tone after thermal modification. It's an excellent all-purpose sauna wood — suitable for both benches and walls — and has a slightly warmer aesthetic than aspen.

Thermo-Radiata Pine is a plantation-grown softwood from sustainably managed forests. Radiata pine is faster-growing and more affordable than European species, making thermo-radiata the budget-friendly thermowood option. After thermal modification, it develops a warm amber color and gains the same rot resistance and dimensional stability as the more premium species. It's best suited for wall paneling, ceilings, and applications where bench-surface softness is less critical.

Thermowood vs. Cedar

The most common comparison is thermowood vs. Western Red Cedar. Both resist rot and moisture naturally — cedar through its natural oils (thujaplicin), thermowood through the modification of its cell structure. The practical differences: cedar has a stronger aromatic scent (which some love and others find overpowering in an enclosed sauna), is typically more expensive per board foot, and comes in wider widths with more natural color variation. Thermowood has a more uniform color and grain, produces almost no scent (ideal if you want the wood to stay neutral so you can use essential oils and aromatherapy without competing smells), and is generally 15–30% less expensive than comparable cedar. For a deep dive on all wood options, read our guide: Best Sauna Wood Types.

Applications

Bench and backrest boards: Thermo-aspen is the standard recommendation. Low density = lower surface temperature at the same air temperature, so it's more comfortable on bare skin at 180°F+. Thermo-alder is the alternative for builders who want a darker, richer tone on benches. For bench construction guidance, our guide covers dimensions, spacing, and structural considerations: How to Build Sauna Benches. Wall paneling and ceilings: Thermo-spruce and thermo-radiata pine are the most common choices — they're harder and more durable for surfaces that don't contact skin directly. The tongue-and-groove profiles install quickly and create a tight, clean interior finish. Exterior cladding (outdoor saunas): Thermo-spruce handles weather exposure well. Its low moisture absorption means minimal seasonal expansion and contraction, and its rot resistance eliminates the need for exterior chemical treatments. Trim, molding, and details: Thermory produces trim profiles in all four species for a matched finish throughout the sauna. Browse our sauna trim collection for options.

Finishing Thermowood

One of the advantages of thermowood is that it can be left unfinished inside a sauna — the thermal modification process already provides the rot and moisture resistance that finishes would otherwise deliver. Many sauna purists prefer the natural feel of untreated thermowood. If you do want to finish it (to preserve color or add a slight sheen), use a sauna-rated paraffin oil or specialized sauna wood treatment — never polyurethane, varnish, or standard wood stain, which will off-gas toxic fumes in sauna temperatures. Our guide covers all the options: How to Finish Wood in a Sauna. Browse sauna wood oils for compatible products.

Frequently Asked Questions

Is Thermory the same as "thermowood"?

Thermory is a brand name — the company that manufactures the wood. "Thermowood" (or "thermally modified wood") is the generic term for the process. It's similar to how "Jacuzzi" is a brand of hot tub. Thermory is the most recognized thermowood brand in the sauna industry, but not all thermowood is Thermory. When you buy Thermory-branded products, you're getting wood processed in their Estonian facility under their specific quality control standards.

Which species should I choose for a DIY sauna build?

For a complete DIY build: thermo-aspen for benches, backrests, and headrests (stays coolest on skin); thermo-spruce for walls and ceiling (harder, more affordable, handles the non-contact surfaces); and thermo-spruce or thermo-radiata pine for exterior cladding if you're building an outdoor sauna. This combination gives you the best performance at each surface while keeping overall material cost down. If you want a single species throughout for a unified look, thermo-alder is the best all-rounder. For complete DIY room kits, see our DIY sauna room kits.

Does thermowood change color over time?

Yes, gradually. Thermo-spruce and thermo-alder will lighten slightly with UV exposure over years (particularly if your sauna has a window). Thermo-aspen tends to be the most color-stable. Applying a sauna-rated wood oil annually can slow color shift and maintain the original tone. The structural properties and rot resistance are not affected by color change.

How does Thermory wood hold up in an outdoor sauna?

Extremely well — outdoor durability is one of the primary reasons thermowood exists. The thermal modification process reduces the wood's equilibrium moisture content from 10–12% (typical for kiln-dried lumber) to 4–6%. This means it absorbs far less water from rain, snow, and humidity, which dramatically reduces the expansion/contraction cycling that causes cracking and warping in untreated outdoor wood. Thermory rates their thermo-spruce and thermo-radiata pine for ground-contact exterior applications without chemical preservatives.

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