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Thermo-alder takes the traditional Scandinavian sauna wood and elevates it through thermal modification — transforming natural alder's soft beige tone into a rich, warm brown with deeper grain definition and significantly improved performance. Where natural alder is prized for its calm, understated character, thermo-alder delivers the same low heat conductivity and smooth surface in a deeper, more refined palette. The color runs throughout the entire cross-section of the board (not just a surface treatment), so cuts and sanding never reveal a different color underneath. All thermo-alder sauna wood we carry is produced by Thermory, the Estonian manufacturer that pioneered commercial thermal modification for sauna applications.
For details on how thermal modification works: Why Thermowood Is the Best Wood for Saunas.
Thermo-alder wall cladding is available in three Thermory profiles, all sold as 6-board packs:
Nickel Gap 1×4 (STS4) — Tight flat-bottomed gap between boards for a clean, contemporary look. This is the standard width for most sauna walls and ceilings. Six boards per pack.
Nickel Gap 1×5 (STS4) — Wider board face for fewer seams and a slightly more expansive feel. Same nickel gap joint profile as the 1×4 but with broader board coverage per piece. Ideal for larger sauna rooms or anyone who prefers the look of wider planks. Six boards per pack.
Large Gap 5/4×3 (STEP) — Thicker boards with pronounced gap between them for dramatic shadow lines. The 5/4" thickness gives a more substantial, dimensional presence on the wall. Works well as a full wall treatment or as an accent wall paired with nickel gap on the remaining surfaces. Six boards per pack.
Black Wax 1×5 Nickel Gap (STS4) — Thermo-alder with a heat-rated black wax surface coating. Creates a striking dark finish while retaining the wood's natural grain texture underneath. Use for a full dark-toned sauna interior or as a high-contrast accent wall paired with natural thermo-alder. The black wax is a surface treatment (unlike the thermal modification color, which is structural) — it will not penetrate to the core if deeply sanded, but it's engineered for sauna heat exposure. Six boards per pack. Browse all options: Black Wax Sauna Wood.
Thermory thermo-alder bench boards are milled flat (no tongue-and-groove) and sold in multi-board packs:
1×4 Bench (SHP, 5-board pack) — Standard thickness bench board for seating surfaces. Smooth on all faces for comfortable skin contact. Laid across bench frames with gaps between boards for air circulation and drainage.
5/4×4 Bench (SHP, 4-board pack) — Thicker bench board for applications requiring extra rigidity or a more substantial feel. The added thickness reduces flex on wider bench spans. Four boards per pack.
Alder's naturally low heat conductivity is one of its strongest properties as a bench material — the surface stays comfortable against bare skin even at higher sauna temperatures. Thermal modification enhances this further by reducing the wood's density slightly, which lowers thermal conductivity even more.
Thermory offers factory-assembled thermo-alder bench modules — the fastest path to a finished sauna interior. These arrive ready to mount on your framing with no individual board layout required:
63" Bench Module — Shorter bench for compact saunas or as a lower-tier bench in multi-level layouts.
94.5" Bench Module — Full-length bench for standard sauna widths. Seats two to three adults comfortably.
Corner Piece — L-shaped module that connects two bench runs at a 90° angle. Essential for wrap-around bench configurations.
81" Backrest — Prebuilt vertical backrest panel that mounts behind the bench. Adds comfort and a finished look to the upper wall area behind seating. Also available in thermo-aspen.
Prebuilt modules are ideal for builders who want a professional result with less on-site labor, or for DIY builders who want to simplify the bench construction phase of their project.
Alder has been a sauna wood of choice in Scandinavia and the Baltic states for centuries — valued for its smooth surface, low heat conductivity, and fine even grain. Natural alder performs well in sauna environments on its own (it's naturally water-repellent and resin-free), but thermal modification pushes its performance further. The heat treatment process (400–430°F in the absence of oxygen) reduces moisture absorption, improves dimensional stability through repeated heating and cooling cycles, and increases resistance to decay.
The visual transformation is equally significant. Natural alder's subtle beige-to-honey tone deepens into a warm, consistent brown with more visible grain contrast. The uniformity of color — both across boards and throughout the cross-section of each board — gives thermo-alder a refined, cohesive appearance that many designers and builders prefer for upscale custom sauna projects.
Both share the same base species and fundamental characteristics (low heat conductivity, resin-free, splinter-resistant, smooth grain). The practical differences come down to color, stability, and product availability:
Natural alder (ProSaunas) is lighter in color (calm beige to honey), less expensive per board, and available as individual boards in three wall cladding profiles, five bench sizes, and four trim profiles. It's the choice when you want a lighter palette or plan to mix alder with darker accent woods.
Thermo-alder (Thermory) is deeper in color (warm brown), more dimensionally stable, and available in packaged board sets plus prebuilt bench modules and a black wax finish option. It's the choice for a richer, darker aesthetic with improved long-term performance — particularly in high-use commercial installations or outdoor saunas where moisture exposure is greater.
Both are thermally modified hardwoods with similar golden-to-brown tones, but they come from different base species with different grain characteristics. Thermo-alder has a finer, more even grain with a slightly warmer, reddish-brown undertone. Thermo-aspen has a more neutral golden-brown tone with a smoother, more uniform surface. Thermo-aspen has the wider product range overall (including designer profiles like KYTE and VIRE from Thermory, plus ProSaunas individual boards). Both make excellent sauna interiors and pair well together in two-tone designs. For a full species comparison: Best Sauna Wood Types.
Use our Sauna Wood Calculator to estimate board footage for walls, ceiling, and benches. For Thermory pack products, you'll need to convert from total square footage to the number of packs required — each product listing specifies coverage per pack. Contact us if you need help with the conversion.
Yes — this is common in custom builds. Thermo-alder pairs well with lighter woods like natural aspen or natural alder for contrast. It also blends naturally with thermo-aspen since both share similar warm brown tones. All Thermory profiles use standard tongue-and-groove joints that interlock with other Thermory products of the same profile type.
No — the thermal modification process provides the protection. The reduced moisture absorption handles sauna wet/dry cycles without an applied finish. For bench surfaces that see heavy skin contact, an optional sauna wood oil can maintain the fresh appearance. Read more: How to Finish Wood in a Sauna.
They serve different aesthetics and priorities. Thermo-alder is warm brown with no scent, smoother grain, and better dimensional stability from thermal modification. Cedar is reddish-brown with a strong aromatic scent and natural rot resistance from thujaplicin oils. Thermo-alder typically costs less per linear foot. For a detailed comparison: Thermowood vs. Cedar for Saunas.
Natural thermo-alder is warm brown from the thermal modification process — this color goes all the way through the board. The black wax version applies a heat-rated dark surface coating over the thermo-alder. It creates a much darker, more dramatic look while preserving the wood's grain texture. The wax is a surface treatment, so deep sanding would reveal the brown thermo-alder underneath. Most builders use black wax panels as an accent wall or ceiling feature rather than the entire room.
Shop more: All Sauna Wood · Natural Alder · Thermo-Aspen · Thermo-Spruce · Thermo-Radiata Pine · Cedar · Aspen · Hemlock · Trim & Molding · Wood Oil · Wood Calculator
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