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Thermo-spruce brings a distinctive rustic warmth to sauna interiors that sets it apart from the other thermally modified species. Where thermo-aspen is smooth and contemporary, thermo-spruce has a more traditional character — a slightly coarser grain with visible texture that gives walls a natural, lived-in quality reminiscent of classic Nordic saunas. The thermal modification process transforms ordinary spruce into a golden-brown wood with dramatically improved stability and moisture resistance, while eliminating the resin that makes untreated spruce unsuitable for sauna use. All thermo-spruce sauna wood we carry is sourced from ProSaunas, milled specifically for sauna applications.
Learn about the thermal modification process: Why Thermowood Is the Best Wood for Saunas.
Thermo-spruce wall cladding comes in tongue-and-groove profiles with rounded edges that create a smooth, finished joint between boards. Two profile options:
1×4 Nickel Gap (WOOD673) — A small flat-bottomed gap between boards creates clean lines with subtle shadow definition. Coverage is 3.1" per board. The rounded edges soften the joint between boards for a warm, inviting look that showcases thermo-spruce's natural grain texture.
1×4 Medium Gap (WOOD668) — Wider gap between boards for more defined shadow lines and visual rhythm. Same 3.1" coverage. The broader gap emphasizes the rustic character of the spruce grain by framing each board more distinctly.
Both profiles are recommended for walls and ceilings in traditional (Finnish) saunas. The rounded edge detail is a thermo-spruce signature — it adds a subtle tactile quality and visual softness that complements the wood's natural texture.
5/4×4 Bench (WOOD654) — Thicker bench board (5/4" nominal) in standard 4" width. Milled smooth on all sides for comfortable skin contact. The extra thickness provides rigidity and a substantial feel underfoot and under bench. No tongue-and-groove — boards lay flat across a bench frame with gaps for drainage. The golden-brown color of thermo-spruce bench boards matches the wall cladding for a cohesive interior.
Note: Thermo-spruce's bench lineup is more focused than some other species. If you need additional bench widths or thicknesses, consider pairing thermo-spruce walls with bench material from thermo-aspen (which offers 1×3, 1×4, 5/4×3, 5/4×4, and 5/4×6 bench sizes). Both are thermally modified with similar golden-brown tones that work well together.
Untreated spruce is one of the most common construction woods in Northern Europe, but it has a problem in saunas: resin. At sauna temperatures, natural spruce bleeds sticky resin from its surface — uncomfortable on skin, unattractive on walls, and difficult to clean. Thermal modification solves this completely. Heating the wood to 400–430°F (200–220°C) in the absence of oxygen breaks down the resin-producing compounds and fundamentally changes the wood's cellular structure. The result is a resin-free, dimensionally stable, rot-resistant material that retains spruce's natural grain character while gaining the golden-brown color characteristic of all thermowood species.
What makes thermo-spruce unique among thermowoods is its insulating properties. Spruce has excellent thermal insulation — it holds heat well within the sauna room and doesn't transfer heat aggressively to the touch, making it comfortable for both wall surfaces and bench seating. Its slightly coarser grain compared to aspen or alder gives it a more rustic, textured appearance that many builders prefer for traditional sauna aesthetics.
All three thermally modified species we carry — thermo-aspen, thermo-spruce, and thermo-radiata pine — share the same thermal modification benefits (reduced moisture absorption, improved stability, rot resistance, golden-brown color). The differences are in grain character, texture, and product range:
Thermo-Spruce vs. Thermo-Aspen: Thermo-spruce has a more pronounced, rustic grain pattern and slightly coarser texture. Thermo-aspen is smoother and more uniform — the preferred choice for contemporary, minimalist interiors. Thermo-aspen has a much wider product range (designer profiles, prebuilt bench modules, more bench sizes). Choose thermo-spruce for traditional Nordic character; choose thermo-aspen for modern Scandinavian minimalism.
Thermo-Spruce vs. Cedar: Both have warm tones and visible grain. Cedar is reddish-brown with a strong aromatic scent; thermo-spruce is golden-brown with no scent. Thermo-spruce has better moisture resistance from thermal modification. Cedar has natural rot resistance from thujaplicin oils. For a deeper comparison: Thermowood vs. Cedar for Saunas.
Yes — it doesn't secrete resin (the thermal modification eliminates that), stays cool enough for comfortable skin contact, and doesn't splinter. The 5/4×4 bench board is smooth on all sides. If you need wider or narrower bench boards than the 5/4×4 offered in thermo-spruce, you can pair thermo-spruce walls with thermo-aspen bench material (available in five sizes) — the golden-brown tones are similar enough to blend well.
Use our Sauna Wood Calculator — enter your room dimensions and it estimates board footage for walls, ceiling, and benches. Wall cladding coverage is 3.1" per 1×4 board (less than the nominal width because the tongue is hidden in the groove).
No — the thermal modification process provides the protection. Thermo-spruce's reduced moisture absorption handles the wet/dry cycles of a sauna without any applied finish. If you want to maintain the fresh look on bench surfaces, a sauna wood oil can be applied. Read more: How to Finish Wood in a Sauna.
Thermo-spruce's rot resistance and dimensional stability make it a viable option for exterior cladding, though our wall cladding profiles are designed primarily for interior use. For outdoor sauna exteriors, many builders choose thermowood for its weather resistance. Contact us for guidance on exterior applications.
Spruce has a more visible, pronounced grain pattern than aspen or alder. You can see the growth ring lines more clearly, and the surface has a slightly more textured feel. It's not rough — it's milled smooth — but it has more visual depth and character than the more uniform surfaces of aspen or alder. Think of it as the difference between a modern planed surface and a traditional wood surface with natural personality.
Shop more: All Sauna Wood · Thermo-Aspen · Thermo-Radiata Pine · Thermo-Alder · Cedar · Aspen · Alder · Hemlock · Trim & Molding · Wood Oil · Wood Calculator
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