Abachi (Triplochiton scleroxylon) is the international specification standard for sauna interior timber, chosen for its exceptionally low thermal conductivity that keeps surfaces comfortable to skin even at peak operating temperatures of 90–110°C. This guide covers species selection, profile options, and installation best practice for architects designing residential and commercial sauna environments.
For structural timber specification in the broader context of timber construction, see our Structural Timber Specification Guide.
Why Abachi Dominates Sauna Specification
The physics of sauna comfort comes down to thermal conductivity — the rate at which heat transfers from the timber surface to human skin. Abachi's thermal conductivity (approximately 0.10 W/mK) is among the lowest of any commercially available timber, meaning the bench surface remains comfortable to bare skin even when the surrounding air temperature reaches 90–110°C.
Timber Species Comparison for Sauna Use
| Species | Thermal Conductivity (W/mK) | Density (kg/m³) | Skin Comfort at 90°C | Recommended Use |
|---|---|---|---|---|
| Abachi (Obeche) | 0.10 | 350–420 | Excellent | Benches, all skin-contact surfaces |
| Aspen (Populus tremula) | 0.12 | 400–500 | Good | Benches (budget alternative) |
| Alder (Alnus glutinosa) | 0.14 | 420–530 | Moderate | Wall cladding, non-contact surfaces |
| Western Red Cedar | 0.11 | 330–380 | Good | Wall cladding, aromatic applications |
| Spruce (Picea abies) | 0.13 | 400–500 | Moderate | Budget cladding (resin risk) |
| Thermo Pine | 0.12 | 380–450 | Good | Outdoor sauna cladding |
Abachi's combination of low thermal conductivity, absence of resin, knot-free grain, and splinter-free finish makes it the only timber recommended by major sauna manufacturers (Harvia, Tylö, KLAFS) for direct-skin-contact surfaces.
Available Profiles and Specifications
Modern sauna interiors use a system of complementary profiles that allow architects to create visual hierarchy and functional differentiation across the interior:
Bench Boards
Bench boards are the primary skin-contact surface and must meet the highest specification:
| Dimension | Thickness | Length | Grade Options |
|---|---|---|---|
| 80mm width | 22mm | 2.10 / 2.40 m | Class A · Thermo |
| 85mm width | 22mm | 2.10 / 2.40 m | Class A · Thermo |
| 95mm width | 22mm | 2.10 / 2.40 m | Class A · Thermo |
| 80mm width | 28mm | 2.10 / 2.40 m | Class A · Thermo |
| 85mm width | 28mm | 2.10 / 2.40 m | Class A · Thermo |
| 95mm width | 28mm | 2.10 / 2.40 m | Class A · Thermo |
| 125mm width | 28mm | 2.10 / 2.40 m | Class A · Thermo |
The 22mm thickness is standard for residential saunas. The 28mm thickness is specified for commercial installations, wider bench spans (>600mm between supports), or where a more substantial visual presence is desired.
Wall and Ceiling Cladding — Soft Line Profile
The Soft Line profile features radiused face edges for a tactile, modern aesthetic:
| Dimension | Covering Width | Length | Grade Options |
|---|---|---|---|
| 97 × 16mm | 89mm | 2.10 / 2.40 m | Class A · Thermo |
| 125 × 16mm | 117.77mm | 2.10 / 2.40 m | Class A · Thermo |
The narrower 89mm covering width creates a finer linear rhythm suited to compact saunas and ceilings. The wider 117.77mm covering width suits larger interiors and feature walls.
Architectural Lamellas
For feature walls and accent compositions:
| Profile | Covering Width | Thickness | Grade |
|---|---|---|---|
| Standard lamella | 86mm | 21mm or 26mm | Thermo only |
| Decorative lamella | 91mm | 16mm | Thermo only |
Standard lamellas form primary wall surfaces; decorative lamellas mark feature walls and transition zones. Combining both profiles within a single project creates subtle visual hierarchy.
Class A vs Thermo-Modified: When to Specify Each
| Criterion | Class A | Thermo-Modified |
|---|---|---|
| Colour | Pale ivory, brightens interior | Rich caramel, warm architectural tone |
| Dimensional stability | Good | Excellent (≈50% improvement) |
| Rot resistance | Standard | Enhanced |
| Best for | Dry saunas, bright contemporary aesthetic | Steam rooms, outdoor saunas, dark aesthetic |
| Humidity tolerance | Standard (dry sauna only) | High (steam, outdoor, high-humidity) |
| Cost | Baseline | +15–25% premium |
| Maintenance | None required | None required |
Specification rule of thumb: Use Class A for traditional dry Finnish saunas where a bright, clean aesthetic is desired. Use thermo-modified for any environment with sustained high humidity (steam rooms, bio saunas, outdoor installations) or where the architectural brief calls for a warm, dark material palette.
Installation Best Practice
Wall and Ceiling Cladding
- Vapour barrier: Install aluminium-faced vapour barrier on the warm side of insulation
- Battens: Fix 22×50mm or 25×50mm timber battens at 600mm centres over the vapour barrier
- Ventilation gap: The batten depth creates the required 20mm minimum ventilation gap
- Cladding direction: Vertical installation is traditional; horizontal is contemporary. Both are structurally acceptable
- Fixing: Concealed nailing through the tongue using stainless steel or coated nails
- Expansion gaps: Leave 5–8mm gap at all end walls, corners, and ceiling/floor junctions
- Around heater: Maintain minimum clearances specified by heater manufacturer (typically 500mm)
Bench Construction
- Frame: Construct bench frame from spruce or pine (not visible, lower cost)
- Support spacing: Maximum 600mm between bench board supports for 22mm boards; 800mm for 28mm boards
- Board spacing: Leave 10–15mm gap between bench boards for air circulation and drainage
- Fixing: Screw from below (concealed) using stainless steel screws, or use proprietary bench clips
- Edge treatment: All exposed edges should be rounded (minimum 3mm radius) to prevent splinters
- Removable sections: Design bench sections to be removable for cleaning access
Environmental Conditions
- Operating temperature: 60–110°C (traditional Finnish sauna)
- Relative humidity: 10–30% (dry sauna), 40–100% (steam/bio sauna)
- Heat-cool cycles: 1–3 per day (residential), 5–15 per day (commercial)
- Ventilation: Minimum 6 air changes per hour during operation
Specification Checklist for Architects
When specifying sauna interior timber, include:
- Species: Abachi (Triplochiton scleroxylon)
- Grade: Class A or Thermo-modified (specify per surface)
- Profile: Bench board / Soft Line / Standard lamella / Decorative lamella
- Dimensions: Width × thickness × length
- Moisture content at delivery: 8–12%
- Surface finish: Splinter-free, sanded to minimum 120 grit
- Certification: FSC chain-of-custody
- Quantity: Calculate from interior surface area + 10% waste allowance
- Delivery: Pallet transport, film-wrapped, segregated by grade
Related Products and Resources
- Abachi Sauna Bench Boards — full specification and quote
- Abachi Sauna Cladding — Soft Line — wall and ceiling profiles
- Thermo-Modified Abachi Lamellas — architectural paneling
- Decorative Sauna Lamellas — feature wall profiles
- Structural Timber Specification Guide — broader timber specification context