This guide shows how to build a high-performance, eco-conscious sauna using the Steico Wood Fibre system and HUUM technology. By integrating Steico Flex and Duo Dry, you create a “Massive-Passive” structure that manages moisture naturally, offers superior acoustic insulation, and retains a deep, “soft” heat through its high thermal mass.
The Integrated Wall Anatomy
To protect the timber and wood fibre, the wall must follow a “vapour-closed inside, vapour-open outside” philosophy.
The Layering (Inside to Outside):
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Interior Cladding (15mm): Cedar, Aspen, or Thermo-Alder.
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Horizontal Air Cavity (22mm): Created with timber battens. This is the “drainage plane” that allows the back of your cladding to dry and prevents rot.
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Vapour Barrier: High-temp aluminium sauna foil (minimum 0.05mm). Crucial: Overlap joins by 100mm and seal with high-temp aluminium tape. This is your primary defence against steam entering the wall core.
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Insulation Core: Steico Flex 036 wood fibre batts friction-fitted between timber studs. Cut them 10mm wider than the stud gap for a seamless thermal seal.
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External Sheathing: Steico Duo Dry (40mm). These tongue-and-groove boards provide structural rigidity and act as a secondary thermal break.
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Rainscreen Gap (20mm): Vertical battens to allow the exterior cladding to breathe.
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Exterior Siding: Your choice of timber cladding or modern metal profile.
Sizing and HUUM Heater Selection
The rule of thumb is 1kW of power per of volume. Steico’s density means your sauna will stay warm far longer than a standard mineral-wool build.
The Glass Penalty: Glass is a significant heat sink. For every of glass (doors or windows), add 1.2 to your volume calculation before choosing your heater.
Core Design Principles
Ventilation: The “Convection Loop”
Proper airflow prevents “stale air” and ensures your Steico insulation stays dry.
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The Inlet (Fresh Air): Positioned directly under or behind the HUUM heater, approx. 100mm from the floor. The rising heat from the stones creates a natural siphon, pulling fresh oxygen into the room.
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The Exhaust (Stale Air): Positioned on the opposite wall, approximately 300mm–500mm below the ceiling. This forces the hot air and steam (löyly) to circulate past the bathers before exiting.
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The Drying Vent: A secondary vent in the ceiling, kept closed during use, but opened fully after the session to vent remaining moisture.
Bench Geometry: The “Law of the Stones”
In a Finnish-style sauna, your feet must be above the “cold zone.”
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The Foot Bench: Should be level with, or slightly higher than, the top of the HUUM heater stones.
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The Upper Bench: Should be 1.1m below the ceiling. This keeps the bathers’ heads in the optimal heat pocket.
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Bench Depth: The top bench should be at least 600mm deep to allow for comfortable reclining.
Why Steico + HUUM is the Gold Standard
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Thermal Mass: Steico Flex is dense (). It stores heat energy, meaning the room recovers its temperature almost instantly after the door is opened.
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Hygroscopic Buffer: If any moisture bypasses the foil, the wood fibre can absorb and release it without losing its R-value, unlike mineral wool which sags when damp.
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Acoustics: Wood fibre is a superior sound absorber. Your sauna will be a silent, meditative vault.
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Soft Steam: HUUM heaters hold a massive stone mass (up to 250kg). This creates a “soft” steam that doesn’t burn the lungs, perfectly complemented by the natural feel of the wood-fibre-lined walls.
Construction Expert Tips
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Cutting: Use a dedicated insulation saw (wavy-edged blade) for the Steico Flex to ensure clean, tight-fitting edges.
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Safety Clearance: HUUM heaters get extremely hot. Ensure you maintain the 120mm–150mm safety clearance from your timber cladding. For smaller saunas, a HUUM Safety Rail is essential.
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Smart Control: Pair your heater with the HUUM UKU Wi-Fi controller. Since the Steico mass takes roughly 45–60 minutes to fully “heat-soak,” being able to trigger the heater from your phone is a game-changer.