Materials

    Sauna Materials & Cladding

    The right timber, insulation, vapour foil, and stones — chosen for UK climates and traditional Finnish löyly performance.

    Sauna Materials & Cladding

    Why Sauna Materials Are Different

    A sauna routinely cycles between 20°C and 100°C, with humidity spiking from 5% to 60%+ when water hits the stones. That punishes any timber with high resin, any insulation with a plastic facer, and any metal fixing that conducts heat into bare skin.

    Choose resin-free softwoods for inside, mineral wool with aluminium foil for the wall build-up, and stainless steel for any fixing the bather might touch. Get those three right and the rest is detail.

    Inner Cladding

    The Wood You Sit Against

    Inside the cabin, only resin-free, low-conductivity softwoods belong. Pine and spruce are forbidden in the bench area — they'll burn skin and weep sticky resin.

    Thermo-Aspen

    £35–£60

    Resin: Very Low · Colour: Dark caramel to near-black

    Aspen thermally modified to ~190°C, removing sugars and sap. Modern favourite for inner cladding and benches — completely resin-free, dimensionally stable, and beautifully dark against the timber tones around it.

    Pros

    • Zero resin — never weeps or pops
    • Cool to touch even at 100°C
    • Stunning dark colour without staining
    • Naturally rot- and insect-resistant

    Cons

    • Premium price
    • Slightly more brittle than untreated aspen — pre-drill all fixings

    Western Red Cedar

    £40–£70

    Resin: Low · Colour: Warm honey, ages to silver

    The aromatic classic. Soft, lightweight, naturally rot-resistant. Releases that unmistakable cedar scent when warmed — many holistic builders won't use anything else.

    Pros

    • Iconic aroma when heated
    • Excellent thermal stability
    • Lightweight, easy to fix
    • Natural resistance to rot and insects

    Cons

    • Soft — dents and scratches easily on benches
    • Imported (carbon footprint)
    • Some find the scent overpowering at first

    Nordic Aspen (untreated)

    £25–£45

    Resin: Very Low · Colour: Pale cream

    The traditional Finnish bench timber. Almost no resin, very low thermal conductivity (stays cool on bare skin), pale and clean-looking.

    Pros

    • Stays cool on skin
    • Very low resin
    • Bright, reflects light beautifully
    • Affordable for what you get

    Cons

    • Pale colour shows water marks
    • Not dimensionally stable in damp conditions

    Abachi (Obeche)

    £30–£55

    Resin: Very Low · Colour: Pale yellow

    African hardwood prized for benches. Extremely low thermal conductivity, splinter-free, and a smooth feel against skin.

    Pros

    • The 'invisible bench' — almost no heat felt on contact
    • Splinter-free even after years of use
    • Low resin

    Cons

    • Imported tropical hardwood — check FSC certification
    • Soft and easily marked
    Wall Build-Up

    Insulation & Vapour Foil

    Inside a sauna, this is the system that keeps heat in and moisture out of the structure.

    Recommended: Mineral Wool / Rockwool

    50–70mm of mineral wool (Rockwool, Knauf Earthwool) is the universal choice. It handles temperatures well above 200°C, doesn't off-gas, and is non-combustible — essential when there's a hot stove a few centimetres away.

    Holistic alternative: sheep's wool insulation works beautifully for breathable builds, especially if you're using thicker thermo-treated cladding. It manages moisture vapour without losing performance.

    Never Use Inside a Sauna

    • PIR / Celotex / Kingspan: the foam degrades and the foil facing can off-gas above 80°C
    • Spray foam: off-gases at sauna temperatures
    • Polythene vapour barriers: melt or warp — only aluminium foil belongs here
    • Pine, spruce, fir cladding inside the cabin: high-resin softwoods that pop and weep

    Aluminium Vapour Foil — The Critical Layer

    A continuous aluminium foil layer on the warm side of the insulation reflects radiant heat back into the cabin AND blocks moisture vapour from migrating into the wall build-up. Skip it and you'll be replacing rotten studs in 5 years.

    • Use sauna-grade foil (Thermo-Hanf SaunaFolie, IsoFleece, or branded sauna foil — not loft foil)
    • Tape every joint, overlap, and penetration with aluminium foil tape
    • Leave a 20–25mm batten gap between foil and inner cladding for drying air
    • Continue the foil onto the ceiling — no gaps at the wall/ceiling junction
    Outer Cladding

    The Face Your Sauna Shows

    Outside, you have more freedom. Choose for aesthetic, weather resistance, and how much patina you want.

    UK Larch (outer)

    £15–£30

    Colour: Honey, silvers in 1–2 years

    The go-to outer cladding for UK garden saunas. Naturally durable Class 3, no treatment needed, supports British forestry.

    Western Red Cedar (outer)

    £35–£60

    Colour: Warm brown to silver

    Premium outer cladding — exceptionally durable, lightweight, and dimensionally stable.

    Shou Sugi Ban (Yakisugi)

    £40–£70

    Colour: Charcoal black

    Japanese charred timber. The carbon layer protects against rot, insects, fire, and UV — essentially maintenance-free for 50+ years.

    Stones

    Sauna Stones (Kiuaskivet)

    The hidden hero of every sauna. Wrong stones crack, dust, or fail to hold steam. Right stones turn a hot box into a transcendent löyly.

    The traditional Finnish stones are peridotite and olivine diabase — dense, igneous rocks that store enormous heat and release it as silky, soft steam. Vulcanite (volcanic basalt) is the modern alternative.

    Avoid: river stones (can explode from trapped moisture), limestone or sandstone (crumble), and unbranded "garden centre" sauna stones (often the wrong rock entirely).

    Quantity: follow the heater manufacturer's minimum, then add 15–25kg more for richer steam. A 6kW heater typically takes 20–30kg of stones.

    Stacking: largest at the bottom, smallest at the top. Loose, never packed — heat and steam need to circulate. Re-stack and rinse stones once a year.

    You might also like