Thermally modified sauna timber: what it is, benefits and trade-offs

The short answer

• Thermo timber is ordinary timber changed by heat. Kiln-dried boards are heated to about 180–220 °C in a low-oxygen atmosphere. No chemicals are added.
• It is more stable, resin-free and darker than the same species untreated, and passes heat to the skin a little more slowly.
• It is more brittle, has a faint smoky scent that never fully disappears, and is not termite-resistant.
• It is widely used for sauna benches in Europe, especially thermo-aspen and thermo-alder.
• It is not our standard material. Most of the saunas we build are lined in western red cedar, and we sometimes use Canadian hemlock. We cover thermo timber because you may encounter it.

For how it compares with the other sauna timbers, see our guide to choosing sauna timber.

How it is made

The best-documented process is ThermoWood, developed in Finland. The Finnish ThermoWood Association describes three phases:

1. High-temperature drying to close to 0% moisture content.
2. Heat treatment at 185–215 °C for two to three hours, under protective steam.
3. Cooling and conditioning back to a usable moisture content of about 4–7%.

Heat changes the wood’s chemistry permanently. Because the change runs through the whole board, the colour is the same inside as on the surface: Lunawood notes that dents and sanding do not reveal a paler layer underneath.

Treatment classes

Producers sell two strengths of treatment, under different names and at slightly different temperatures:

ThermoWood’s hardwood classes cover birch and aspen, and Thermory sells thermo-aspen and thermo-alder in its own grades. Ask which class you are being offered, because it affects colour, stability and strength.

Species you will see

• Thermo-aspen: golden to mid-brown. A common bench timber in European saunas, valued for its stability and mark-hiding colour.
• Thermo-alder: hazel to deep brown, used for benches and walls.
• Thermo-spruce and thermo-pine: walls, ceilings and battens. Untreated pine is avoided in hot rooms because of resin; thermal modification removes it.
• Thermo-radiata: Thermory sells intensely modified radiata pine for walls and 26 × 140 mm bench boards. Australian stock is unconfirmed.
• Thermo “magnolia”: Thermory describes its magnolia as the tulip tree, that is yellow-poplar (Liriodendron tulipifera), and sells it for benches and interiors.

Australian sauna timber suppliers list thermo-aspen, thermo-alder and thermo-spruce.

What changes, and what it means in a sauna

Stability

Thermo timber takes up much less moisture. Its equilibrium moisture content is roughly half that of untreated wood after treatment at around 220 °C, and swelling and shrinkage fall by 40–80% depending on species. ThermoWood also reports no drying stress in the finished boards. In a room that dries out hard every session and takes up moisture again between them, that is the material’s main advantage.

No resin

Resin flows out during treatment, so thermo timber does not bleed at sauna temperatures, even in species such as pine that are resinous untreated. The side effect is that large knots can loosen, because the resin that held them in has gone.

Heat feel

The ThermoWood Handbook reports thermal conductivity 20–25% lower than untreated softwood: 0.099 W/m·K for Thermo-D in a VTT test, against 0.12 for untreated timber under the Finnish code. Thermory claims up to 30%; we use the trade association’s figure. Density also falls as treatment temperature rises. A Czech supplier lists Finnish thermo-aspen at about 420 kg/m³, against about 450 kg/m³ for natural European aspen; that is a supplier figure, and no comparable figure was found for thermo-alder.

Colour

Treatment darkens the timber throughout, which hides sweat and water marks better than pale natural timber. Oiling darkens it further. Outdoors, thermo timber greys under UV; Lunawood says silvering shows within three to six months. Lunawood also notes that some pigment may come out of new boards in humid spaces such as saunas, which is worth knowing if the towels are pale.

Decay and insects

Decay resistance improves. ThermoWood rates Thermo-S at European durability class 3 and Thermo-D at class 2, and Lunawood quotes EN 350 class 2 for LunaThermo-D. These are European classes and do not translate to the Australian 1–4 scale. A French test reported in the ThermoWood Handbook found resistance to longhorn beetle, common furniture beetle and lyctid borers.

Termites are a different matter. The ThermoWood Handbook says test results show the material “is unable to resist attack from termites”, Lunawood says its timber is not termite-resistant, and Virginia Tech’s research centre reports no termite resistance gain. A Thermory-commissioned test found reduced feeding, not prevention. Never treat thermo timber as termite-resistant, particularly on an outdoor sauna in Queensland.

The trade-offs

Brittleness

Splitting strength falls by 30–40% in ThermoWood tests, bending strength drops at the Thermo-D level, and the timber is more prone to cracking under sudden loads. Virginia Tech lists it for non-structural uses only. For benches that means:

• adequate board thickness and supported spans;
• rounded edges;
• sealed end grain, because ThermoWood warns that fast wetting and drying can split bench ends;
• frames in another timber, with Finnish bench maker Pieksäwood suggesting spruce, pine or thermo-aspen.

Scent

Thermo timber smells faintly smoky, probably from furfurals produced in treatment. Measured emissions are only a fraction of pine’s, but the scent is noticeable. Thermory says it weakens over time but never fully disappears, and that heating with good airing, and oiling, help it fade faster. Washing untreated thermo timber with water strengthens the scent.

Workshop dust

Thermo timber dust is finer than ordinary softwood dust and may trouble people with asthma during machining. Lunawood advises machining it like a hardwood and taking care with feed rollers to avoid cracking. Like cedar dust, this is a fabrication issue, not a concern for bathers.

Care and finishes

• No paint or varnish on hot-room surfaces. Thermory says not to use them, because the wood needs to breathe; Harvia rules out lacquer inside the sauna.
• Seal bench ends. The ThermoWood Handbook allows oil, wax or varnish on ends; oil or wax is the option consistent with every manufacturer’s guidance.
• Bench oil suits thermo timber. Harvia says its paraffin oil especially suits heat-treated wood. Tikkurila’s sauna wax has a tinted version for thermally modified timber, but it should not go over oiled surfaces. Choose one system per surface.
• No pressure washer or running water, towels on benches, and water only on the heater, as Thermory advises.

Sources disagree on whether thermo timber needs any treatment at all; Finnish builder’s guide Rakentaja.fi says it does not, while Thermory recommends treating at least yearly. Follow the supplier’s instructions for the specific product. Our care guide covers routines in general.

Thermo timber compared with natural timber

Western red cedar already offers much of what thermal modification adds: very low movement, no resin and low density. Thermo timber’s advantages are greatest over species that start less stable or more resinous, such as aspen, alder, spruce and pine.

Sourcing

Lunawood says its timber is PEFC-certified wood from Northern Finland, and Thermory offers PEFC, FSC or Nordic Swan certified wood on request. Those statements describe the producers’ supply, not a particular Australian consignment, so ask the importer for chain-of-custody documents for the specific order. The ThermoWood Handbook calls thermo timber hygienic for sauna benches, but we found no test data behind that, so treat it as a claim.