The facade has to withstand sun, rain, frost, temperature changes, UV radiation and constant exposure to the site. At the same time, it must retain architectural character and the feeling of a natural material. Inside, the sauna works with temperatures up to 90 °C and steam; outside, with continental or Mediterranean climate extremes. A material that connects these two worlds is not decoration — it is a technical decision.
The classic solutions are two compromises: chemical impregnation, which requires sanding and coating every two years, or tropical hardwood (ipe, teak) with a high ecological, ethical and regulatory burden.
The third route — thermal modification of Nordic wood — uses heat and steam as the only “treatment”. The wood changes chemically at the level of its cellular structure, without chemicals and without foreign substances. After treatment, the material lasts for decades outdoors without coatings or chemical protection.
Lunawood is a Finnish thermowood manufacturer that has standardised this process. In KUBIQ projects, Lunawood carries the exterior cladding role — allowing the sauna to remain natural, warm and architecturally clean on the outside, with far more predictable behaviour than ordinary wood.
This guide explains what Lunawood is, how it is made, which classes exist, how it ages, what not to expect, and why it is used in outdoor wellness architecture.
Step 01What Lunawood is
Lunawood is a Finnish manufacturer of thermally modified Nordic wood, founded in 2000 and one of the founders of the International ThermoWood Association. Production is based in Iisalmi in central Finland, where the wood is sourced geographically from PEFC-certified sustainably managed Nordic forests.
Lunawood Thermowood is produced through a process that uses heat and water vapour, without adding chemicals to the wood. This means the properties of the wood are not changed by a surface coating, but through the structure of the material itself.
In other words: Lunawood is not ordinary wood that has been “coated” on the outside. It is wood that has undergone a controlled thermal process and therefore behaves differently from untreated wood.
What sets Lunawood apart from other thermo manufacturers:
- Continuous kiln system — production in a continuous flow, not in batch loads. Result: more even treatment depth, more consistent colour and predictable mechanical properties of every board.
- BRE-accredited 30-year service life for exterior facades (Building Research Establishment, UK).
- Durability class 2 according to BS EN 350 — the second highest resistance class, without chemical additives.
- 20-year manufacturer warranty for projects in the EU.
- PEFC certificate and EPD (Environmental Product Declaration) documenting the carbon footprint across the full life cycle.
Step 02How thermowood is made
Thermal modification (Finnish *lämpökäsittely*) is a process in which wood is exposed to high temperature in a controlled water-vapour atmosphere — without oxygen and without chemical additives.
Water vapour has a triple role: it prevents the wood from igniting at extreme temperatures, controls the drying process and acts as a medium that prevents structural cracking.
Standard process:
- Drying (50–100 °C) — controlled removal of moisture from the wood
- Thermal modification (185–215 °C) — the wood is heated in a steam atmosphere where the actual chemical transformation takes place
- Cooling and conditioning — gradual lowering of temperature and equalisation of moisture
The full cycle lasts 36 to 96 hours, depending on the wood thickness and target treatment class.
### What happens to wood at the molecular level
At 185–215 °C, wood undergoes several permanent changes:
- Hemicellulose breaks down. Hemicellulose is the wood component that absorbs the most moisture. By removing it, wood permanently loses hydrophilicity — its ability to absorb moisture is significantly reduced, and with it the swelling and shrinkage of boards.
- Resin leaves the wood. Nordic pine and spruce naturally contain resin that runs outdoors and leaves marks. Thermal modification eliminates resin — the facade never shows resin stains in summer heat.
- Sugars caramelise. The wood gains its characteristic honey-brown to dark-brown colour through the full thickness, not only on the surface.
- Biological nutritive value disappears. Fungi and insects lose interest — the wood no longer contains the nutritional components that attract them.
### Difference between chemical impregnation and thermal modification
It is the difference between protection over the wood and changing the wood itself.
Step 03Lunawood classes: Thermo-S and Thermo-D
Thermal modification is carried out in two basic levels, depending on the final application:
### Thermo-S (Stability)
- Treatment at 190 °C
- Emphasis on stability while preserving mechanical strength
- Colour: honey-brown, medium intensity
- Application: interior, furniture, semi-protected exterior
- Durability class 3 according to BS EN 350
### Thermo-D (Durability)
- Treatment at 212 °C
- Emphasis on durability, maximum dimensional stability
- Colour: dark brown, consistent through the full thickness
- Application: facades, terraces, exterior cladding exposed to full weather conditions
- Durability class 2 according to BS EN 350 — the second highest resistance class
KUBIQ engineering standard: for all exterior facades and exterior cladding on wellness spaces, Thermo-D class is used. This enables integration of hidden fixing clips without visible screws, with confidence that the panels will not twist, deform or crack under atmospheric exposure.
### Wood species in the Lunawood line
Lunawood produces thermally modified products from two Nordic species:
- Nordic pine — the traditional base for facades and terraces. Density approximately 420 kg/m³ at 6–8 % moisture content. Characteristic visible grain.
- Nordic spruce — finer structure, less visible grain, smoother visual impression. More often used in design-led 3D profiles such as Luna Triple.
Step 04Luna Triple 32×140 — the KUBIQ choice
In the KUBIQ Eclipse line and studio projects, Luna Triple 32×140 mm is used as standard. It is an award-winning 3D profile made from Nordic spruce, with two deep grooves that simulate the appearance of fine wooden lamellae, but in the form of a solid tongue-and-groove panel.
Why Luna Triple:
- Deep grooves create rhythm and shadow — the facade is not flat and ordinary; it has a vertical structure that changes throughout the day
- Hidden joints (hidden clip fastening) — no visible screws, cleaner architectural form
- Winner of the Architecture MasterPrize 2021 in the sustainable products category
- Compatible with a prefabricated panel — the KUBIQ 162 mm wall system (Lunawood 32 mm exterior + 75 mm stone wool insulation + vapour barrier + 15 mm thermo-alder interior)
In the KUBIQ visual language, this matters because an outdoor sauna should not look like a garden shed. It should look like a designed wellness space.
The alternative — Luna Arctic Triple 32×140 Brushed — is used for projects that require an instant grey tone from the day of installation, without passing through the honey-coloured phase. Brushed surface treatment plus factory silver-grey coating.
Step 05How Lunawood ages
Lunawood is natural wood. That means it ages.
But the point is not that wood never changes colour. The point is that the change happens predictably and in an aesthetically acceptable way. Under UV radiation and rain, the surface lignin in the wood goes through natural oxidation and changes colour through recognisable phases.
### Ageing phases
New (0–3 months). Warm honey-brown to darker brown shade, depending on wood species and treatment class. A gentle satin sheen that disappears within the first few months.
3–6 months. Slight lightening, the surface becomes more uniform. The beginning of the patination process.
6–12 months. Natural greying becomes visible, especially on the southern and western sides of the facade.
1–2 years. A uniform silver-grey patina begins to dominate. Subtle tonal differences may be visible between exposed and sheltered sides of the object.
2+ years. Stable patina — a lasting and aesthetically pleasing appearance. The patina is superficial (less than 0.1 mm); the wood underneath remains structurally healthy and resistant.
The speed of patination depends on orientation, climate and the site's microclimate:
- Southern and western facades — grey fastest (full UV exposure)
- Northern facades — slower, can retain the honey tone longer
- Under a canopy or overhang — slowest, can remain brown for years
This creates subtly different tones on the same object, which is an aesthetic part of the wood's character — not a defect. In premium outdoor wellness architecture, natural ageing is often part of the object's character.
Step 06If you do not want natural greying
Three strategies for managing the aesthetic:
Accept the natural patina (wabi-sabi approach). The most common choice in premium outdoor wellness. The wood ages organically, requires minimal maintenance (washing with water 1× per year), and perfectly communicates the philosophy of architecture coexisting with nature.
Retaining the honey colour. If you want to permanently retain the warm initial shade, the facade should be treated immediately after installation with pigmented UV-protective oils (e.g. *SiOO:X* system). The process requires periodic renewal every few years.
Instant grey architecture (Luna Arctic line). For projects that want a stable grey tone from day one, without transitional brown phases. Factory-treated profiles with a silver-grey coating that gradually washes out over the years while the natural greying of the wood develops underneath.
The lowest maintenance means letting the wood patinate naturally. The greatest control over colour means regularly maintaining the surface with an appropriate coating. There is no universally better option. It depends on whether the user wants natural patina or retention of a specific tone.
Step 07Luna Arctic — grey architecture from day one
The natural greying described in the previous step takes time. Southern facades take months, northern ones even years. For projects that want a grey tone immediately — without passing through honey and transitional brown phases — there is Luna Arctic.
Arctic is not a separate material. It is a Lunawood variant with the same thermal base (Thermo-D Nordic pine or spruce), the same durability and the same maintenance profile. The difference is in the surface treatment: a factory silver-grey coating plus a brushed texture that emphasises the grain.
For this line, KUBIQ uses the Luna Arctic Triple 32×140 Brushed profile — the same format as the classic Luna Triple described in Step 04, only in grey.
How Arctic behaves over time. The factory grey coating is not a permanent “varnish” that has to be renewed. Over the years, it gradually washes out, while the natural grey of the wood itself develops underneath. The result: the transition from factory grey to natural grey is continuous and almost invisible — there is no moment when the facade “loses” colour and waits to grey again.
When Arctic makes sense:
- The project requires a stable, architectural grey tone from installation day.
- The investor does not want to watch the transition through honey and brown phases.
- Grey needs to fit into an existing grey palette on site (stone, concrete, metal).
When it does not make sense: if the warm, honey-toned initial character of natural wood is desired — then the classic Luna Triple is the route, with or without coating to retain colour.
Step 08Lunawood as a base for other treatments
The thermal stability of Lunawood makes it a preferred base for further surface treatments:
- Shou Sugi Ban (charred wood) — carbonised surface on a thermally modified base. Covered in the Shou Sugi Ban guide [/vodic/shou-sugi-ban]. KUBIQ Eclipse Charred Black uses this combination.
- UV-protective coatings (SiOO:X and other variants) — extend the honey tone without compromising dimensional stability.
- Brushed treatment — mechanical, emphasises the grain and gives a more tactile feel. Standard for premium Lunawood lines.
All these variants upgrade Lunawood; they do not change its character. The base remains thermally modified Nordic pine or spruce.
Step 09What Lunawood is NOT
To keep communication realistic, it is important to clearly state what Lunawood is not:
- It is not a material that never changes. It ages — just differently and more predictably than ordinary wood.
- It is not immune to micro-cracking. Over years of exposure to extremes, fine microscopic longitudinal cracks will appear on the surface. This is a normal physical reaction of wood, not a sign of decay. Lunawood explicitly mentions it as part of the material's character.
- It is not completely waterproof. Wood still absorbs water — less than untreated wood, but not zero. That is why the prefabricated 162 mm wall system requires a vapour-open, waterproof membrane behind the ventilation layer.
- It is not a load-bearing structural element. Thermal treatment at 212 °C reduces the elasticity and bending strength of wood. The wood becomes more brittle. That is why Lunawood is used exclusively as premium exterior and interior cladding, while the load-bearing structure is made from high-strength KVH/BSH beams.
- It is not universally better than every wood. Tropical woods (ipe, kumaru) have higher density and longer life without treatment, but they are expensive and regulatorily problematic. Cedar (Western Red) has a different character and natural resistance, but lower dimensional stability. Lunawood is a premium choice for a Nordic profile and predictability — not universally “the best”.
- It is not a substitute for correct facade ventilation. Even the best material can perform poorly without proper ventilation, drainage, spacing and joint execution.
Step 10Lunawood vs. alternatives
A quick orientation when considering different facade materials for outdoor wellness:
Ordinary wood. More affordable, but requires more maintenance — sanding, coating, periodic renewal. Low dimensional stability.
Chemically impregnated wood. Standard solution for rustic applications. Not acceptable from a safety perspective for saunas (toxic emissions when heated). Aesthetic compromise.
Cedar (Western Red). Naturally resistant, pleasant smell, classic character. Lower dimensional stability, more expensive import, requires periodic oil treatment.
Ipe / Kumaru / tropical hardwoods. Maximum density, 30+ years without treatment. High price, regulatory issues, ethical concerns. Use is declining.
WPC / composite. Maintenance-free, predictable appearance. Visually dead, does not age, does not breathe. Large thermal expansion in an outdoor sauna context.
Compact facade panels (HPL). Stable modern appearance. They do not have the natural character of wood — a different visual language.
Lunawood Thermo-D. Chemical-free, dimensionally stable, predictable 25–30 year durability without treatment, BRE accreditation, EU origin, PEFC certification. Premium position with authentic wooden character.
The choice depends on the project, location, budget and aesthetic preference. For KUBIQ outdoor wellness architecture, Lunawood is chosen as standard because it provides predictable behaviour and authentic wood character without compromising long-term durability.
Step 11Common mistakes in understanding thermowood
Thinking that thermowood never ages. It ages — just differently and more predictably. Colour change and gradual patination are a natural part of the life cycle.
Thinking natural patina means decay. In a correctly executed facade, greying is a surface-level visual change — the wood underneath remains structurally healthy and fully resistant.
Using the wrong type of wood in the wrong place. Wood for a facade, wood for a sauna bench and wood for a load-bearing structure do not have the same requirements. Lunawood for the facade, thermo-alder for the sauna interior, KVH/BSH for the load-bearing structure. Different roles, different materials.
Ignoring installation details. Even the best material can perform poorly without proper ventilation, vapour-permeable membranes, expansion gaps and joint execution. Material is part of a system, not a solution on its own.
Site preparation [/vodic/priprema-terena] covers the technical details related to installation.