Lunawood thermowood

The facade has to withstand sun, rain, frost, temperature changes, UV radiation and continuous site exposure while retaining architectural character and the feel of a natural material. Interior and exterior layers work under different conditions, so material selection is a technical decision rather than decoration.

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 without adding chemical preservatives to the timber. Performance and permitted exterior use depend on the declared class, profile, installation detail and manufacturer conditions.

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:

• Controlled factory process — performance is assessed from the current declaration for the actual product, not from an assumption that every board is identical.

• 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.

• Manufacturer warranty of up to 20 years for accepted and registered projects that meet Lunawood conditions — not automatic for every installation.

• 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 project direction: Thermo-D and Lunawood are frequent starting points for timber exteriors, but they are not mandatory on every project. Profile, fixing and ventilated-façade details are confirmed for the site and current product documentation.

Wood species in the Lunawood line

Lunawood produces thermally modified products from two Nordic species:

• Nordic pine — a traditional base for facade and terrace profiles, with characteristic visible grain. Exact properties are checked in the selected product data sheet.

• 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

Luna Triple 32×140 mm is one confirmed KUBIQ façade profile, not the universal cladding for every project. It is a Nordic-spruce 3D profile with two deep grooves and LunaThermo-D classification.

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

• Coordinated with the actual wall build-up — KUBIQ has no single universal panel; cladding, substructure, ventilation layer, membranes and insulation are confirmed in the project specification

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

The natural tone changes with UV, precipitation, orientation and shade. It does not follow a universal calendar and may differ between exposed and sheltered surfaces; the expected appearance and any coating strategy are agreed for the actual site.

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 finish and natural patina change with exposure and maintenance. Current product documentation defines the expected colour change and renewal method; we do not promise an invisible transition on every facade.

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].

• 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. Each wall build-up therefore needs designed protection, ventilation and drainage layers for the actual façade and site.

• 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. Thermally modified without added chemicals, dimensionally stable and supported by manufacturer documentation, PEFC data and a BRE assessment for defined uses. Actual service life depends on profile, installation, exposure and maintenance.

The choice depends on the project, site, investment framework and desired ageing. Lunawood is a frequent KUBIQ starting point for timber exteriors, but remains a project option rather than a mandatory standard.

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.