- ThermoWood is timber heat-treated at 190–215°C using steam — no chemicals. The process permanently improves durability, dimensional stability, and biological resistance.
- Thermo-D (212°C) — the exterior grade. Durability Class 2. 30-year BRE endorsed service life for cladding and decking. Interior and exterior use.
- Thermo-S (190°C) — the interior grade. Improved stability only. Interior applications only. Not suitable for exterior cladding.
- Thermo Pine (Pinus sylvestris) — denser, warmer honey-brown, standard exterior cladding. Thermo Spruce (Picea abies) — lighter colour, popular for sauna interiors.
- All ThermoWood cladding supplied by Timber Cladding Specialists is Thermo-D grade — the exterior specification.
- Equilibrium moisture content reduced from 12% to 4–6% — dimensional movement reduced by approximately 60% versus standard softwood.
ThermoWood is one of the most widely specified exterior cladding materials in the UK — but it is also one of the most misunderstood. Buyers frequently encounter references to Thermo-D, Thermo-S, Thermo Pine, and Thermo Spruce without a clear explanation of what these terms mean or how they affect what they should be ordering. This guide explains the ThermoWood classification system from first principles — what the treatment classes mean, which species are used and why, and what the technical data actually tells you about performance in UK exterior conditions.
What Is ThermoWood?
ThermoWood is timber that has been thermally modified using heat and steam in a controlled kiln environment, at temperatures between 180°C and 215°C, without any chemical additives. The high temperature permanently alters the cellular structure of the wood — reducing its equilibrium moisture content, improving dimensional stability, and upgrading its biological durability class. ThermoWood is a registered trademark of the International ThermoWood Association (ITWA) and may only be produced by licensed member companies. Products produced outside this licensing framework are thermally modified timber but are not entitled to use the ThermoWood name.
The thermal modification process was developed by VTT Technical Research Centre of Finland and has been refined commercially since the 1990s. The key insight behind thermal modification is that most of the properties that make softwood unsuitable for long-term exterior use — high moisture absorption, susceptibility to fungal decay, dimensional instability — are rooted in the wood's organic chemistry. By heating the timber to above 180°C in a steam atmosphere, these compounds are broken down permanently, transforming a standard softwood into a material with performance characteristics similar to naturally durable hardwoods.
We supply Lunawood ThermoWood — produced by Lunawood Oy in Finland, the world's largest thermally modified timber manufacturer. All Lunawood ThermoWood is certified to the SFS 5765 ThermoWood standard and carries PEFC and FSC certification. Browse our full ThermoWood cladding range.
Thermo-D vs Thermo-S — The Two Treatment Classes
All ThermoWood is produced in two standard treatment classes distinguished by the heat treatment temperature applied. Understanding which class you are ordering — and which class is appropriate for your application — is the most important specification decision in the ThermoWood range.
- Treated at 212°C
- Durability Class 2 — Durable (BS EN 350)
- Use Class 3 — above ground, weather exposed
- 30-year BRE endorsed service life
- Interior and exterior use
- Sauna, cladding, decking, facades
- Richer, darker brown tone
- No chemical preservative required
- Treated at 190°C
- Improved stability — not full durability
- Use Class 1–2 — interior/sheltered
- Not BRE endorsed for exterior use
- Interior applications only
- Flooring, panelling, furniture
- Light, attractive golden-brown tone
- Lower cost than Thermo-D
Thermo-S should never be used for exterior cladding or decking. The lower treatment temperature does not achieve the biological durability required for above-ground exterior exposure in UK weather conditions. Always confirm that any ThermoWood ordered for exterior use is Thermo-D grade. All ThermoWood cladding and decking supplied by Timber Cladding Specialists is Thermo-D as standard.
Thermo Pine vs Thermo Spruce — Species Comparison
ThermoWood is produced from two primary Scandinavian softwood species — Scots Pine (Pinus sylvestris) and Norway Spruce (Picea abies). Both can be treated to either Thermo-D or Thermo-S grade, and both are available in the full range of cladding profiles. The choice between them affects appearance, density, and the most appropriate application.
| Property | Thermo Pine (Pinus sylvestris) | Thermo Spruce (Picea abies) |
|---|---|---|
| Colour after treatment | Warm honey-brown — deeper tone | Lighter golden-brown |
| Density (Thermo-D) | 362–404 kg/m³ | Slightly lower |
| Grain character | Pronounced — visible knots, warm character | Finer, more consistent grain |
| Best exterior use | Cladding, decking, facades, garden rooms | Cladding — suits lighter aesthetic |
| Best interior use | Sauna — warm tone suits sauna aesthetic | Sauna — paler, cooler appearance |
| Most common UK use | Exterior cladding and decking | Interior sauna panels and T&G |
For exterior cladding in the UK, Thermo Pine is the most widely specified species — its denser grain, warmer tone, and slightly better durability characteristics make it the preferred choice for architects and specifiers. Thermo Spruce is more commonly used for sauna interior linings and interior panelling where a lighter, more neutral colour palette is wanted. Both species are available in our ThermoWood range across all standard profiles including ThermoWood tongue and groove for sauna interiors and ThermoWood shiplap for traditional residential projects.
Technical Performance Data — Thermo Pine Thermo-D
The following data is drawn from our ThermoWood Pine Thermo-D Technical Data Sheet, tested to European and DIN standards. Download the full data sheet below for use in building control submissions and NBS specifications.
Performance vs Standard Softwood
| Property | Standard softwood (pine KD) | ThermoWood Pine Thermo-D | Standard |
|---|---|---|---|
| Expected service life (cladding) | 10–15 years | 30 years (BRE endorsed) | |
| Durability class | Class 4–5 (Not durable) | Class 2 (Durable) | BS EN 350 |
| Use class | Class 1–2 | Class 3 — weather exposed | BS EN 335 |
| Chemical preservative required | Yes | No | |
| Equilibrium moisture content | 11–12% | 4–6% | EN 13183-1 |
| Dimensional movement | High | Reduced by ~60% | |
| Density | 434–470 kg/m³ | 362–404 kg/m³ (lighter) | DIN 52182 |
Dimensional Stability — Swelling and Shrinkage
| Property | Pine Kiln-Dried | ThermoWood Pine Thermo-D | Standard |
|---|---|---|---|
| Max swelling — tangential | 8.60% | 3.22% | DIN 52184 |
| Max swelling — radial | 4.42% | 1.50% | DIN 52184 |
| Max shrinkage — tangential | 7.26% | 3.62% | DIN 52184 |
| Max shrinkage — radial | 4.00% | 1.79% | DIN 52184 |
Mechanical Properties — Important Note
| Property | Pine Kiln-Dried | ThermoWood Pine Thermo-D | Standard |
|---|---|---|---|
| Modulus of Elasticity (MOE) | 8,529 MPa | 7,411 MPa | DIN EN 408 |
| Modulus of Rupture (MOR — bending) | 76 MPa | 31–42 MPa | DIN EN 408 |
| Impact Bending Strength | 0.43 MPa | 0.16 MPa (lower — more brittle) | TS 2477 |
| Compressive Strength | 42 MPa | 44 MPa | TS 2595 |
The thermal modification process reduces some strength properties compared to kiln-dried pine — particularly impact bending strength and modulus of rupture. ThermoWood is more brittle and more susceptible to splitting at board ends than untreated softwood. Pre-drilling near board ends is recommended throughout. ThermoWood is not a structural timber and should not be used in structural applications where these strength properties are relied upon.
Fire Performance — Thermo-D
| Property | Pine KD | ThermoWood Pine Thermo-D | Standard |
|---|---|---|---|
| Fire class — uncoated | D | D (same as natural timber) | EN 13823 |
| Fire class — with retardant treatment | — | A2 / B achievable | EN 13823 |
| Smoke production | S2 | S1 | |
| Flaming droplets | d0 | d0 |
Uncoated ThermoWood Pine Thermo-D has the same Euroclass D fire rating as standard timber — it is a combustible material. Where enhanced fire performance is required — for buildings over 11m, projects close to boundaries, or where building control or fire engineers specify improved performance — factory-applied fire retardant treatment is available, achieving Euroclass A2 or B. See our fire retardant treatment service and our Approved Document B guide for full details.
ThermoWood Applications — Where Thermo-D Is Specified
Thermo-D grade ThermoWood is appropriate for a wide range of exterior and interior applications where its combination of durability, dimensional stability, and zero maintenance requirement provides clear advantages over standard softwood or naturally durable species.
- Exterior cladding — all profiles from shadow gap to triple shadow gap, shiplap, feather edge, and rainscreen. 30-year service life, zero maintenance
- Exterior decking — smooth and ribbed profiles. Exceptional dimensional stability minimises board movement and gapping over the service life
- Sauna interior and exterior — the resin-free, low thermal conductivity characteristic of ThermoWood makes it the best interior sauna cladding species. Thermo-D is equally suitable outside on the sauna exterior. See our sauna timber guide
- Garden rooms and garden offices — zero maintenance, contemporary shadow gap profiles, and consistent quality from UK stock make ThermoWood the most popular premium garden room cladding
- Rainscreen facades — including TIGA secret fix system with ThermoWood TIGA profiles. See our rainscreen cladding guide
How to Specify ThermoWood — NBS Language
For architects and specifiers writing NBS clauses or building control submissions, the correct specification language for our ThermoWood Pine Thermo-D cladding is:
Thermally modified pine (Pinus sylvestris) Thermo-D grade cladding, treated to 212°C in accordance with SFS 5765 ThermoWood standard. Durability Class 2 (BS EN 350). Expected service life 30 years above ground (BRE endorsed). Supplied by EMI Interiors Ltd t/a Timber Cladding Specialists. Profile: [specify profile and dimensions]. PEFC and FSC certified. No chemical preservative treatment required. A4 stainless steel fixings throughout. Install as ventilated rainscreen system with minimum 25mm clear cavity and breather membrane to substrate.
For any exterior UK cladding or decking project — specify ThermoWood Pine Thermo-D. This is the exterior-grade treatment class, Durability Class 2, 30-year BRE endorsed service life, suitable for cladding, decking, and sauna. For interior sauna lining where a lighter colour is wanted — Thermo Spruce Thermo-D is the preferred choice. Never specify Thermo-S for exterior applications. All cladding in our ThermoWood range is Thermo-D as standard — we will confirm the treatment class and species on every order confirmation. Download our full technical data sheet above for building control submission.
What is the difference between Thermo-D and Thermo-S?
Thermo-D (212°C) is the exterior grade — Durability Class 2, 30-year BRE endorsed service life, suitable for cladding, decking, and sauna. Thermo-S (190°C) is the interior grade — improved stability only, not suitable for exterior use. All ThermoWood cladding supplied by Timber Cladding Specialists is Thermo-D.
What is ThermoWood?
ThermoWood is timber thermally modified at 180–215°C using steam — no chemicals. The process permanently reduces equilibrium moisture content to 4–6%, improves dimensional stability by 60%, and upgrades biological durability from Class 4–5 to Class 2. It carries a 30-year BRE endorsed service life for exterior cladding and decking with no preservative treatment required.
What is the difference between Thermo Pine and Thermo Spruce?
Thermo Pine (Pinus sylvestris) is denser, has a warmer honey-brown tone, and is the most specified species for exterior cladding and decking. Thermo Spruce (Picea abies) is lighter in colour and slightly less dense — popular for sauna interiors where a paler tone is wanted. Both are available in Thermo-D grade for exterior use.
How long does ThermoWood last?
ThermoWood Thermo-D carries a BRE endorsed expected service life of 30 years for above-ground exterior applications — cladding and decking — when correctly installed with a ventilated cavity, minimum 150mm ground clearance, and A4 stainless steel fixings. This is achieved without any chemical preservative treatment.
Is ThermoWood stronger than untreated timber?
Not in all properties. Thermal modification reduces bending strength (MOR) from 76 MPa to 31–42 MPa and lowers impact bending strength — ThermoWood is more brittle than untreated softwood. Pre-drilling at board ends is essential. For durability, stability, and low maintenance, ThermoWood significantly outperforms untreated timber. It should not be used in structural applications.
ThermoWood Pine Thermo-D — UK Stock, Nationwide Delivery
We supply ThermoWood Pine and Spruce Thermo-D cladding in all standard profiles from UK stock — shadow gap, triple shadow gap, shiplap, feather edge, rainscreen, and tongue and groove. Full technical data sheet available. PEFC and FSC certified. Nationwide delivery in 7–14 days.