Thermowood — timber that has been kiln-treated at temperatures between 185°C and 215°C in a steam environment — has established itself as a premium alternative to both treated softwood and composite cladding in the UK market. The high-temperature process drives out moisture, breaks down the sugars that fungi feed on, and produces a dimensionally stable, naturally durable timber that requires no chemical preservative treatment.
In 2026, Thermowood cladding supply costs range from £35 to £90 per m² depending on species and profile. Installed, including battening, breathable membrane, fixings, and labour, expect £80 to £160/m².
What Is Thermowood and How Is It Made?
Thermowood is a trademarked brand name owned by Thermory (Estonian) and licensed to producers across Scandinavia and northern Europe. The generic term is thermally modified timber (TMT). The modification process:
- Heating phase — timber is raised to 130°C in a steam environment to prevent cracking
- Modification phase — temperature rises to 185–215°C; chemical structure of the wood changes (hemicelluloses break down, reducing hygroscopicity)
- Cooling and conditioning phase — temperature gradually reduced to bring moisture content back to 5–7%
The result is timber that is significantly more stable and durable than untreated equivalents. The darker colouration is a natural result of the process, not a stain.
Common Species and Profiles
| Species | Durability class (EN 350) | Typical supply cost (m²) |
|---|---|---|
| Thermowood Pine (Scots or Radiata) | Class 2–3 | £35–£55/m² |
| Thermowood Ash | Class 2–3 | £50–£75/m² |
| Thermowood Spruce | Class 2 | £35–£50/m² |
| Thermowood Alder | Class 2–3 | £45–£65/m² |
| Thermowood Accoya (acetylated, related) | Class 1 | £80–£130/m² |
Common profiles include:
- Cove and bead (rounded shadow gap)
- Tongue and groove (vertical or horizontal)
- Open-jointed rainscreen (ventilated, horizontal or vertical)
- Shiplap and rebated drop profiles
Standard board widths run 90–142 mm with finished thicknesses of 18–26 mm for cladding applications.
Installation Costs
Thermowood cladding must be installed on a properly ventilated rainscreen system per BS 8000-13 and good practice guidance from the TRADA (Timber Research and Development Association). This means:
- Breathable membrane (vapour permeable, windtight) — £2–£5/m²
- Counter battens (if required for insulation or service cavity) — £5–£12/m²
- Vertical battens for cladding support (35 × 50 mm or 50 × 50 mm, treated softwood or similar TMT) — £4–£9/m²
- Fixings — stainless steel A4-grade secret fixings or face-fix screws; stainless steel is non-negotiable with TMT to prevent staining — £2–£5/m²
- Labour — cladder day rate £200–£320/day; a typical semi-detached front elevation (40–50 m²) takes 2–4 days
| Cost element | Range per m² of finished cladding |
|---|---|
| Thermowood boards | £35–£90 |
| Battens and membrane | £10–£20 |
| Fixings (stainless) | £3–£7 |
| Labour | £25–£50 |
| Total installed | £73–£167 |
Thermowood vs Alternatives
| Material | Supply cost (m²) | Installed cost (m²) | Expected service life | Maintenance |
|---|---|---|---|---|
| Thermowood | £35–£90 | £80–£160 | 30–50 years | Periodic oil/oil-wax coat every 5–10 years (optional for aesthetics) |
| Treated softwood (tanalised) | £15–£35 | £55–£100 | 15–25 years | Paint or stain every 3–5 years |
| Siberian larch (untreated) | £30–£60 | £75–£140 | 25–40 years | Optional finish only |
| Western red cedar | £35–£65 | £80–£150 | 20–35 years | Periodic oiling |
| Fibre cement cladding | £20–£50 | £65–£130 | 40–60 years | Minimal; repaint every 10–15 years |
| Composite/PVC cladding | £25–£60 | £60–£120 | 20–40 years | Wash down; no painting |
Thermowood sits in the upper-mid tier on cost but towards the top on service life among timber options. Its advantage over conventional treated softwood is the absence of biocide chemicals, which appeals to specifiers seeking FSC-certified, low-toxicity materials.
Greying and Finishing
Left unfinished, all Thermowood species will weather to a silver-grey patina within 6–18 months of exposure to UV light and rain. This is an aesthetic consideration rather than a structural one — the material remains durable when grey. Many architects specify Thermowood precisely for this silver-grey weathered look on contemporary or barn-style buildings.
If the original darker brown colour is preferred, a UV-stabilising oil or oil-wax finish (Osmo, Rubio Monocoat, or similar) applied every 5–10 years will maintain the tone and add a degree of surface protection.
Do not paint Thermowood with conventional masonry or wood paints — the very low moisture content causes paint films to adhere poorly and peel.
Planning and Building Regulations
Cladding to external walls is generally permitted development on a house, unless:
- The property is a listed building (any external alteration requires listed building consent)
- The property is in an Article 4 direction area that removes permitted development rights
- The cladding would change the material facing a highway in a conservation area (requires planning permission in most cases)
Under Approved Document B (Fire Safety), cladding systems on buildings over 11 m high must meet strict European Classification standards. For typical residential properties under 11 m, there are no cladding-material fire restrictions beyond ensuring combustible materials are not used near openings where Building Control requires a fire stop. Always confirm with your local authority building control (LABC) or an approved inspector if there is any doubt.
Thermal performance: replacing a masonry wall with a cladding system in a renovation context requires the resultant wall to meet current Approved Document L standards (maximum 0.18 W/m²K for an external wall). This typically means incorporating insulation behind the rainscreen, which adds cost but improves the building’s energy rating.
Finding Suppliers and Installers
Thermowood is widely available through specialist timber merchants, many of whom offer profile cutting. Thermory, Lunawood, and Brimstone (UK distributor) are among the established suppliers. When specifying:
- Confirm the modification class (Class S for superior, Class D for standard) — Class S has higher thermal modification and better durability
- Check the durability class per EN 350 for the species specified
- Request an FSC or PEFC certificate of conformity if chain-of-custody documentation is required for planning conditions
For installers, seek members of the Cladding Manufacturers Association or contractors with demonstrable experience in rainscreen cladding systems and an understanding of BS 8000-13 workmanship requirements.
