Thermally modified wood is solid timber that has been cooked in a kiln until its chemistry changes. Designers use it for cladding, decking, interior panelling and wet-room joinery, where they want a dark, stable, chemically untreated board.
This guide sets out what the ThermoWood handbook, four EPDs and peer-reviewed reviews say, and where they stop. Most numbers come from manufacturers or their trade association, so we label them as such.
What is thermally modified wood and how is it made?
Heat, steam and no added chemicals
The International ThermoWood Association defines thermal modification as a method in which heat and steam change the chemical properties of wood, with a minimum modification temperature of 160 °C, and states that the changes are permanent.[1] The kiln cycle has three phases. The timber is dried at high temperature, held at the modification temperature, then cooled while steam and water bring the moisture content back up to 4–7 %.[1] The association says no chemicals are used, and Thermory's EPD describes its process the same way.[1][2] These are company and association statements. A 2008 BioResources review synthesises the heat-treatment literature on properties, chemical changes, uses and quality control.[7] A Holzforschung review notes that supporting scientific and industrial LCA data on the process are scarce.[8]
Thermo-S and Thermo-D
ThermoWood is a registered trademark that only association members may use. It defines two classes: S for stability and D for durability.[1] For pine and spruce, Thermo-S is modified at 190 °C and Thermo-D at 212 °C, each ±3 °C. Hardwoods use 190 °C for Thermo-S, and the handbook gives each species its own Thermo-D temperature: 208 to 212 °C for most hardwoods, 190 °C for iroko, 225 °C for western hemlock and 230 °C for radiata pine.[1] Other makers set their own recipes: Thermory states 215 °C for its pine.[2] Ask for the temperature and the durability class of the exact product, not the trade name.
Standard
CEN/TS 15679:2007 gives definitions and characteristics for thermally modified timber.[5] BSI describes it as covering characteristics assessed after treatment, a test method for resistance to wood-destroying fungi, and type testing and factory production control, and lists it as "Current, Under Review".[6] We did not read the standard text.
How does thermally modified wood perform?
Moisture, stability and weather
Heating lowers the equilibrium moisture content. At 220 °C it is halved against untreated spruce, and it stays far below the 20 % at which saprotrophic fungi become active, which the handbook ties to the improved durability.[1] Radial and tangential swelling fall too, so boards keep their dimensions without a surface finish.[1] Left untreated outdoors, the wood stays drier than unmodified timber but turns grey under UV, and superficial mould can appear and be wiped off.[1] The handbook recommends a surface treatment for weather-exposed use, says linseed oil is unsuitable because it enables fungal growth, and does not recommend the wood for contact with soil or water.[1]
Durability classes
The handbook's table pairs EN 350 durability classes with EN 335 use classes and recommends a class for each. Under use class 3 (outdoors, above ground, exposed to weather, listed against durability class 2) it separates limited wetting, where windows and doors take Thermo-D or Thermo-S, from prolonged wetting, where cladding and decking take Thermo-D only. Under use class 2 (indoors but possibly exposed to moisture and condensation, listed against durability class 3) cladding, ceilings and saunas take either class, and so does interior cladding and furniture under use class 1. The table recommends no product for use classes 4 and 5, which cover contact with soil, fresh water or salt water.[1] Resistance is reported for longhorn beetle, common furniture beetle and Lyctus; for termites the handbook says the studies to date do not prove resistance.[1] HJT-Holz's EPD uses a reference service life of 30 years, which it attributes to the manufacturer.[4] We found no independent long-term field monitoring.
Strength loss
Strength drops as temperature rises. In the handbook's EN 408 tests, spruce bending strength measured 74.2 N/mm² untreated, 65.0 as Thermo-S and 47.5 as Thermo-D; pine went from 60.7 to 45.1 and 38.1. In the same table the modulus of elasticity of pine stayed near 9,000 N/mm², while spruce fell from 13,658 to 10,133 N/mm² at Thermo-D.[1] Screw-holding strength of spruce fell from 22.0 to 18.2 and 14.9 N/mm², the handbook cites a 30–40 % reduction in splitting strength, and spruce modified at 220 °C lost about 25 % of its impact strength.[1] The handbook says ThermoWood products are not available as strength-graded timber and must not be used for load-bearing structures.[1] For structural timber use glulam or cross-laminated timber, and keep the thermally modified board for the skin.
Heat flow and indoor air
The handbook reports 20–25 % lower thermal conductivity for modified Nordic softwood, but gives no absolute value.[1] On emissions, its VTT-sourced test of pine measured total VOC of 235 µg/m²h after modification at 230 °C against 1,486 for unmodified pine, with acetic acid the main compound.[1] Dust from machining is dry and fine, and the handbook says a respirator must be used when it is generated.[1]
How does thermally modified wood behave in fire?
The handbook says modified timber has a smaller fire load and lower smoke production than standard timber because of its lower density and extractive content, and then adds that these factors may not be used directly in fire design.[1] It requires application-specific fire planning. It also says fire-retardant treatment can lift the wood to class C or B, with B-s1, d0 as the highest achievable.[1] We found no Euroclass for an untreated board in the documents we read. The handbook adds that wood panelling and cladding made of thermally modified wood does not automatically receive class D-s2,d0 like untreated wood, and that manufacturers are responsible for testing.[1] The regulation behind this, Delegated Regulation (EU) 2024/1399, gives its no-test classes only to wood that has had no treatment other than kiln drying, and it does not mention thermally modified wood by name, so the link to thermally modified wood is the handbook's reading.[16] One large project shows the route: the Pelgulinna school below used Thermory pine with the Woodsafe fire retardant.[10]
How much carbon does thermally modified wood hold?
What the EPDs show
Three EN 15804+A2 EPDs, all manufacturer-owned and verified by third parties, give a negative A1–A3 total because the tree's carbon uptake is counted as a biogenic term. Per m³, Thermory's pine (432 kg) shows −588 kg CO₂e, made of +147 fossil and −736 biogenic. Its spruce (413 kg) shows −564, made of +174 and −739.[2][3] HJT-Holz's Thermo-D spruce and pine (430 kg) shows −673, made of +67 fossil and −740 biogenic. The same EPD declares Thermo-S spruce and pine at −676 (+64 fossil, −740 biogenic) and Thermo-D radiata pine (470 kg) at −621 (+187 fossil, −809 biogenic).[4]
The spread in fossil carbon is the useful signal. HJT-Holz's fossil figure for Thermo-D spruce and pine is lower than Thermory's even though its kilns run on light fuel oil, but its radiata pine, whose raw material comes from New Zealand, is higher than both; its EPD notes a 66 % variation in A1–A3 fossil GWP between the products it declares.[4] Thermory's spruce EPD, an average of two factories, reports −17 / +26 % variation.[3]
Read the number with care
The negative total does not mean the board removes carbon from the atmosphere for good. Each EPD models the end of life. In Thermory's pine EPD, C3 adds +740 kg CO₂e, almost all of it biogenic, under a Danish scenario of 82 % recycling and 18 % energy recovery. HJT-Holz models incineration, with C3 at +767.[2][4] The EPDs come from different programme operators (EPD Hub and Rakennustieto RTS) and PCR versions, and they state that EPDs from different programmes may not be comparable.[2][4] Ask what transport distance (A4) each assumes: 1,390 km to Denmark for Thermory pine and 1,228 km on average for HJT-Holz.[2][4]
Two peer-reviewed papers help. The Holzforschung review compiles LCA studies and finds that few complete ones exist.[8] A 2021 Coatings study ran a cradle-to-gate-with-options assessment of coated and thermally modified facades: thermal modification had a significant impact at the product stage, but the uncoated modified facade had the lowest impact over a 30-year use stage, and recoating frequency mattered.[9] Both findings come from abstracts.
Which standards and certificates apply?
CEN/TS 15679 is the European definition for thermally modified timber, and EVS lists it as valid from 09.01.2009.[5] For CE marking, the handbook points to EN 14915 for wood panelling and cladding and EN 14342 for wood flooring, and notes that thermally modified timber does not typically carry a CE marking of its own.[1] The ThermoWood logo is tied to an annual audit by an accredited inspection body and an annual test of finished products, and the handbook says certification may be revoked if a plant fails the requirements.[1] The handbook we read is the updated edition of October 2025 from the association's current site; the first edition appeared in April 2021.[1]
Where is thermally modified wood used, and what are the limits?
Exterior cladding
Best for: architects · facade designers
The handbook recommends a minimum thickness of 19 mm for outdoor wall and ceiling cladding and 26 mm for patios, and says profiled boards need no back grooves because of their stability.[1] Specify Thermo-D, decide on a finish or accept greying, and seal cut ends.[1] Fixings matter: the Pelgulinna facade uses stainless steel.[10]
Decking and garden structures
Best for: landscape architects · self-builders
The handbook's use-class table recommends Thermo-D for decking and cladding under prolonged wetting (use class 3.2).[1] Contact with soil or water is outside the handbook's recommendation.[1]
Interior panelling, floors and saunas
Best for: interior designers
Recommended minimums are 14 mm for indoor wall and ceiling cladding and 26 mm for floorboards.[1] The handbook's table recommends Thermo-D or Thermo-S for cladding, ceilings and saunas under use class 2 and for interior cladding and furniture under use class 1, and it recommends mineral oil for sauna walls, ceilings and benches.[1]
Product and industrial designers
Best for: product designers
We found no sourced data on thermally modified wood in product design. The handbook only notes that milling resembles working with hard, brittle hardwoods.[1]
Where has thermally modified wood been used in real projects?
The projects below are documented by manufacturer pages, the trade association or a contractor. We found no independent performance monitoring for them.
Pelgulinna State Upper Secondary School, Tallinn
Architect: Arhitekt Must · completed 2023
Thermory's case study lists Benchmark thermo-pine with Woodsafe fire-retardant treatment, quotes architect Ott Alver giving 3,600 m² of facade cladding for the building (it does not say how much of that is Thermory pine), and states that the timber has to last at least 25 to 30 years.[10] The contractor Merko records the client as Riigi Kinnisvara, a 2023 completion, a four-storey building, and load-bearing elements and finishing layers in cross-laminated timber.[11]
Project Ö, Finnish archipelago
Architects: Aleksi Hautamäki and Milla Selkimäki · 2019
A private island retreat. The Lunawood catalogue page on ArchDaily lists pine and spruce ThermoWood for exterior cladding, decking and interior panels, and describes the result as dimensionally stable and low maintenance. It does not state a modification class, and the claims are the manufacturer's.[12]
Mouriz School Center, Paredes
Architect: Atelier Nuno Lacerda Lopes (CNLL) · 2010
The 2021 edition of the ThermoWood handbook credits Lunawood and the Centro Escolar de Mouriz by CNLL in a photo caption dated 2010; the 2025 edition no longer carries it.[14] ArchDaily lists the building with Lunawood as manufacturer but does not say what the material is used for or which treatment it has.[13]
Where to buy thermally modified wood in Europe
Suppliers selected by our editors and checked against company documents and certificate records where available.
| Supplier | Products | Country | Certifications |
|---|---|---|---|
| Thermory AS | Thermally modified pine and spruce boards without surface coating | Estonia (plants at Loo and Reola) | EPD Hub HUB-4211 (pine, published 27 Oct 2025, library shows expiry 26 Oct 2030; the PDF says 27.10.2030) and HUB-4822 (spruce, published 9 Jan 2026, expiry 8 Jan 2031), both found in the EPD Hub library on 8 October 2026.[2][3] |
| HJT-Holz Oy | Rough-sawn ThermoWood: Thermo-S and Thermo-D in spruce and pine, Thermo-D in radiata | Finland | RTS EPD RTS_441_25, verified 29.10.2025, published 26.11.2025, valid to 26.11.2030, found in the RTS registry node (rts.lca-data.com, checked 8 October 2026); the copy on thermoholz.fi leaves these fields blank. Listed as a ThermoWood Producer on the association's members page (checked 8 October 2026), in line with the EPD's own statement.[4][17] |
| Lunawood | ThermoWood cladding, decking and interior panels | Finland (mills in Kaskinen and Iisalmi, per its EPD) | RTS EPD "RTS 180 22", verified 13 May 2022, valid to 12 May 2027, covering Thermo-S and Thermo-D rough and planed products; four entries in the RTS registry node (rts.lca-data.com, checked 8 October 2026). Listed as a ThermoWood Producer on the association's members page (checked 8 October 2026). It declares results per kg (A1–A3 GWP-total −1.34 to −1.46 kg CO₂e/kg), under an older RTS PCR from 2020, so do not compare it directly with the per-m³ EPDs above. PEFC chain of custody is a company statement in the EPD.[15][17] |
EPD numbers and dates were matched against the EPD Hub library and the RTS registry node on 8 October 2026. Thermory does not appear on the ThermoWood Association's members page, so we do not describe its product as ThermoWood. Confirm they are still valid before specifying. Listing policy: editorial selection, not paid.
Frequently asked questions about thermally modified wood
What is thermally modified wood?
Timber heated in a kiln with steam to at least 160 °C, without added chemicals, so that its chemistry changes permanently. The result is darker, more stable and more resistant to decay.
What is the difference between Thermo-S and Thermo-D?
Thermo-S is modified at 190 °C for Nordic softwoods and favours stability; Thermo-D at 212 °C favours durability. In the handbook's use-class table only Thermo-D is recommended for outdoor cladding and decking under prolonged wetting; Thermo-S is recommended for windows and doors under limited wetting, saunas and interiors.
Is thermally modified wood strong enough for structure?
No. Spruce bending strength fell from 74.2 to 47.5 N/mm² at Thermo-D in the handbook's tests, and the handbook says it must not be used for load-bearing structures.
How long does thermally modified wood last outdoors?
HJT-Holz's EPD uses a 30-year reference service life, attributed to the manufacturer. We found no independent long-term monitoring, and the handbook does not recommend soil or water contact.
Is thermally modified wood fireproof?
No. We found no Euroclass for untreated product, and the ThermoWood handbook says that, under EU Regulation 2024/1399, it does not automatically receive the D-s2, d0 class that untreated wood can get. With a fire retardant the handbook says classes C or B are possible, up to B-s1, d0.
Is thermally modified wood carbon negative?
In the production stage, the Thermory and HJT-Holz EPDs show negative totals of −564 to −676 kg CO₂e per m³ because of stored biogenic carbon. The same EPDs model that carbon returning at end of life, so the net depends on disposal.
Does thermally modified wood need a finish?
Outdoors it greys under UV without one, and the handbook recommends surface treatment for exposed use. Linseed oil is unsuitable because it enables fungal growth.
The bottom line on thermally modified wood
Thermally modified wood is a chemical-free way to make local softwood stable and decay-resistant enough for cladding, decking and interiors. Its limits are real: lost strength, no structural use, a fire class that needs treatment, and carbon figures that depend on end-of-life assumptions. Ask each supplier for the modification temperature, the EN 350 class of the exact product, the current EPD and the fire test for your build-up.
Related materials
Sources
- International ThermoWood Association (October 2025). ThermoWood® Handbook, second updated edition (the first edition was April 2021). Source of the definition, process phases, classes and temperatures, density, bending and screw-holding data, EN 350 / EN 335 table, fire-retardant classes, fire note on Regulation 2024/1399, emissions test, CE marking, surface treatment and storage. Trade-association document citing VTT research (association claims); full text read from the association's own download page. PDF (thermowood.com)
- Thermory AS (2025). Environmental Product Declaration, Thermally modified pine boards without surface coating, EN 15804+A2, declared unit 1 m³ (432 kg), EPD Hub HUB-4211, published 27.10.2025, valid until 27.10.2030, verifier Sergio Ballen Zamora for EPD Hub. Source of GWP values, 215 °C modification, A4 distance and end-of-life scenario. Manufacturer-owned, third-party verified; full text read. The EPD Hub library lists it as HUB-4211, published 27 Oct 2025, expiry 26 Oct 2030. PDF (thermory.com) · EPD Hub library entry
- Thermory AS (2026). Environmental Product Declaration, Thermally modified Spruce boards without surface coating, EN 15804+A2, declared unit 1 m³ (413 kg), EPD Hub HUB-4822, published 09.01.2026 (last updated 13.01.2026), valid until 08.01.2031, verifier HaiHa Nguyen for EPD Hub; two factories (Loo, Reola). Manufacturer-owned, third-party verified; full text read. The EPD Hub library entry matches. PDF (thermory.com) · EPD Hub library entry
- HJT-Holz Oy (2025). Environmental Product Declaration, Rough sawn ThermoWood®, EN 15804+A2, declared unit 1 m³ (430 kg for spruce and pine), programme operator Rakennustieto Oy (RTS), author Hanna Lento (Sweco Finland), verifier Sigita Židonienė (UAB Vesta Consulting), verification date 29.10.2025, registration number RTS_441_25, published 26.11.2025, valid until 26.11.2030 (these three fields are blank on the cover of the thermoholz.fi copy and filled in the RTS registry copy; the rest of the text is identical). Source of GWP values, kiln fuel, product sizes, 30-year reference service life and end-of-life scenario. Manufacturer-owned, third-party verified; full text read. PDF (RTS registry copy) · PDF (thermoholz.fi)
- Estonian Centre for Standardisation and Accreditation (EVS), catalogue page for CEN/TS 15679:2007, "Thermal Modified Timber - Definitions and characteristics": scope "gives definitions and characteristics for Thermally Modified Timber"; valid from 09.01.2009. Independent catalogue page; the standard text was not read. evs.ee
- BSI Knowledge, catalogue page for DD CEN/TS 15679:2007, published 29 February 2008, status "Current, Under Review". Independent catalogue page; the standard text was not read. knowledge.bsigroup.com
- Esteves, B.M. & Pereira, H.M. (2008). "Wood modification by heat treatment: A review." BioResources 4(1):370–404. Peer-reviewed review of heat-treatment effects on properties, chemistry, uses and quality control. Abstract read via Crossref; full text not accessed; no figures from it are used. doi:10.15376/biores.4.1.370-404
- Candelier, K. & Dibdiakova, J. (2020). "A review on life cycle assessments of thermally modified wood." Holzforschung 75(3):199–224. Peer-reviewed review; abstract read via Crossref, full text not accessed. doi:10.1515/hf-2020-0102
- Búryová, D. & Sedlák, P. (2021). "Life Cycle Assessment of Coated and Thermally Modified Wood Façades." Coatings 11(12):1487. Peer-reviewed LCA (cradle-to-gate with options, EN 15804+A2, SimaPro). Abstract read via Crossref; full text not accessed. doi:10.3390/coatings11121487
- Thermory, case study "Pelgulinna State Upper Secondary School, Arhitekt Must" (27 October 2023): Benchmark thermo-pine with Woodsafe fire-retardant treatment, 3,600 m² of facade cladding, 25–30 year service-life statement, stainless-steel fixings. Manufacturer page; claims are the manufacturer's. The 3,600 m² figure is a quote from architect Ott Alver about the building's facade cladding, and the 25–30 year figure is in the case-study text. thermory.com
- Merko Group, project page "Pelgulinna State gymnasium": client Riigi Kinnisvara AS, architect Arhitekt Must OÜ, construction 2022, completed 2023, four storeys, footprint 3,773 m², cross-laminated timber structure and finishes, timber sun shielding. Contractor page; it does not name the facade species. group.merko.ee
- Lunawood on ArchDaily, product catalogue page "Project Ö": architects Aleksi Hautamäki and Milla Selkimäki, Finnish archipelago, 2019; Luna panel, decking and interior products in pine and spruce; manufacturer statements on stability, no chemicals and a 20-year warranty offer. Manufacturer catalogue page; no modification class stated; not independently verified. archdaily.com
- ArchDaily, project page "Mouriz School" by Atelier Nuno Lacerda Lopes, 2010, 3,220 m², manufacturer listed: Lunawood. The page gives no text on the cladding material. archdaily.com
- International ThermoWood Association (April 2021). ThermoWood® Handbook, first edition. Used only for the photo caption crediting Lunawood and the Centro Escolar de Mouriz by CNLL (2010), which the 2025 edition no longer carries. Trade-association document; read from a copy on the association's file server. PDF (file host)
- Oy Lunawood Ltd (2022). Environmental Product Declaration, Lunawood Thermowood® of Nordic Pine and Spruce (Rough Thermo Timber and Planed Thermowood®, Thermo-S and Thermo-D), RTS EPD no. RTS 180 22, Building Information Foundation RTS, EN 15804+A2 and RTS PCR 26.8.2020; author Tarmo Räty (Luke), verifier Sigita Židonienė (Vesta Consulting), verified 13 May 2022, valid until 12 May 2027 (the cover gives an issue date of 24.5.2022). Results are per kg. Manufacturer-owned, third-party verified; results tables and general information read. Listed in the RTS registry node (rts.lca-data.com): four Lunawood entries, valid until 2027, checked 8 October 2026. PDF (RTS registry copy)
- European Commission (2023). Commission Delegated Regulation (EU) 2024/1399 of 10 November 2023 on the conditions for classification, without testing, of solid wood panelling and cladding with regard to their reaction to fire, OJ L, 22.5.2024. Recital 3 and Annex footnote 5 define untreated wood as wood not subject to any treatment other than kiln drying. The text does not mention thermally modified wood. Primary legal text, read in full. eur-lex.europa.eu
- International ThermoWood Association, members page (thermowood.com/members), checked 8 October 2026: HJT-Holz Oy and Oy Lunawood Ltd are listed under ThermoWood Producers; Thermory is not listed. Trade-association web page. thermowood.com