Thermally Modified Siberian Larch
Also known as thermo Siberian larch, thermally modified Siberian larch · Botanical Larix sibirica
Revision history
- — Data re-verified in site-wide audit (formulas, sources, durability classes)
- — Working properties and comparable-species data cross-checked against listed sources
About Thermally Modified Siberian Larch
Thermally Modified Siberian Larch is Siberian larch (Larix sibirica) modified at around 212°C. At 1,100 lbf (4,893 N) and 0.59 g/cm³ it is markedly harder and denser than European larch, which is what slow growth in a hard climate does: very tight rings and a genuinely tough board.
Note: this is a modified product, and the panel above is modelled rather than measured. The hardness figure is the base species, because thermal modification changes hardness very little. For movement, the shrinkage and the fibre saturation point both fall after treatment, and that is why modified timber moves less across the same change in moisture. Published strength reductions vary by source; the table above uses our library figures for this pair. Modification also lowers bending strength and toughness relative to the untreated timber. Use this panel to size and to compare species, and take structural, fire, durability or movement figures from your supplier's declaration for the product you are actually buying.
This is the one product in the range where the raw material was already a serious cladding timber. Siberian larch is rated moderately durable, Class 3, under EN 350 untreated, and it has been the default premium cladding softwood in northern Europe for decades. Radial shrinkage of 1.4% and tangential of 2.9% after treatment are around a third of the natural figures.
At 590 kg/m³ a cubic metre of Thermally Modified Siberian Larch weighs about 590 kg air-dry — that figure, the 2.9% tangential and 1.4% radial movement and the 1,100 lbf hardness are the three inputs the calculators below actually consume. Select Thermally Modified Siberian Larch in any of them to get species-specific results for weight, moisture, shrinkage and hardness.
Working properties
Siberian larch is hard, dense and resinous, and its knots are like glass. It was never a pleasure to work and treatment does not improve its temper. How the board behaves once it is out of the kiln is set by the process itself, which is described in full in our guide to how ThermoWood is made. What follows is specific to this timber, drawn from the International ThermoWood Association, VTT Technical Research Centre of Finland and published thermal-modification research.
Durability & use classification
Two things to check before you specify this. First, Siberian larch does not appear in Table 1 of the ITWA ThermoWood Handbook. Thermally modified Siberian larch is traded, but under the generic CEN/TS 15679 term Thermally Modified Timber, not as ITWA ThermoWood, so a "Thermo-D" label on it is a producer designation rather than an ITWA classification. Second, and more practically: Siberian larch is a Russian timber. Sanctions introduced from 2022 have restricted Russian wood into the EU, the UK and the US, and supply has been complicated and legally fraught since. Ask where the log came from and when, and get the paperwork. For reference, ITWA gives Class 2, durable under EN 113 for genuine Thermo-D and states the material is not intended for load-bearing structures. See our guide to how ThermoWood is made.
The technical case for modifying it is narrower than for pine or spruce, because it starts at Class 3 rather than Class 5. You are buying one durability class and a large gain in stability, not the transformation you get with ash or birch. Given the sourcing position, it is worth pricing modified Nordic spruce or pine against it before committing: several Finnish producers market exactly that swap.
Typical applications & suitability
Indoor Outdoor — Siberian larch is the premium larch feedstock — very slow-grown in a hard climate, so it is denser and straighter-grained than European larch, with tighter rings. Modification adds the durability and stability its natural form only partly delivers.
Documented end-uses: exterior cladding and rain screens, decking, garden structures, flooring and interior panelling.
Comparable species
Species closest to Thermally Modified Siberian Larch by density, hardness, stiffness and bending strength, computed directly from the Vriksai species dataset rather than hand-picked.
Trade alternatives. Compare ThermoWood Larch (European, less dense) and the untreated European Larch. For untreated durable cladding alternatives, Western Red Cedar or Sweet Chestnut.
Alternatives to Thermally Modified Siberian Larch
Thermally Modified Siberian Larch measures 1,100 lbf Janka at 0.59 g/cm³, EN 350 durability Class 2, movement rated Moderate. Thermally Modified Siberian Larch is a modified timber: its durability comes from process, its strength from the base species. Substitutes are matched on both, not on the treated figure alone. Read the numbers against the Janka scale and EN 350 durability classes.
Thermal modification lowers hardness slightly, so if the treated figure is short of what the job needs, these are the harder options at comparable service durability.
Lower-hardness options at similar treated durability — usually cheaper per m³ and faster through the shop.
Modification is already doing the stability work here. These achieve equal or lower movement by species or by deeper treatment. Size the gaps with the shrinkage predictor and EMC calculator.
If Thermally Modified Siberian Larch is unavailable and the specification cannot change, these are the nearest substitutes across hardness, density, movement, stiffness, bending strength and durability class together — not on any single property. Lower distance means a closer match.
Computed from measured properties only — hardness, density, movement, stiffness, bending strength and EN 350 class. Price, availability and appearance are never inputs. Figures are representative values, not consignment absolutes: two species within a few percent are equivalent until verified on the material. Full method and the rules the engine cannot break.
Comparing modified timber to unmodified is a comparison of process as much as species — treatment class sets the durability, the base species sets the strength. Compare directly before substituting.
Compare Thermally Modified Siberian Larch side by side
Modified against unmodified, and modification class against class — the comparisons that matter for treated timber. Every pairing shows the measured figures side by side; full matrix in the comparison index.
Calculators relevant to Thermally Modified Siberian Larch
Includes the thermal-modification tools, since Thermally Modified Siberian Larch behaves by treatment class as much as by species. Full set in the tool index.
Understand the figures
Thermal conductivity — k-value and insulation behaviour · ThermoWood species guide — Thermo-D/Thermo-S treatment classes · Janka hardness scale — how the lbf figure is measured · Wood density reference — oven-dry vs air-dry basis.
Further reading. What is ThermoWood? · ThermoWood vs Teak · Wood moisture content explained.
Common questions about Thermally Modified Siberian Larch
A note on the strength figures. This species is not on the ITWA ThermoWood species list, so no association-published strength or hardness figures exist for it. The values above are for the untreated base species and are marked unverified — thermal modification reduces strength, typically in the region of 10–30% depending on class and species, with bending strength affected more than hardness. Thermally modified timber is not strength-graded and must not be used for load-bearing structures. Take any design value from your supplier's declaration for the actual product.
unverified no published source for this figure · ITWA International ThermoWood Association / VTT · calc derived from the figures above