Thermally Modified Western Red Cedar
Also known as thermo cedar, thermally modified western red cedar · Botanical Thuja plicata
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 Western Red Cedar
Thermally Modified Western Red Cedar is western red cedar (Thuja plicata) modified at around 212°C. At 350 lbf (1,557 N) and 0.38 g/cm³ it is the softest and lightest board on this site, modified or not, and its thermal conductivity of 0.102 W/m·K is the lowest figure in our whole dataset.
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.
It is also the product in this range that needs the hardest questions asked of it. Western red cedar heartwood is already rated durable under EN 350 without any treatment at all, and it is already one of the most dimensionally stable softwoods in commerce. Radial shrinkage of 1% and tangential of 2% after treatment are low, but natural cedar was not far off that to begin with.
At 380 kg/m³ a cubic metre of Thermally Modified Western Red Cedar weighs about 380 kg air-dry — that figure, the 2.0% tangential and 1.0% radial movement and the 350 lbf hardness are the three inputs the calculators below actually consume. Select Thermally Modified Western Red Cedar in any of them to get species-specific results for weight, moisture, shrinkage and hardness.
Working properties
Cedar is soft, straight-grained and effortless to cut. It is also acidic enough to corrode ordinary steel, and brittle after treatment in a timber that was already fragile. 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
Be careful with this one. Western red cedar does not appear in Table 1 of the ITWA ThermoWood Handbook, so a board sold as "Thermo-D western red cedar" is not ITWA ThermoWood. It is thermally modified timber under the generic CEN/TS 15679 description, and the class label is the producer's own. More to the point, the usual reason for modifying a timber does not really apply here. Cedar heartwood is already durable under EN 350 and already stable, so the durability gain that justifies the process on spruce or ash is largely a gain cedar had anyway. What you do get is the darker through-colour and a further reduction in movement. What you give up is strength: modification makes an already soft, low-strength timber more brittle still. For reference, ITWA gives Class 2 under EN 113 for genuine Thermo-D and states ThermoWood is not intended for load-bearing structures. See our guide to how ThermoWood is made.
If you want cedar, natural western red cedar is cheaper, better understood and already durable. If you want the dark modified look and maximum stability, Thermo Pine or Thermo Spruce will do it for less. The honest case for this board is narrow: you specifically want cedar's lightness and insulating value, and you specifically want it darker and more stable than nature made it.
Typical applications & suitability
Indoor Outdoor — The most questionable value case in the range, and worth being straight about: western red cedar is already durable (Class 2) untreated, thanks to its natural thujaplicins.Moderate dimensional stability and a darker, uniform colour, but not a durability upgrade, and it costs some of the strength of an already soft, low-strength wood.
Documented end-uses: exterior cladding and rain screens, soffits, garden structures, sauna interiors and interior panelling.
Comparable species
Species closest to Thermally Modified Western Red Cedar by density, hardness, stiffness and bending strength, computed directly from the Vriksai species dataset rather than hand-picked.
Trade alternatives. The untreated original is Western Red Cedar — already Class 2, lighter, and cheaper. Choose the modified version only if stability or uniform colour is genuinely the requirement. For a Class 1 modified softwood, Accoya.
What modification did to Western Red Cedar
Thermally Modified Western Red Cedar is not a species — it is Western Red Cedar after heat treatment. Both sets of figures are in our library, so the effect of the process on this timber can be stated exactly rather than described in general terms.
| Property | Western Red Cedar | Thermally Modified Western Red Cedar | Change |
|---|---|---|---|
| Tangential movement | 5.0% | 2.0% | -60% |
| Radial movement | 2.4% | 1.0% | -58% |
| EN 350 durability | Class 2 | Class 2 | — |
| Bending strength (MOR) | 52 MPa | 45 MPa | -13% |
| Stiffness (MOE) | 7.7 GPa | 7.5 GPa | -3% |
| Janka hardness | 350 lbf | 350 lbf | +0% |
| Density | 0.32 g/cm³ | 0.38 g/cm³ | +19% |
On this species the process buys a 60% cut in tangential movement and no change in durability class, paid for with 13% of its bending strength. A normal trade-off for this treatment class: fine for cladding and exterior joinery, size structural work off the treated figure, never the untreated one.
The mechanism — hemicellulose breakdown, why durability is through-section rather than surface-deep, and why the strength loss happens — is set out once in our guide to thermal modification and the ThermoWood species reference, rather than repeated on each treated species page. Untreated figures: Western Red Cedar.
Alternatives to Thermally Modified Western Red Cedar
Thermally Modified Western Red Cedar measures 350 lbf Janka at 0.38 g/cm³, EN 350 durability Class 2, movement rated Moderate. Thermally Modified Western Red Cedar 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.
If Thermally Modified Western Red Cedar 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 Western Red Cedar 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 Western Red Cedar
Includes the thermal-modification tools, since Thermally Modified Western Red Cedar behaves by treatment class as much as by species. Full set in the tool index.
Understand the figures
Wood density reference — oven-dry vs air-dry basis · Moisture content — EMC, FSP and service conditions · Strength properties — MOR, MOE and what they govern · Durability & fire — EN 350 classes and E84 ratings.
Further reading. What is ThermoWood? · ThermoWood vs Teak · Wood moisture content explained.
Common questions about Thermally Modified Western Red Cedar
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