ThermoWood Spruce Thermo-D

About ThermoWood Spruce Thermo-D

ThermoWood Spruce Thermo-D is Norway spruce (Picea abies) modified at about 212°C. Alongside Scots pine it is one of the two Nordic softwoods the ThermoWood process was built around. At 380 lbf (1,690 N) and about 0.45 g/cm³ it is the softest and lightest board in the range, and its thermal conductivity of 0.117 W/m·K is the lowest here.

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.

That low conductivity is not a footnote. It is why Thermo Spruce is the traditional sauna timber: it stays comfortable to touch at bench temperature, and having had its resin driven off for good, it will not weep on anyone sitting on it. Radial shrinkage of 1% and tangential of 2.2% are about a quarter of untreated spruce.

At 420 kg/m³ a cubic metre of ThermoWood Spruce Thermo-D weighs about 420 kg air-dry — that figure, the 2.2% tangential and 1.0% radial movement and the 380 lbf hardness are the three inputs the calculators below actually consume. Select ThermoWood Spruce Thermo-D in any of them to get species-specific results for weight, moisture, shrinkage and hardness.

Working properties

Spruce starts cleaner than pine. Less resin, smaller and tighter knots, a paler and more even ground colour. After treatment it is also the most fragile thing in the range, and that governs everything you do with it. 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

Norway spruce is in Table 1 of the ITWA ThermoWood Handbook, so genuine, marked ThermoWood Spruce is available. As a Thermo-D product, treated near 212°C, the ITWA rates it durable, Class 2 under EN 113. There is a specific reason modification matters more for spruce than for almost any other timber: untreated spruce is not durable, and it is also famously refractory, meaning preservative will not penetrate it properly. Pressure treatment barely works. Thermal modification is one of the few routes that gets spruce outdoors at all. As with the whole range, ITWA states ThermoWood is not intended for load-bearing structures, and ThermoWood is a trademark. The generic term under CEN/TS 15679 is Thermally Modified Timber. See our guide to how ThermoWood is made.

Set against that, 380 lbf is genuinely soft. A dropped spanner marks it, a chair leg marks it, and a busy hallway floor will look worn in a year. Use it where things do not get dropped on it: cladding, ceilings, sauna walls and benches, and garden joinery.

Typical applications & suitability

Indoor Outdoor — Spruce is the other core ThermoWood feedstock, and modification matters more here than almost anywhere: untreated spruce is both non-durable and difficult to treat with preservative, so thermal modification is one of the few routes that gets it outdoors at all.

Documented end-uses: exterior cladding and rain screens, decking, sauna interiors, garden structures, interior panelling and ceilings.

Comparable species

Species closest to ThermoWood Spruce Thermo-D by density, hardness, stiffness and bending strength, computed directly from the Vriksai species dataset rather than hand-picked.

Trade alternatives. The untreated original is Norway Spruce. Compare ThermoWood Pine (its usual stablemate) or Accoya for a Class 1 modified alternative.

What modification did to Norway Spruce

ThermoWood Spruce Thermo-D is not a species — it is Norway Spruce 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.

On this species the process buys a 73% cut in tangential movement and a 2-class gain in durability, paid for with 5% of its bending strength. An unusually small strength penalty for a thermal treatment on this species.

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: Norway Spruce.

Compare ThermoWood Spruce Thermo-D 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 ThermoWood Spruce Thermo-D

Includes the thermal-modification tools, since ThermoWood Spruce Thermo-D 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.