The short answer
- Nordic spruce is the traditional Finnish wall and ceiling timber: pale, fine-grained and economical.
- It is usually knotty and contains some resin, though considerably less than pine. Resin can bleed at knots when heated.
- Use it for walls, ceilings and hidden bench frames, not for seats or backrests, and keep knotty boards away from the area directly above the heater.
- Thermo-spruce is resin-free and much more stable, with a darker colour and a mild smoky scent.
- Neither form is termite-resistant, which matters in Queensland for anything outside the hot room.
What we use
Most of the saunas we build are lined in western red cedar, and we sometimes use Canadian hemlock for a pale, scent-free room. We do not routinely build with spruce, but it is the classic lining in Finnish and European rooms. If you want the traditional Nordic look, we can discuss whether spruce or thermo-spruce suits your project.
Origin and names
Nordic spruce is Norway spruce (Picea abies), from northern and central Europe. In the timber trade it is sold as European or Nordic whitewood. That name can also cover European silver fir (Abies alba), so ask for the botanical name.
Do not confuse it with “Nordic redwood” or “European redwood”, which is Scots pine, a more resinous timber generally avoided in hot rooms.
Appearance and knots
Spruce is creamy white with hints of yellow or red, and has a fine, even, straight grain. The manufacturer Thermory describes it as one of the lightest-coloured conifers. It gives the bright, simple interior many people associate with a Finnish sauna.
Spruce is usually knotty, so grade selection is critical. For walls and ceilings, choose a grade with small, tight knots. Knots are denser than the surrounding wood and hold more resin, which is why they should be kept off anything you sit or lean on.
Resin
Spruce contains resin, but less than pine. Finnish sources prefer it to pine for exactly that reason, describing its resin content as considerably lower. Even so, resin can soften and bleed at knots under sauna heat.
That has two practical consequences:
- Keep knotty spruce off benches, backrests and headrests.
- Avoid knotty spruce on the ceiling directly above the heater, the hottest part of the room.
Thermal modification drives the resin out, so thermo-spruce does not bleed.
Aroma
Dry spruce has no characteristic scent. Thermo-spruce has the mild smoky note of all thermally modified timber, which fades but does not disappear completely.
Thermal feel and density
| Timber | Published density | Notes |
|---|---|---|
| Norway spruce | About 405 kg/m³ | The Wood Database |
| Western red cedar | About 340–380 kg/m³ | Range across published sources |
| European aspen | About 450 kg/m³ | For comparison |
Spruce is a light timber, a little denser than cedar and lighter than aspen. On walls and ceilings, its knots and resin matter more than its density.
Thermal modification lowers heat transfer further. The ThermoWood Association measured about 0.099 W/m·K for its Thermo-D pine and spruce, against 0.12 for untreated timber under the Finnish building code.
Stability and moisture movement
Spruce moves moderately. Norway spruce shrinks about 3.9% radially, 8.2% tangentially and 12.9% by volume from green to dry, against 6.8% by volume for western red cedar. Real movement in a well-dried, ventilated room is a fraction of these laboratory figures, but timber should still be delivered dry and acclimatised on site before fixing.
Thermo-spruce is far more stable: thermal modification typically reduces swelling and shrinkage by 40–80%, depending on species.
Durability and termites
Norway spruce heartwood is rated EN 350 class 4, slightly durable, a European classification. We found no Australian durability or termite rating for it. Inside a heated, ventilated room that is acceptable; outdoors, untreated spruce is a poor choice.
Thermal modification improves decay resistance. Lunawood rates its LunaThermo-D pine and spruce, treated at 212 °C, as EN 350 class 2 (durable). But thermally modified timber is not termite-resistant: the ThermoWood Handbook and Lunawood both say so. In Queensland, where subterranean and drywood termites occur, termite management for the sauna structure and subfloor is a matter for the designer and certifier.
Where spruce works
Walls and ceilings
This is spruce’s natural role. It is the traditional Finnish panel and is used for walls and ceilings by manufacturers such as Klafs and Tylö. Klafs’ untreated spruce and hemlock element saunas carry Germany’s Blue Angel eco-label. Thermory says spruce is generally preferred for exterior walls.
Benches and backrests
Spruce is not a standard seat timber. Finnish guidance does list pale, low-resin spruce among bench timbers, but most manufacturers and retailers prefer aspen, alder or abachi for seats because of spruce’s knots and resin.
Bench frames are a different matter. Spruce suits the hidden supports beneath the seats, where its resin and knots are out of sight. One Finnish bench maker recommends spruce, pine or thermo-aspen for frames.
Commercial rooms
Spruce suits commercial walls and ceilings, with knots kept out of contact zones and away from the heater.
Thermo-spruce
Thermo-spruce is made in Finland by producers such as Lunawood. The ThermoWood classes for softwood are:
- Thermo-S (190 °C), which the ThermoWood Association lists for uses including sauna benches.
- Thermo-D (212 °C), listed for sauna and bathroom furnishing.
Lunawood, which makes thermo-pine and thermo-spruce from PEFC-certified Northern Finnish timber, describes its Thermo-D grade as “also suitable for humid spaces like spa and sauna”. It uses the sapwood face of spruce as the visible side, because the straight grain there reduces delamination.
Compared with natural spruce, thermo-spruce is darker, resin-free and much more stable. The trade-offs are those of all thermo timber: splitting strength falls by 30–40% in ThermoWood tests, so it needs careful detailing; it has a smoky scent; it greys in sunlight, within 3–6 months outdoors according to Lunawood; and Lunawood notes that minor pigment release is possible in humid spaces such as saunas, which is worth knowing with pale towels.
Ageing and care
- Spruce darkens with use, like all sauna timber, faster where steam hits.
- Walls can be left untreated, or finished only with sauna-specific products. Sources differ on whether walls need any protection at all; follow the timber supplier’s instructions.
- No varnish or lacquer inside the hot room.
- Dry the room after each session with the vent open and the door ajar.
See our care guide for routine cleaning.
Advantages and disadvantages
Strengths: traditional Finnish look; pale, fine and even grain; light; generally economical; considerably less resin than pine; spruce and thermo-spruce are both listed by Australian sauna timber suppliers.
Trade-offs: usually knotty; resin can bleed at knots in the heat; not suited to seats or backrests; moves more than cedar; slightly durable only and not termite-resistant; the knotty look is not to every taste.
How spruce compares with western red cedar
- Look: spruce is pale and creamy; cedar warm brown and varied. Both come in knotty grades for walls.
- Resin: cedar is described by its trade association as pitch- and resin-free; spruce has some resin, which can bleed at knots.
- Feel: cedar is lighter, at about 340–380 kg/m³ against about 405 for spruce.
- Stability: cedar moves least of the mainstream sauna species; spruce moves moderately.
- Scent: cedar is strongly aromatic; dry spruce is close to scent-free.
- Benches: clear cedar makes good benches; spruce is best kept to frames.
- Durability: Australian references rate cedar heartwood termite-resistant; spruce has no Australian rating and thermo-spruce is not termite-resistant.
- Cost: spruce is generally the more economical lining.
For the full picture, see our western red cedar guide and the timber hub.
Specifying spruce
Ask for Picea abies by name, the knot grade for each surface, the moisture content at delivery and, for thermo-spruce, the treatment class. Specify clear, lower-density timber for benches and backrests, and keep knotty boards out of the zone directly above the heater.
Compliance and licensed trades
In commercial and multi-residential buildings, linings may be subject to National Construction Code fire hazard requirements, and European fire classes quoted for thermo timber are not Australian classifications. Confirm timber selection with the designer and building certifier, and follow the heater manufacturer’s clearances to timber.