The short answer
- Alder is a smooth, fine-grained, resin-free timber used for sauna benches and walls in Europe and North America.
- It has a warmer colour than aspen and darkens and reddens with age.
- Thermo-alder is thermally modified alder: hazel to deep brown, much more stable and good at hiding marks, but more brittle and with a mild smoky scent.
- It is a little denser than aspen and cedar, so it can feel slightly warmer to sit on.
- Neither form is durable or termite-resistant, so both belong inside the hot room.
What we use
Most of the saunas we build are lined in western red cedar, and we sometimes use Canadian hemlock. We do not routinely build with alder, but it is an established sauna timber offered by several European manufacturers. If you are drawn to alder or thermo-alder, we are happy to discuss whether it suits your room.
Origin and names
Two species are sold as sauna alder:
- Black or common alder (Alnus glutinosa), from Europe, North Africa and temperate Asia. This is the usual European sauna alder.
- Red alder (A. rubra), from western North America.
Alder belongs to the birch family and is botanically a hardwood. Some suppliers call it a softwood; that is incorrect.
Appearance, knots and resin
Natural alder is light tan to reddish-brown, with a straight grain and a fine, uniform texture. It finishes smoothly.
Thermo-alder is hazel to deep brown and even in tone.
Knots: sauna grades are sold clear. As with every species, knot-free is a grade, so specify it for benches and backrests.
Resin: alder is resin-free in practice, and thermal modification removes any that remains.
Aroma
Natural alder has no characteristic scent. Thermo-alder carries the mild smoky note of all thermally modified timber, which fades but does not disappear completely.
Thermal feel and density
| Timber | Published density | Notes |
|---|---|---|
| Black alder | About 380–600 kg/m³, typically about 490 | Wide natural range |
| Red alder | About 450 kg/m³ | The Wood Database average |
| Thermo-alder | Not published | Thermal modification lowers density and heat transfer |
| Western red cedar | About 340–380 kg/m³ | Range across published sources |
Alder is somewhat denser than aspen and cedar, so it passes heat to the skin a little faster. It is nonetheless listed among the recommended bench timbers in Finnish sauna-design guidance. Choose well-dried, clear boards, because dry, knot-free timber is what keeps a bench comfortable.
Thermal modification lowers heat transfer: the ThermoWood Association reports conductivity about 20–25% lower than untreated softwood. We found no specific density figure for thermo-alder.
Stability and moisture movement
- Natural alder moves moderately. Red alder’s full green-to-dry shrinkage is 12.6% by volume, against 6.8% for western red cedar.
- Thermo-alder is markedly more stable. Thermal modification typically reduces swelling and shrinkage by 40–80%, depending on species.
These laboratory figures are for ranking species; movement in a well-dried, ventilated room is a fraction of them.
Durability and termites
Alder is rated non-durable to perishable. Inside a heated, well-ventilated sauna that is manageable, because decay needs timber to stay wet. Outdoors it is a real limitation. Claims that alder is “hardy and water-repellent” are marketing, not established properties.
Thermal modification improves decay resistance but does not make timber termite-resistant; the ThermoWood Handbook and Lunawood both say so. We found no Australian termite rating for natural alder. In Queensland, where subterranean and drywood termites both occur, neither form suits exterior or structural use, and termite management for the sauna structure is a matter for the designer and certifier.
Where alder works
Benches, backrests and headrests
Alder is a long-standing bench timber. Tylö offers alder and thermo-alder benches, and Thermory’s sauna range includes thermo-alder bench boards. Its fine texture finishes smoothly against the skin.
Thermo-alder is more brittle than natural alder. ThermoWood tests show splitting strength falls by 30–40% after modification, so benches need adequate thickness, supported spans, rounded edges and sealed end grain. Neither form should be used for structural bench frames.
Walls and ceilings
Both forms are used for panelling. Natural alder gives a warm, light interior that deepens with time; thermo-alder a dark, even one. Thermory sells thermo-alder wall panels in its sauna range.
Commercial rooms
Natural alder is a moderate choice for heavy use. Thermo-alder’s dark, even tone suits busy rooms, because it hides marks well, provided bench ends are sealed and slats can be removed and replaced.
Thermory’s sauna catalogue lists thermo-alder bench boards and panels in both its medium and intense treatment grades. Ask which grade you are being offered, because it affects colour, stability and strength.
Ageing and care
- Alder darkens and reddens over time. Expect the room to warm in tone as it is used.
- Paraffin oil suits alder well. Harvia says its bench oil particularly suits alder and heat-treated wood, and recommends treating benches one to three times a year. Oiling deepens the colour, so test on the underside first.
- One system per surface. Tikkurila says its sauna wax is not for oiled timber; choose oil or wax, not both.
- Towels, cleaning along the grain and light sanding keep benches looking good.
- No varnish or lacquer in the hot room.
- Seal the end grain of thermo-alder bench boards with oil or wax.
See our care guide for routine cleaning.
Advantages and disadvantages
| Alder | Thermo-alder | |
|---|---|---|
| Strengths | Fine, smooth, even grain; resin-free; no scent; warm tone | Very stable; dark, even colour that hides marks; resin-free; slightly lower heat transfer |
| Trade-offs | Denser than aspen or cedar, so slightly warmer to touch; moves more than cedar; non-durable; darkens and reddens | More brittle, so ends must be sealed; smoky scent; density not published; not termite-resistant |
The dust of alder can occasionally irritate the eyes, skin and airways of people machining it. That is a workshop matter, not a concern for bathers.
How alder compares with western red cedar
- Feel: cedar is lighter (about 340–380 kg/m³) than alder (about 450–490 typically), so clear cedar is slightly cooler to sit on.
- Stability: cedar moves least of the mainstream sauna species. Natural alder moves nearly twice as much in laboratory figures; thermo-alder is far more stable.
- Scent: cedar is strongly aromatic. Alder has no scent; thermo-alder a mild smoky one.
- Look: both are warm-toned. Cedar is more varied from board to board; alder is finer and more even.
- Durability: Australian references rate cedar heartwood termite-resistant. Alder and thermo-alder are not rated resistant.
- Surface: cedar is very soft and dents. Alder finishes smoothly; thermo-alder resists marking visually but is brittle at edges.
More detail is in our western red cedar guide and the timber hub.
Specifying alder
Name the species (Alnus glutinosa or A. rubra), a clear grade for anything you touch, the moisture content at delivery and, for thermo-alder, the treatment grade. Australian sauna timber suppliers list both alder and thermo-alder. Ask for chain-of-custody documents if certified supply matters to you.
Compliance and licensed trades
In commercial and multi-residential buildings, linings may be subject to National Construction Code fire hazard requirements, and overseas fire classifications are not Australian ones. Confirm timber selection with the designer and building certifier, and follow the heater manufacturer’s clearances to timber.