Cedar vs. Spruce vs. Aspen: Choosing the Right Sauna Wood

Sauna wood is chosen by job. Thermo spruce sauna wood is heat-treated Nordic spruce used on outdoor shells. Untreated spruce, western red cedar, and aspen answer different jobs: indoor walls, an aromatic North American cabin, and the boards you sit on.

Match the board to the climate and to the part of the room. A shell that can take weather is not the same product as a bench that stays tolerable on bare skin. Several kits we sell already mix those jobs in one cabin.

What thermo spruce sauna wood is

Thermo spruce sauna wood starts as Nordic spruce. It then goes through thermal modification: elevated heat in the presence of steam, with no chemical preservative added in that process.¹ The industry method uses two standard classes for Nordic softwoods. Thermo-S is treated near 190°C. Thermo-D is treated near 212°C. Hardwoods such as aspen use their own class temperatures.² A cabin stave is not labeled with those class names on every product page. Treat the figures as the process, not as a stamp on one kit.

The heat changes the board through its thickness. Resin is driven off. The wood takes up less moisture, moves less, and resists decay better than the same species left untreated. It also conducts heat less readily, which is why heat-treated boards are used on seats as well as on shells.¹² The color darkens through the piece. That is a side effect of the process, not a stain.

Those gains have limits. It is still wood. Standing water and a room that never dries will still damage it. The association does not recommend the material for contact with soil or water.²

On the outdoor traditional kits listed under thermo-spruce, the shell is thermo-spruce. The seats in that pairing are thermo-aspen, which stays cooler on skin.¹¹¹²¹³ Shop thermo-spruce wood saunas for that outdoor shell.

Spruce sauna wood without the thermo step

Spruce sauna wood, left untreated, is used as wall and ceiling board. A current indoor cabin line uses vertical spruce panelling inside the room.⁸ The look is lighter. Knots stay visible. Resin is still in the tree.

That is why untreated spruce and thermo spruce are different products. The first is a bright interior wood with the original resin and moisture movement. The second has had that resin cooked out and the moisture behavior changed.¹²

Untreated spruce is not banned from a hot room. It is also not only an indoor species on this site. One outdoor cabin we list uses 1.65-inch Nordic spruce wall beams and grade-A aspen benches.¹⁰ See Nordic spruce wood saunas for that kit. Do not buy untreated spruce as if it were thermo spruce. Do not assume every spruce cabin is indoor-only.

Grade still matters. Boards that weep resin at bathing temperature belong off the bench.

Western red cedar sauna

Western red cedar is Thuja plicata. The heartwood is rated resistant to very resistant to decay. The wood is light. It is not immune to termites.³ The scent is part of why North American buyers search for a western red cedar sauna.

On this site a cedar cabin is often two species. Infrared and infra-steam rooms we carry use a clear grade western red cedar interior and a heat-treated Canadian aspen exterior.¹⁴ The cedar is what you see and smell inside. The aspen is the weather face. Shop western red cedar wood saunas for those rooms.

Eastern white cedar is a different tree, Thuja occidentalis. Its heartwood is also rated resistant to very resistant to decay, and the wood has its own cedar odor.⁵ The two white-cedar cabins we list are built from that species, not from western red cedar. They are listed under eastern white cedar wood saunas.

Aspen sauna

An aspen sauna, in practice, is usually aspen where you sit. Aspen is light and soft. Untreated aspen heartwood is rated slightly resistant or nonresistant to decay, so it is a poor outdoor shell on its own.⁴ That is why it shows up on benches and backrests.

A Finnish interior line sells benches in aspen and in heat-treated aspen, alongside alder and abachi.⁷ Thermal modification is used on aspen for the same reasons it is used on spruce: less moisture movement and lower thermal conductivity on the seat.¹⁹ In the outdoor pairing we stock, thermo-aspen is the interior and the bench wood. Thermo-spruce is the exterior.¹³

The “Canadian aspen” collection label is not an all-aspen traditional room. On this catalog that label is the aspen exterior on the cedar-interior infrared set already covered above.¹⁴ If you want aspen on the skin, look at the seats inside a thermo-spruce kit, or at the SaunaLife G2 pairing of spruce walls and aspen benches.¹⁰¹³

Which wood for which job

• Outdoor traditional cabin: thermo-spruce shell and a cooler seat wood. Start at the thermo-spruce collection. The SaunaLife line is where that pairing is spelled out on the brand page.
• Cedar aroma inside an infrared or infra-steam room: western red cedar interior.¹⁴ Shop western red cedar wood saunas.

Mixing species in one room is normal. The kits above already do it. Put the weather-resistant board on the outside. Put the cooler seat board where you sit.

Basswood is a common US bench question. It is not a cabin line we merchandise. Pressure-treated yard lumber does not belong in the hot room. The pine cabin collection on this site is empty, so do not plan a pine sauna from that page.

What wood will not fix

The wrong species will not save a wet room. Heartwood ratings describe decay resistance when the board can dry. They do not replace an inlet and an outlet, or a door left open after the session. That failure mode is in Sauna Ventilation: The Finnish Two-Vent Method.

A heater maker we carry tells owners not to lacquer the interior of the hot room. It recommends paraffin oil on benches if the wood will be treated at all, and a towel on every sit.⁶ Those habits matter more than swapping one wall species for another after the cabin is up.

Sources

1. International ThermoWood Association, “What is ThermoWood,” thermowood.com/thermowood
2. International ThermoWood Association, ThermoWood product classes (Thermo-S near 190°C and Thermo-D near 212°C for Nordic softwoods; application limits including soil and water), thermowood.fi/1 and ThermoWood handbook / brochure 2025–2026
3. USDA Forest Products Laboratory, “Western redcedar (Thuja plicata),” American Woods tech sheet, fpl.fs.usda.gov
4. USDA Forest Products Laboratory, “Aspen (Populus spp.),” American Woods tech sheet, fpl.fs.usda.gov
5. USDA Forest Products Laboratory, “Northern white-cedar (Thuja occidentalis),” American Woods tech sheet, fpl.fs.usda.gov
6. Harvia Support, “Maintaining your sauna,” updated 29 January 2026, support.harvia.com
7. Harvia, Formula 22 bench set, heat-treated aspen, harvia.com
8. Harvia, Panorama Large indoor sauna, harvia.com
9. Harvia, “The Ronald McDonald House sauna renovation project,” harvia.com
10. SaunaLife, Garden-Series Model G2, saunalife.com
11. SaunaLife, Ergo Elegance-Series Model EE8G, saunalife.com
12. SaunaLife, Cube-Series Model CL12GCP, saunalife.com
13. My Sauna Plus, SaunaLife collection FAQ, mysaunaplus.com/collections/saunalife
14. Finnmark Designs, Trinity Combination wood-type specification and Infrared Sauna Wood page (clear “A” grade Canadian Western Red Cedar interior; Thermal Plus Canadian Aspen exterior; heat and steam only), Trinity spec and Infrared Sauna Wood