Thermo-Aspen or Cedar for Your Sauna? How to Choose

Cedar and thermo-aspen at a glance

The two woods differ most in what you see and smell. They are close on feel, and the care routine is the same.

Look: reddish cedar or warm brown thermo-aspen

Thermory describes western red cedar’s rich reddish color and naturally varied tones. It says thermal modification gives a rich, warm brown tone, and that the final color depends on the species and on treatment temperature and time Thermory catalog, pp. 5, 6. The USDA describes cedar heartwood as reddish or pinkish brown to dull brown USDA Wood Handbook, p. 2-17.

Thermory USA notes slight shade differences between production batches and suggests opening several packages to match boards on the wall. It adds that wood surfaces may darken over time, faster where hot steam reaches them Thermory USA guide, pp. 2, 4.

For the palest room, plain aspen has what Thermory calls a clean light tone. Mixed rooms work too: Thermory’s project photos include cedar walls with alder benches, and alder walls with thermo-aspen benches Thermory catalog, pp. 4, 13, 28. The thermo-aspen vs plain aspen post compares the two aspens.

What each wood smells like

Thermory describes western red cedar as valued for its aromatic scent Thermory catalog, p. 5. Thermo-aspen has a characteristic scent of its own. Thermory USA says that scent weakens over time but never completely disappears. Long heats with the room aired disperse it faster, a suitable wax reduces it, and washing untreated boards with water strengthens it Thermory USA guide, p. 4.

If scent matters most to you, smell a sample of each wood before you decide. The VOC test post covers what the emission tests on thermally modified wood found.

Feel on bare skin

Thermory says western red cedar stays cool in high heat, and that aspen stays comfortably cool and has a smooth texture. It adds that thermally modified wood stays cooler to the touch on benches and walls Thermory catalog, pp. 4, 5, 6.

Test figures come from ITWA’s handbook, mostly on pine and spruce. For Thermory boards, use Thermory’s catalog. The handbook reports 20 to 25% lower thermal conductivity in thermally modified Nordic softwood than in standard timber, and names saunas among the uses this suits ITWA handbook, p. 28. Less conductive wood passes heat to your skin more slowly.

If a bench still runs hot, Thermory’s fix is a seat cover or a dry towel Thermory catalog, p. 39. The bench comfort post goes deeper.

Movement with sauna humidity

The USDA describes western red cedar as having very low shrinkage USDA Wood Handbook, p. 2-17. Thermory says thermal modification gives its wood excellent dimensional stability under large swings in temperature and humidity Thermory catalog, p. 6.

In VTT tests on spruce reported by ITWA, modification at 428°F about halved the moisture the wood settles at in a given humidity, and swelling fell significantly ITWA handbook, pp. 26, 27.

Every sauna wood still moves as the room heats and dries. Thermory asks for a yearly check of bench screws for that reason Thermory catalog, p. 39. Thermory USA calls it best practice to keep paneling in the room for a few weeks before installing it, and to finish any wet work first Thermory USA guide, p. 2. The framing and air-gap guide covers the ventilated space behind the boards.

Care is the same for both

After each session, Thermory asks you to wipe benches with a damp cloth, skip running water, and leave the heater on for at least half an hour with the vent open and the door closed. Before first use and at least once a year, it recommends a breathable wood protector on all surfaces and sauna wax on walls and floor grids Thermory catalog, p. 39. Never use linseed oil, which the ThermoWood® Handbook says enables fungal growth ITWA handbook, p. 33.

Decay fungi need moisture. The USDA says keeping wood at 20% moisture content or less can prevent their growth, and it rates western red cedar heartwood as resistant to decay USDA Wood Handbook, p. 14-6 and Table 14-1. ITWA reports that the equilibrium moisture content of thermally modified timber stays well below 20% at any relative humidity ITWA handbook, p. 26. The bench oiling guide has the finishing steps, and can a sauna get moldy? covers airing.

Choosing your room

• If an aromatic scent and a reddish color matter most, shortlist western red cedar.
• For a warm brown room with a smooth surface, shortlist thermo-aspen and pick a profile on the profiles page.
• For the palest room, look at plain aspen.
• Put samples side by side in your own light. Order a sample of Thermory thermo-aspen, and ask your cedar supplier for one too.
• Plan the room in the room planner and check covering widths on the spec guide.

Thermo-aspen wall cladding and bench boards are listed on the Sauna Marketplace thermo-aspen page. For short answers about specific boards, see the questions page or the buyer’s guide.