Does Thermo-Aspen Off-Gas in a Sauna? What the VOC Tests Found

What goes into the wood?

Heat and steam. For its sauna materials, Thermory uses a high-pressure chamber in which the wood is heated up to 356°F while pressure rises to about 130 psi Thermory catalog, p. 6. The handbook defines thermal modification as heat and steam at 320°F or higher, and says the resulting changes in the wood are permanent ITWA handbook, p. 4. The process page covers the kiln stages.

The handbook also reports an EU research test from 1998 in which water leached from thermally modified spruce showed no toxic effect on a small freshwater crustacean or on marine bacteria ITWA handbook, p. 12.

Does thermally modified wood off-gas?

All the pine in the test gave off VOCs, and the modified samples gave off less. VTT, the Technical Research Centre of Finland, measured unmodified pine and pine modified for four hours at 356°F or at 446°F, seven to eight weeks after treatment ITWA handbook, p. 30.

Most of unmodified pine’s emissions were terpenes. Terpenes belong to the wood’s extractives, and most extractive compounds evaporate during thermal modification ITWA handbook, pp. 12, 30. ITWA’s conclusion, in the English edition’s words: ThermoWood® is “a safe choice for indoor applications in terms of its emission levels” ITWA handbook (English), p. 30.

Where the scent comes from

The handbook says the smoky scent of thermally modified timber probably comes from compounds called furfurals. The scent is clearly noticeable and can seem stronger than that of standard timber, yet the VOCs released are only a fraction of those from standard pine ITWA handbook, p. 30. A stronger smell does not mean higher total emissions.

Thermory USA’s installation guide says the characteristic scent of thermally modified wood weakens over time but never disappears completely. Its advice for managing it Thermory USA guide, p. 4:

• Heat the sauna for long sessions while airing it. The scent disperses faster that way.
• Treat the wood with a suitable wax or maintenance product, which reduces the scent.
• Do not wash untreated boards with water, which strengthens the scent.

Check the boards when they arrive, before you cut. Thermory USA asks that any concern about scent or quality be raised before the wood is sawn, installed, or treated Thermory USA guide, pp. 1, 4. To see and smell the wood before you buy, order a sample.

Heat, humidity, and finish

The handbook places saunas in use class 2, indoor use potentially exposed to moisture and condensation, and recommends both ThermoWood® classes there ITWA handbook, p. 29. Like standard timber, thermally modified wood takes up and gives back moisture, which evens out swings in room humidity, though less, because its equilibrium moisture content is lower ITWA handbook, p. 30. Its equilibrium moisture content stays well below the 20% at which decay fungi become active ITWA handbook, p. 26.

That covers moisture drawn from the air, not liquid water. Throw water only on the heater, never on walls or benches, and do not let it pool on the wood Thermory catalog, p. 39. For the surface side of the question, see can a sauna get moldy?

A finish that seals the wood too well stops moisture moving between the room air and the boards. For sauna walls, ceilings, and benches, the handbook recommends mineral oil and rules out linseed oil, because linseed oil enables fungal growth ITWA handbook, pp. 30, 33. For its boards, Thermory recommends a breathable wood protector on all wooden surfaces, and sauna wax or a similar coating on walls and floor grids Thermory catalog, p. 39. For Thermory boards, follow Thermory. The bench oiling guide has the steps.

Checklist: from delivery to the first heats

Most steps below come from Thermory USA’s installation guide Thermory USA guide, pp. 1, 2, 4. Where a step comes from another source, it is cited in place.

• Check the boards on arrival, and raise any scent or quality concern before sawing, installing, or treating them.
• Cut and sand with good dust extraction and a respirator. The handbook describes the dust as dry and fine and calls for a respirator whenever dust is generated ITWA handbook, p. 31.
• Fasten the paneling with a staple gun and stainless steel clips. The fasteners post has the details.
• Finish with sauna wax or a similar coating made for sauna use Thermory catalog, p. 39. Never use linseed oil, which the handbook says enables fungal growth ITWA handbook, p. 33.
• Run long heats with the room aired to disperse the scent faster.
• After each session, leave the heater on for at least half an hour with the air vent open and the door and windows closed.
• Stay within the heater maker’s maximum temperature. Thermory USA warns that excessive heat may pose a fire hazard or cause the wood to warp.

For planning the room, the buyer’s guide covers species and care, and the profiles page compares the wall boards. The spec guide holds the coverage math, and the designer opens each profile in a 3D sauna plan. Cutting advice is in cutting, sanding, and storing thermo-aspen. Thermory thermo-aspen boards are listed on the Sauna Marketplace thermo-aspen page.