Building it yourself
Measure the room’s inside dimensions, run the estimate above, then lay out walls and benches before you order.
- Lay out the room in SaunaDesignPro (opens in a new tab)
- Thermory USA installation guide (opens in a new tab)
Thermally modified aspen for saunas
What is thermo-aspen?
Thermo-aspen is thermally modified aspen: aspen boards treated with heat and steam in a controlled kiln. The process turns the wood a warm brown and makes it more stable through changes in heat and humidity. Thermory’s catalog calls aspen a preferred sauna wood because it is resin-free, smooth, and stays comfortably cool.
Source: Thermory catalog, pp. 4, 6
From Sauna Marketplace, which sells Thermory thermo-aspen. Shop thermo-aspen (opens in a new tab)
Industry test data (ITWA, mostly pine and spruce)
What heat and steam change in wood
Test figures come from ITWA’s handbook, mostly on pine and spruce. For Thermory boards, use Thermory’s catalog. The handbook, from the International ThermoWood Association, lists aspen in both ThermoWood® classes but reports none of these values for aspen.
- Minimum modification temperature
- 320°F
- ITWA’s threshold for thermal modification. The change to the wood is permanent.
- Lower heat conduction
- 20 to 25%
- Thermally modified Nordic softwood versus standard timber, a property the handbook names as suiting saunas.
- Moisture the wood holds
- About half
- Spruce modified at 428°F versus untreated spruce. It stays well below the 20% moisture content at which decay fungi become active.
- Lower VOC emissions
- About 84%
- ThermoWood® pine modified at 446°F versus unmodified pine, in VTT tests.
Plan your room
How much thermo-aspen do you need?
Multiply the net wall and ceiling area in square feet by 12, then divide by the board’s installed cover in inches. Add 5 to 10% for cutting loss, the range Thermory USA gives for wall paneling. STS4 1×4 covers 3.15 in, so each square foot takes about 3.81 linear feet before loss.
Coverage calculator
Room length
Room width
Ceiling height
Estimated STS4 1x4
885linear feet
- Walls
- 182 sq ft
- Ceiling
- 42 sq ft
- Net area
- 211 sq ft
- Rows per wall
- 27
211 sq ft × 12 ÷ 3.15 in cover = 804 ft, plus 10%.
A first-pass estimate for wall and ceiling paneling only. Benches, trim, framing, and bundle rounding are separate, and the manufacturer's drawings govern the installed cover.
Thermory thermo-aspen wall panels, narrowest cover first
STEP 5/4×32.20 in installed coverStepped, with a deep square shadow at each joint
VIRE 1×43.11 in installed coverSawtooth, with angled facets
STS4 1×43.15 in installed coverFlat, with a slim groove at each joint
KYTE 1×43.15 in installed coverRaised ribs with narrow grooves between them
STS4 1×54.33 in installed coverFlat, with a slim groove at each joint, on a wider board
Cover widths come from the Thermory catalog. The swatches show each face’s texture, and Thermory USA notes slight shade differences between production batches, so check a sample before you order.
Sources: Thermory catalog, pp. 8, 9, 10; Thermory USA guide, p. 2
Before you build
What to know before you build
Four details make or break a thermo-aspen room.
- Use stainless fasteners. For its paneling, Thermory USA recommends stainless steel clips set with a staple gun and says finishing nails are not suitable. For ThermoWood® products, the handbook sets Type 304 (A2) stainless as the minimum for sauna cladding and benches, because the wood is acidic, about pH 4, and other fasteners stain it.Sources: Thermory USA guide, pp. 2, 3; ITWA handbook, pp. 12, 44Fastener guide
- Leave a ventilated gap. The handbook’s sauna drawings and Thermory USA’s guide both keep battens no more than 23-1/2 in apart. The drawings leave an air gap of at least 13/16 in behind the panels. Thermory USA calls for battens at least 13/16 in thick and 1-13/16 in wide, run across the boards, and recommends about 15-3/4 in between them. A 1x furring strip is 3/4 in thick, just under 13/16 in; 5/4 stock, about 1 in thick when dry, clears it.Sources: ITWA handbook, p. 38; Thermory USA guide, p. 2; USDA Wood Handbook, Table 6-6, p. 6-11Framing guide
- Choose a sauna finish. Finish with a sauna wax or protector made for saunas, as Thermory USA recommends for its paneling. Never use linseed oil, which the ThermoWood® Handbook says enables fungal growth.Sources: Thermory USA guide, p. 4; ITWA handbook, p. 33Finish guide
- Frame the benches, and finish wet work first. Thermory USA says to add bench supports behind the paneling, which must not carry the benches’ weight. The supports carry the load, and thermo-aspen bench boards, such as Thermory’s SHP, sit on them. Thermory USA also says to finish all wet work before paneling and to keep the bundles unopened in the room for a few weeks.Sources: Thermory USA guide, pp. 2, 4; Thermory catalog, p. 16Storing and cutting guide
Two ways in
Building your own sauna, or specifying one?
Specifying for a client
Take profile sizes, installed cover, and fixing details from Thermory’s documents. Read the ITWA handbook as background on thermally modified wood.
Read the spec guideHow it’s made
A controlled kiln process, with a schedule for each species
Thermory USA says it tailors a separate process to each species by controlling temperature and moisture.
For its sauna materials, Thermory’s catalog describes a high-pressure chamber. The wood is heated to as much as 356°F while pressure rises to about 130 psi, with only heat and steam. Thermory says this controlled chamber lets the wood change without over-drying, which reduces internal stress and keeps cracking and warping to a minimum. Final color depends on the species, the temperature, and the time.
Sources: Thermory USA brochure, p. 3; Thermory catalog, p. 6
How thermo-aspen is made, step by stepFrom the journal
Guides for sauna builders
How Much Thermo-Aspen Do I Need? Sauna Coverage Math
Multiply the net wall and ceiling area by 12, divide by the board’s installed cover in inches, and add 5 to 10% for cuts. A 5 by 7 foot room with a 7 foot ceiling and one door takes about 797 linear feet of STS4 1×4.
Read the articleThermo-Aspen or Cedar for Your Sauna? How to Choose
Cedar gives an aromatic, reddish room; thermo-aspen gives a warm brown one. How they compare on look, scent, feel on skin, movement, and care.
Read the articleThermo-Aspen vs Plain Aspen: What Do Heat and Steam Change?
Thermal modification uses heat and steam to permanently change the wood’s color and how it takes up moisture. It stays aspen: soft, smooth, and fine-grained.
Read the articleIs thermo-aspen waterproof?
No. Thermal modification lowers how much water wood takes up, yet the ThermoWood® Handbook does not recommend ThermoWood® products for direct contact with soil or water. In a sauna, Thermory says to throw water only on the heater, not on walls or benches, and to keep it from pooling. After each session, dry the room with the heater on and the vent open.
Does thermo-aspen smell?
Yes, a smoky scent, most noticeable when the sauna heats. The ThermoWood® Handbook says it probably comes from compounds called furfurals, and that it seems stronger than untreated wood’s even though VOC emissions are far lower. Thermory USA says the scent weakens over time but never disappears completely. Long heating with airing speeds that up, and a suitable wax or maintenance product can reduce it.
Will thermo-aspen change color over time?
Yes, gradually. Thermory USA says wooden sauna surfaces may darken over time, and faster where hot steam reaches them. A breathable protector applied before first use guards against moisture and dirt and makes cleaning easier, per Thermory’s catalog.
Should I order a thermo-aspen sample before a full room?
Yes. A sample shows tone, texture, profile depth, and machining under your own lighting. Thermory USA notes slight shade differences between production batches, so treat the sample as a guide and check the delivered order before cutting.