The thermo-aspen
buyer’s guide.

01 · Choose

Choose the wood for the room you want.

Start with tone and touch, then pick a profile. Thermory’s June 2026 catalog calls aspen a preferred sauna wood because it is resin-free, smooth, and stays comfortably cool, and it offers aspen natural or thermally modified. Thermory catalog, p. 4

For benches, the number to know is heat flow. In the ThermoWood® Handbook from the International ThermoWood Association (ITWA), thermally modified Nordic softwood conducts heat 20 to 25% less than standard timber, a property the handbook names as suiting saunas. It gives no aspen value. ITWA handbook, p. 28 Thermory’s catalog says its modified wood stays cooler to the touch on benches and walls. Thermory catalog, p. 6

Test figures come from ITWA’s handbook, mostly on pine and spruce. For Thermory boards, use Thermory’s catalog.

Then pick the face. The profile comparison sets STS4, STEP, KYTE, and VIRE side by side, with the installed cover of each.

02 · Estimate

Estimate from installed cover, not board width.

Linear feet equal the net wall and ceiling area in square feet, times 12, divided by the board’s installed cover in inches. Add 5 to 10% for cuts; Thermory USA’s guide puts wall paneling loss in that range, depending on board length. Thermory USA guide, p. 2

Cover is narrower than the board, because each tongue sits inside the next board’s groove. Thermory’s catalog gives STS4 thermo-aspen a covering width of 3.15 in for the 1×4 and 4.33 in for the 1×5. Thermory catalog, p. 8

A worked example: a 5 by 7 foot room with a 7 foot ceiling has 203 sq ft of walls and ceiling. Less one 13 sq ft door, that leaves 190 sq ft. At a 10% allowance, it takes about 797 linear feet of STS4 1×4 or 580 of STS4 1×5. Bench boards, trim, battens, and fasteners are separate counts.

03 · Build

Build on battens with stainless fasteners.

Sauna paneling hangs on battens over a ventilated gap, not flat on the wall. The handbook drawings and Thermory USA’s guide both call for battens no more than 23-1/2 in apart, and both set 13/16 in as the minimum behind the panels: the handbook for the ventilated gap, Thermory USA for batten thickness. Thermory USA also asks for battens at least 1-13/16 in wide and recommends about 15-3/4 in between them. ITWA handbook, p. 38 Thermory USA guide, p. 2

Battens 16 in on center sit close to that recommendation and within the maximum; 24 in on center exceeds it. 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.

Use stainless steel for every clip, staple, nail, and screw. For its paneling, Thermory USA’s guide recommends a staple gun with stainless steel clips. Thermory USA guide, p. 2 For ThermoWood® products, Table 10 of the handbook sets class A2 (Type 304) as the minimum for sauna wall and ceiling cladding and for sauna benches, and names Type 316 (A4) as the acid-resistant step up. The reason is acidity: ThermoWood® products measure about pH 4, against 4.5 to 5 for standard wood, and other fasteners react with the wood and stain around the fastener. ITWA handbook, pp. 12, 44

Carry the benches on supports fixed behind the paneling, because Thermory USA says the paneling must not carry their weight. Thermory USA guide, p. 4 Thermo-aspen bench boards, such as Thermory’s SHP, sit on that support. Thermory catalog, p. 16

Before installing, Thermory USA’s guide recommends storing paneling for a few weeks in the room where it will go, at 59 to 72°F, in unopened packages. Thermory USA guide, p. 2

04 · Care

Protect the wood before first use. Skip linseed oil.

Thermory’s catalog calls for a breathable wood protector on all wooden surfaces before first use, and sauna wax or another suitable coating on walls and floor grids. Treat again at least once a year, and more often with heavy use. Thermory catalog, p. 39

For ThermoWood® products, the handbook recommends mineral oil for interior walls, ceilings, and benches in saunas, and rules out linseed oil because it enables fungal growth. It adds that acidity keeps some finishes, especially some water-based ones, from adhering well, so check with the finish maker. ITWA handbook, p. 33

Dry the room after every session. Thermory recommends leaving the heater on for at least half an hour with the vent open and the door and windows closed. Throw water only on the heater, and keep it from pooling on wooden surfaces. Thermory catalog, p. 39

The 20% line explains the routine. Decay fungi become active above 20% moisture content, and in ITWA data thermally modified timber stays well below that in air of any relative humidity, so pooled water is the thing to manage. ITWA handbook, p. 26

From plan to order

Put the room into SaunaDesignPro.

Pick a thermo-aspen profile on the designer page to open it in SaunaDesignPro. Add the room dimensions, benches, heater, openings, and ventilation there. The spec guide holds the claim table and field checklist for a contractor or designer.