How to Frame Sauna Paneling: Battens, Air Gap, Thickness

What the handbook drawings specify

The handbook draws one sauna ceiling and one sauna wall, built the same way. A frame carries battens, panels are fastened to the battens, and arrows show air moving through the gap behind the boards, upward on the wall. The drawings label the panels Thermo-S or Thermo-D, the handbook’s two product classes: S for stability, D for durability ITWA handbook, pp. 18, 38. The drawings are not species-specific, and the handbook’s species table lists aspen for ThermoWood® products in both classes ITWA handbook, p. 9. The Thermo-S vs Thermo-D post explains how the classes differ, and the process page covers how thermal modification works.

Read the 23-1/2 in limit conservatively, as center to center. A 16 in on-center layout fits inside it. A 24 in on-center layout does not.

The panel minimum matches the handbook’s general advice for indoor wall and ceiling cladding. The same page notes that ThermoWood® profile boards may skip relief grooves on the back, because the wood’s dimensional stability makes them unnecessary ITWA handbook, p. 16.

Compare the handbook’s drawings of a ceiling in a dry space and an ordinary interior wall. They show the same panel thickness and the same 23-1/2 in batten limit, with no dimensioned air gap ITWA handbook, pp. 36, 37. The sauna versions add the minimum gap and the back-ventilation arrows. That fits the handbook’s use-class table, which places saunas in use class 2: indoors, but potentially exposed to moisture and condensation ITWA handbook, p. 29.

For benches, the drawing shows boards at least 1-1/16 in thick carried on their own joists ITWA handbook, p. 39. Plan that support in the framing, as the checklist below sets out.

Why a 3/4 in furring strip falls just short

A 1×2 or 1×3 furring strip is nominal 1 in stock, which dresses to 3/4 in thick when dry. The handbook drawings and Thermory USA’s guide both call for at least 13/16 in behind the panels ITWA handbook, p. 38 Thermory USA guide, p. 2. So a 1x strip on its own leaves the gap about 1/16 in shallow.

Thermory’s guide also sets a minimum batten width of 1-13/16 in Thermory USA guide, p. 2. A 1×2 misses both numbers. Going by the USDA’s dressed lumber sizes, two common stock sizes clear both USDA Wood Handbook, Table 6-6, p. 6-11:

• Use 5/4 stock. Nominal 1-1/4 in lumber dresses to 1 in thick, and a nominal 3 in face dresses to 2-1/2 in.
• Rip battens from 2x lumber, which dresses to 1-1/2 in thick. It clears the gap with room to spare and takes more depth from the room.

Plan the finished room from the face of the paneling. Thermory lists its thermo-aspen STS4 panels at 0.59 in thick in most widths Thermory catalog, p. 8. On 1 in battens, that build-up is about 1-5/8 in per wall, so each interior dimension shrinks by about 3-1/4 in. Run that arithmetic before you size the heater or cut bench lengths. The 3D sauna designer opens thermo-aspen profiles in SaunaDesignPro for layout.

Behind the battens sits the vapor barrier, covered in the Sauna Marketplace guide to sauna vapor barriers. One handbook rule ties the layers together: pick nail and screw lengths that do not pierce an air or moisture barrier ITWA handbook, p. 44.

What Thermory’s US guide says about battens

Thermory USA publishes an installation guide for its sauna wall paneling. Its batten rules, in the order the guide gives them, with its inch figures in US fractions Thermory USA guide, p. 2:

• Install the underlay battens first, at least 13/16 in thick and 1-13/16 in wide.
• Horizontal paneling goes on vertical battens. Vertical paneling needs horizontal battens.
• The battens “must ensure air movement behind the wall paneling across the entire extent of the paneling.”
• Battens must be straight and level before any paneling goes up.
• Recommended spacing is about 15-3/4 in. Spacing must not exceed 23-1/2 in.

The handbook drawings and Thermory USA’s guide both call for at least 13/16 in of depth behind the panels and no more than 23-1/2 in between battens ITWA handbook, p. 38. The guide adds a minimum width and a recommended spacing. Its 15-3/4 in is a little tighter than a 16 in on-center layout. For Thermory panels, lay the battens out at the guide’s figure.

The handbook makes the same point about ThermoWood® products: install them according to the manufacturer’s instructions ITWA handbook, p. 44.

Order of work, tongue direction, and fastening

Finish every wet job first. Thermory’s guide warns that wet work after paneling makes the boards absorb moisture and swell. If the ceiling is paneled too, do it first and cover the walls after. Leave a sufficient airing gap between the first and last ceiling rows and the walls Thermory USA guide, pp. 2, 3.

Horizontal paneling

The guide installs horizontal paneling from top to bottom, tongue side up and groove side down. The guide files this under hidden fixing: staples go in from the groove side. On adjacent walls, work one row at a time around the whole room, so board ends line up in the corners. Check the first row with a level, then again at least every three rows Thermory USA guide, pp. 2, 3.

Vertical paneling

Leave a gap of at least 7/16 in at the adjacent corner or board for the first and last panels, and cover it later with corner moulding Thermory USA guide, p. 3. The handbook adds a general point for clip-fixed vertical cladding. Keep the boards from sliding down, with support at the bottom or nails or screws at the top or bottom ITWA handbook, p. 45.

Fasteners

The guide recommends a staple gun and stainless steel clips, and says finishing nails are not suitable. Set the staple depth so the clips neither stand proud of the surface nor drive in far enough to damage the wood Thermory USA guide, pp. 2, 3. For ThermoWood® products, the handbook’s corrosion table sets Type 304 stainless as the minimum for sauna cladding and sauna benches ITWA handbook, p. 44. The stainless fasteners post covers grades and why they matter.

The profiles page compares the thermo-aspen profiles, and the spec guide collects the install figures in one place.

Framing checklist before the first board

Work through this list with the frame open, before any panel is fixed. Items without a handbook page come from Thermory USA’s guide Thermory USA guide, pp. 2, 3, 4.

• Every wet operation is finished and dry.
• The vapor barrier is in place, and fastener lengths are chosen so nothing pierces it ITWA handbook, p. 44.
• Battens leave a ventilated gap of at least 13/16 in, the minimum in both sources ITWA handbook, p. 38. For Thermory panels they are also at least 1-13/16 in wide.
• Battens run perpendicular to the paneling: vertical battens for horizontal boards, horizontal battens for vertical boards.
• Spacing follows the recommended 15-3/4 in for Thermory panels and is never more than 23-1/2 in, the maximum in both sources ITWA handbook, p. 38.
• Battens are straight and level, and air can move behind the whole lining.
• Bench supports are built into the framing. The paneling must not carry the weight of the benches.
• Panels have sat in the room for a few weeks, in unopened packages, ideally at 59 to 72°F.
• The ceiling goes up before the walls.

Order enough panels in one go. Thermory’s guide puts material loss at about 5 to 10%, depending on length, and notes slight shade differences between production batches Thermory USA guide, p. 2. The coverage post works through the quantities, and the questions page answers shorter install questions.