Cedar, Aspen, or Thermally Modified Wood: Choosing Sauna Cladding

The interior cladding is the wood you'll actually touch, lean against, and smell every time you step into a sauna, which makes it one of the more personal decisions in a build. It also has to survive conditions that would warp, resin-weep, or rot most ordinary lumber — sustained heat, high humidity, and the occasional direct splash from löyly water. Three materials dominate home sauna interiors, and each makes a real tradeoff.

Cedar: aromatic and naturally durable

Western red cedar has been a default sauna cladding choice for decades, and its appeal is straightforward: a rich, warm scent that intensifies with heat, natural oils that resist rot and insect damage without any chemical treatment, and an attractive reddish-brown grain that darkens handsomely over years of use. Cedar is also naturally low-density, so it doesn't retain heat aggressively against bare skin the way denser hardwoods can.

The tradeoffs are cost and resin. Cedar generally sits at the higher end of sauna cladding pricing, and while its natural oils are part of its rot resistance, they can also bleed slightly in the heat, especially from lower-grade boards with more knots. Clear or knotty-clear grades cost more but minimize this.

Aspen: pale, low-resin, and traditional in Finland

Aspen is the traditional choice in much of Finland, prized specifically for its very low resin content — it won't weep sap onto bare skin or benches the way a resinous softwood might, and it stays a consistent pale, almost white color rather than darkening dramatically over time. It's also typically lighter on the wallet than cedar.

What aspen gives up is that distinctive cedar aroma and cedar's naturally higher rot resistance; aspen relies more on the sauna's own ventilation and drying cycle between uses to stay in good condition over the years, and it's worth choosing boards from a supplier who understands sauna-grade milling rather than general construction-grade aspen.

Thermally modified wood: engineered for the job

Thermally modified wood — typically spruce, ash, or pine heated to high temperatures in a low-oxygen kiln without chemicals — is the newest of the three mainstream options and has gained real traction in modern builds. The heat treatment permanently alters the wood's cell structure, driving out much of the resin and sugars that feed rot and warping, and leaving a noticeably more dimensionally stable board that resists cupping and checking even through repeated heat-and-humidity cycles.

It tends to have a deeper, more uniform brown color straight out of the box (no waiting years for it to darken), and many builders find it the most forgiving option to install because it moves less after installation. The main downside is that it's a newer, more specialized product, so it isn't stocked everywhere, and pricing sits closer to cedar than to standard softwood.

Board thickness, coverage, and estimating quantity

Whichever species you choose, sauna cladding is typically milled as tongue-and-groove boards in a nominal 5/8 inch to 3/4 inch thickness, profiled to interlock cleanly along the walls and ceiling. Ordering by board feet — a volume measurement equal to one square foot of coverage at one inch of thickness — is the standard way suppliers price lumber, because it lets a board of any width or length be compared on equal footing.

A practical estimate starts with your total wall and ceiling surface area, subtracts the door opening, and adds a waste allowance of around 10% for trimming, corner cuts, and the coverage lost to the tongue-and-groove joint itself. Multiplying that adjusted area by your board's nominal thickness gives the board feet to order — a shortcut that happens to be mathematically exact, not an approximation.

A note on finishing

Whatever wood you choose, resist the urge to stain, paint, or varnish the interior. Sauna heat can cause finishes to off-gas or become uncomfortably hot to the touch, and part of the appeal of all three of these woods is that they're built to be left bare, relying on their own natural oils or heat-treated stability rather than a surface coating.

Use our Cladding Board-Feet Calculator to turn your own room's dimensions into an order-ready board-foot quantity, waste allowance included.