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. Four 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.

Hemlock: the fourth quiet option

Hemlock doesn't get mentioned as often as cedar or aspen, but it's a long-standing sauna cladding choice for the same underlying reason as the other two: low resin content, a pale, even color, and a mild scent that doesn't compete with löyly the way cedar's stronger aroma can. It tends to sit in the middle of the price range between aspen and cedar, and like aspen it's often milled specifically for sauna use rather than general construction, so sourcing sauna-grade hemlock from a supplier who understands the application is worth the extra step over a generic lumberyard order. It's a reasonable default when cedar feels like overspending but plain aspen isn't quite the look you're after.

Why the wood species matters more in a sauna than almost anywhere else in a house

Two physical properties explain why cedar, aspen, hemlock, and thermally modified wood dominate sauna interiors while so many other woods don't belong anywhere near one. The first is resin content: a resinous softwood like ordinary pine has sap pockets that soften and can weep, bubble, or even feel hot enough to mark bare skin once the wood is running at sauna temperatures for an extended period — exactly the opposite of what you want touching a bather's back or thighs. The four woods above were chosen specifically because they're naturally low in resin, or, in thermally modified wood's case, have had much of that resin driven out by the heat-treatment process itself.

The second property is thermal conductivity — how readily a material conducts heat into whatever touches it. Wood generally conducts heat far more slowly than metal, tile, or stone, which is exactly why a wooden bench in a 180°F room is tolerable to sit on bare-skinned while a metal fitting at the same air temperature would burn on contact. Low-density softwoods conduct even more slowly than denser hardwoods, which is part of why oak, maple, and other dense hardwoods are rarely used for sauna cladding or benches despite being perfectly durable woods in general furniture and flooring use — they simply feel hotter to bare skin at the same air temperature.

What to actively avoid

Beyond resinous pine, two categories of wood have no place inside a sauna, and it's worth being direct about why. Any pressure-treated or chemically preserved lumber is manufactured for exterior and structural use, where the preservative chemicals are meant to stay put in a damp, room-temperature environment — not to be repeatedly heated well past 150°F for hours over years, which risks off-gassing those chemicals into the air you're breathing in an enclosed room. Painted or varnished wood carries the same problem from a different direction: a surface coating that seals the wood can soften, off-gas, or become uncomfortably hot at sauna temperatures, and it also defeats the purpose of choosing a naturally suitable species in the first place. Bare, untreated, sauna-appropriate wood species are the standard for a reason, not a rustic aesthetic choice. Engineered wood products — plywood, MDF, particleboard — belong outside the hot zone entirely for a related reason: the adhesives that hold them together aren't formulated for sustained sauna heat and humidity, and can soften, delaminate, or off-gas well before a comparable solid board would show any wear at all. If a design calls for engineered sheet goods anywhere near the room (subfloor, backing behind the cladding), that's a detail worth confirming against the product's own rated temperature range rather than assuming it's fine because it's hidden from view.

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. For a concrete sense of scale: a 6-by-6-foot room with a 7-foot ceiling, a 12 sq ft door opening, standard 3/4in board thickness, and that same 10% waste allowance works out to 158.4 board feet, ceiling included. Step up to 1-inch-thick boards on that identical room and the figure rises to 211.2 board feet — thickness scales the order size directly, since board feet is, by definition, area times thickness.

What's behind the boards

The cladding itself is only the visible layer. Behind it, a correctly installed sauna wall needs a vapor barrier — typically a foil-faced layer installed with the foil side facing into the hot room — to stop the high humidity generated during a session from migrating into the wall cavity and insulation, where it can condense, degrade the insulation's performance, and eventually cause rot inside a wall you can no longer see. Getting the foil's orientation and seam-taping right is a construction detail worth following exactly as your insulation and vapor-barrier product's own installation instructions specify, since an installed-backwards vapor barrier can trap moisture rather than blocking it. This is a good example of where the beautiful, tactile choice (which wood, what grain) sits on top of a less visible but equally important construction choice that a builder or contractor experienced with sauna-specific assemblies should get right.

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 four 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. If you're also sizing the heater and bench for the same room, the Sauna Room Size Reference lays out all three side by side for eight common footprints.