Sauna Wood Thermal Conductivity: Why Cedar Feels Cool at 90°C

You can sit on a western red cedar bench at 90°C for 15 minutes without discomfort. Press your back against the same bench and it feels warm, not hot. Now touch a stainless steel screw head protruding from that bench (at the same 90°C) and you will pull your hand away in less than a second with a contact burn.

Both objects are at the same temperature. The difference isn’t temperature but the rate at which each material transfers its thermal energy into your skin. This rate is governed by two related material properties: thermal conductivity and thermal effusivity. Understanding these properties explains why wood is the only appropriate material for sauna contact surfaces, why some wood species are better than others, and why exposed metal, tile, and even knots in wood can cause burns.

What Is Thermal Conductivity and Why Does It Matter for Saunas?

Thermal conductivity (k) measures how fast a material transfers heat. Western red cedar at 0.11 W/m-K transfers heat 145 times slower than stainless steel (16 W/m-K), which is why you can sit on a 90°C cedar bench without burning.

Thermal conductivity, denoted k, measures how readily a material conducts heat through its bulk. The unit is W/m-K (watts per meter-kelvin). A high-k material transfers heat rapidly. A low-k material resists heat flow.

The governing equation is Fourier’s Law of heat conduction:

q = k * (T_hot - T_cold) / L

Where q is the heat flux (W/m^2), k is the thermal conductivity, (T_hot - T_cold) is the temperature difference across the material, and L is the thickness of the material.

For a sauna bench scenario: the bench surface is at 90°C, your skin surface is at 33°C, and the contact is essentially surface-to-surface (L approaches zero for the interface, but in practice we consider the thin boundary layer).

Thermal Conductivity of Common Sauna Materials

The numbers are stark. Stainless steel conducts heat 145 times faster than cedar. Aluminum, used in some heater guards and fixtures, conducts heat nearly 2,000 times faster. This is why a single exposed screw head, bolt, or metal bracket at 90°C can cause an instant contact burn while the surrounding wood at the same temperature feels merely warm.

What Is Thermal Effusivity and Why Does It Determine Contact Comfort?

Thermal effusivity determines the instantaneous contact temperature when your skin touches a hot surface. Cedar at 90°C produces a contact temperature of only 43.5°C (comfortable), while stainless steel at the same temperature produces 82.5°C (instant burn).

Thermal conductivity alone doesn’t fully explain the sensation of touching a hot material. The more relevant property for contact comfort is thermal effusivity, denoted e, which determines the interface temperature when two materials at different temperatures are brought into contact.

Thermal effusivity is defined as:

e = sqrt(k * rho * c_p)

Where k is thermal conductivity, rho is density (kg/m^3), and c_p is specific heat capacity (J/kg-K).

When your finger (at 33°C) touches a bench (at 90°C), the instantaneous contact interface temperature T_contact is determined by:

T_contact = (e_bench * T_bench + e_skin * T_skin) / (e_bench + e_skin)

A material with low thermal effusivity produces a contact temperature closer to your skin temperature (it “feels” cooler. A material with high thermal effusivity forces the contact temperature closer to its own temperature) it “feels” hotter.

Thermal Effusivity of Sauna Materials

Using the contact temperature equation for cedar vs. Steel at 90°C (with skin at 33°C and skin effusivity of approximately 1,194 J/m^2-K-s^0.5):

Cedar: T_contact = (240 * 90 + 1,194 * 33) / (240 + 1,194) = 43.5°C. Warm but well below the pain threshold of approximately 48°C.

Stainless steel: T_contact = (7,937 * 90 + 1,194 * 33) / (7,937 + 1,194) = 82.5°C. This exceeds the burn threshold (approximately 48°C for pain, approximately 60°C for tissue damage) immediately and will cause a contact burn within fractions of a second.

This calculation demonstrates precisely why material selection for sauna contact surfaces isn’t optional. It is a safety requirement dictated by physics.

Why Does Wood Work So Well as a Sauna Bench Material?

Wood’s cellular structure traps air (k = 0.026 W/m-K) inside its cells, creating a composite material with very low effective thermal conductivity. And lower-density woods contain more trapped air, making them even better insulators against skin contact burns.

Wood is an exceptional sauna material because of its cellular structure. The bulk of a wood sample consists of air-filled cells. Air has a thermal conductivity of only 0.026 W/m-K. About four times lower than the wood cell walls themselves. The result is a composite material whose effective thermal conductivity is dominated by its trapped air content.

Lower-density woods contain more air per unit volume and therefore have lower thermal conductivity. This is the fundamental relationship:

This is why the tropical hardwood ipe (density ~1,000 kg/m^3, k ~0.25 W/m-K), despite being beautifully durable and rot-resistant, is a terrible choice for sauna benches. It would feel painfully hot at normal sauna operating temperatures.

The Effect of Moisture Content

Thermal conductivity of wood increases with moisture content because water (k = 0.60 W/m-K) is 20-25 times more thermally conductive than air (k = 0.026 W/m-K). As moisture replaces air in the cell cavities, the effective thermal conductivity rises.

In a sauna operating at 80-90°C with low baseline humidity (10-15% RH), the equilibrium moisture content of the wood is extremely low. Typically 2-4% by mass. This is well below the 8-12% range typical of indoor furniture and contributes to the low thermal conductivity of sauna wood in service.

However, wood that is repeatedly soaked (from excessive löyly water running down walls, or splash from a cold shower inside the sauna) will have locally elevated moisture content and will feel hotter in those areas. This is one practical reason to manage water use and ensure proper drainage and ventilation.

Why Are Knots a Problem on Sauna Bench Surfaces?

Knots are 2-4 times more thermally conductive than surrounding clear wood due to higher density, resin content, and cross-grain orientation. Producing contact temperatures of 55-65°C versus 43°C for clear cedar, which exceeds the pain threshold and can cause minor burns.

Knots are the remnants of branch connections in the tree trunk. They differ from the surrounding clear wood in several important ways:

1. Higher density. Knot wood is significantly denser than clear wood. Typically 700-1,000 kg/m^3 for softwood knots vs. 350-450 kg/m^3 for the surrounding clear wood.
2. Higher resin content. Knots in species like pine and spruce contain concentrated resin. At sauna temperatures, this resin softens and can seep to the surface, creating a sticky, extremely hot spot. Resin has a thermal conductivity higher than air-filled wood and, when liquid, makes direct thermal contact with skin far more efficiently than the rough, porous surface of clear wood.
3. Cross-grain orientation. Wood has anisotropic thermal conductivity. It conducts heat roughly 2-2.5 times faster along the grain than across the grain. Knots have their grain oriented perpendicular to the board surface, meaning heat conducts from the bench interior to the surface more efficiently through a knot than through the surrounding cross-grain wood.

The combined effect is that a knot on a sauna bench can have an effective thermal conductivity 2-4 times higher than the adjacent clear wood. At 90°C, clear cedar feels like 43°C to the touch (as calculated above), while a dense, resinous knot in the same board might produce a contact temperature of 55-65°C. Above the pain threshold and potentially causing a minor burn with prolonged contact.

This is why sauna-grade wood is sold specifically as “clear” or “knotless” grade. It isn’t an aesthetic preference. It is a thermal safety specification.

What Materials Should You Use for Each Sauna Surface?

Use low-conductivity, knotless wood (cedar, aspen, abachi) for all skin-contact surfaces like benches and backrests, ensure zero exposed metal hardware, and reserve higher-conductivity materials only for non-contact surfaces like walls and floors.

Benches and Backrests

These are primary contact surfaces where skin presses against the material for extended periods (10-20 minutes continuously).

Recommended: Western red cedar, aspen, abachi, thermally modified spruce. All have k below 0.12 W/m-K and thermal effusivity below 280 J/m^2-K-s^0.5.

Acceptable: Spruce (clear grade), hemlock. Slightly higher conductivity but still comfortable at standard sauna temperatures up to 85-90°C.

Not recommended: Pine (resin risk at sauna temperatures), alder (borderline conductivity), any hardwood with density above 550 kg/m^3.

Board thickness for benches should be 22-28 mm. Thicker boards have more thermal mass, which means they stay hot longer between sessions but take longer to reach operating temperature. The thickness doesn’t affect the initial contact feel (which depends on surface properties, not bulk thickness), but very thin boards (less than 15 mm) can flex under load and may not provide adequate structural support.

Walls and Ceiling Panels

Walls and ceiling aren’t primary contact surfaces (bathers don’t press bare skin against them for extended periods), so thermal conductivity is less critical than for benches. However, the material still matters:

• Backrest panels on walls should use the same species as the bench. These are contact surfaces.
• General wall paneling can use any common softwood, including pine, spruce, or cedar. Pine is acceptable on walls because the resin doesn’t contact skin directly and typically stabilizes after the first few sessions.
• Ceiling panels should avoid very resinous species, as resin at ceiling temperatures (100-110°C) tends to drip.

Headrests

Headrests contact the back of the head and neck, where skin is sensitive and hair is thin. They should use the same low-conductivity species recommended for benches. Many bathers prefer abachi for headrests because of its exceptionally low effusivity.

Floor

The sauna floor operates at a much lower temperature (25-35°C) than the benches and walls, so thermal conductivity isn’t a comfort concern. Floor material selection is driven by moisture resistance, slip resistance, and durability rather than thermal properties. Common choices include:

• Concrete with tile (functional, easy to clean, requires drain).
• Pressure-treated wood (durable but higher conductivity, acceptable since floor temperature is low).
• Cedar duckboards over concrete (combines drainage with a traditional aesthetic).

Hardware and Fixtures

Every piece of metal in the sauna interior is a potential burn hazard. The design imperative is simple: no metal should be accessible to bare skin contact.

• Bench screws should be installed from below (through the support structure into the bench boards) so that screw heads are on the underside, not the sitting surface.
• Where screws must be on the contact surface, they should be countersunk and plugged with wood plugs of the same species.
• Hinges, hooks, and brackets should be recessed into the wood or positioned where skin contact is impossible.
• Thermometer and hygrometer housings should be wood, not metal.
• Light fixture covers should be wood or heat-resistant glass, never metal.

The only exception is the heater itself, which is expected to be hot and should be protected by a railing or guard that prevents accidental contact.

How Does Thermal Modification Reduce Wood Conductivity?

Thermal modification heats wood to 180-230°C in a low-oxygen atmosphere, decomposing hemicellulose to reduce density and create micro-voids. Lowering thermal conductivity by 10-25% so that thermally modified spruce (0.09 W/m-K) matches or exceeds abachi’s performance.

Thermal modification (also called thermo-treatment or heat treatment) is a process in which wood is heated to 180-230°C in a low-oxygen atmosphere for several hours. This causes permanent chemical and physical changes:

1. Hemicellulose decomposition. The hemicellulose fraction of the cell walls breaks down, reducing the wood’s ability to absorb and hold moisture. Equilibrium moisture content drops by 40-60%.
2. Density reduction. The mass loss from hemicellulose decomposition (typically 5-15%) reduces density, which lowers thermal conductivity.
3. Cell wall porosity increase. The decomposition creates additional micro-voids in the cell walls, further reducing thermal conductivity.

The net result is a wood with thermal conductivity 10-25% lower than the same species in its untreated state. Thermally modified spruce, for example, drops from approximately 0.12 W/m-K to approximately 0.09 W/m-K. Matching or exceeding abachi’s performance.

Additional benefits include improved dimensional stability (less expansion and contraction with humidity changes), enhanced rot resistance, and a rich brown coloration that many builders find attractive.

Drawbacks include increased brittleness (the wood becomes more prone to splitting if fasteners aren’t pre-drilled) and reduced bending strength (important for structural applications, less relevant for paneling and bench boards).

Thermally modified wood is increasingly popular for sauna interiors, particularly in Europe where species like spruce and pine are locally available and can be modified at lower cost than importing tropical species like abachi or cedar.

How Do You Calculate Heat Flux for a Sauna Bench-Skin Contact Scenario?

The initial contact heat flux is determined by the effusivities of both materials and decreases over time as the surface equilibrates, settling into a steady-state flux of approximately 285 W/m^2 for cedar at 90°C. Comparable to direct summer sunlight and well below the pain threshold.

For those who want to calculate the actual heat transfer rate for a specific wood-skin contact scenario, the complete surface heat flux equation at the instant of contact is:

q_0 = (T_bench - T_skin) * e_bench * e_skin / (e_bench + e_skin) * (1 / sqrt(pi * t))

Where t is the time since contact began (seconds) and other variables are as defined above.

This equation shows that the heat flux is highest at the instant of contact (t approaches zero, q approaches infinity in the theoretical limit) and decreases over time as the bench surface cools locally and the skin surface warms. In practice, the contact temperature stabilizes within 1-2 seconds for low-effusivity materials like wood.

For a sustained contact scenario (sitting on a bench for 10 minutes), the initial transient gives way to a quasi-steady-state condition where the heat flux is determined by the bulk thermal conductivity of the wood and the temperature gradient through the board thickness. At this point, the low thermal conductivity of the wood (not just the surface effusivity) maintains comfort by limiting the sustained heat transfer rate to approximately:

q_steady = k * (T_bench_interior - T_skin) / L_effective

For cedar at 90°C interior, 33°C skin, through 22 mm of board:

q_steady = 0.11 * (90 - 33) / 0.022 = 285 W/m^2

This is a moderate heat flux. Comparable to the solar radiation on a hot summer day (approximately 300-400 W/m^2 direct sunlight on exposed skin). It is sufficient to feel warm but well below the approximately 5,000 W/m^2 threshold that causes pain within seconds.

For a complete comparison of sauna wood species including durability, appearance, cost, and availability, see our best sauna wood guide.

What Is the Bottom Line on Sauna Wood Thermal Conductivity?

Material selection for sauna contact surfaces is a thermal safety decision dictated by physics. Low-density softwoods like cedar (k = 0.11 W/m-K) keep contact temperatures below the 48°C pain threshold, while metals at the same temperature cause instant burns.

The reason you can sit comfortably on a 90°C wooden bench isn’t magic (it is the physics of thermal conductivity and thermal effusivity. Low-density softwoods like cedar (k = 0.11 W/m-K) transfer heat to your skin slowly enough that the contact interface temperature stays below the pain threshold of 48°C. Metals at the same temperature produce contact temperatures above 80°C and cause instant burns. Choose bench wood by density and conductivity: cedar, aspen, and abachi are excellent. Pine is risky due to resin. Hardwoods above 550 kg/m^3 are uncomfortable. Use clear (knotless) grade because knots are denser and more conductive, creating localized hot spots. Ensure zero exposed metal on any contact surface. The physics is non-negotiable) material selection for sauna contact surfaces is fundamentally a thermal safety decision.