What these sources do and do not cover
The figures below come from the Wood Database, a reference site for lumber properties, and from Wikipedia for thermally modified wood. None of the pages we loaded rate a wood for sauna use. We found no sourced data on how a wood feels on skin at sauna temperatures, so we make no claim about that. What we can compare is rot resistance, density, hardness, and shrinkage, which matter for an outdoor building.
Species names matter. "Spruce," "hemlock," and "pine" cover many trees. The rows below are for the species the Wood Database lists: Sitka spruce, western hemlock, and eastern white pine. Ask your sauna maker which species it uses.
Side-by-side table
Density is average dried weight at 12 percent moisture. Janka hardness is a standard test of how hard a wood is. Lower means softer. Shrinkage is the total change from green to oven-dry, so a lower number means more stable.
| Wood | Decay resistance | Weight (lb per cu ft) | Janka (lbf) | Volumetric shrinkage |
|---|---|---|---|---|
| Western red cedar | Durable to very durable | 23.0 | 350 | 6.8% |
| Northern white cedar | Durable to very durable | 22.0 | 320 | 7.2% |
| Eastern white pine | Moderate to low | 25 | 380 | 8.2% |
| Quaking aspen | Non-durable | 26 | 350 | 11.5% |
| Sitka spruce | Slightly resistant to non-resistant | 27 | 510 | 11.5% |
| Western hemlock | Non-durable | 29 | 540 | 12.4% |
Sources: Western red cedar, northern white cedar, eastern white pine, quaking aspen, Sitka spruce, and western hemlock. Thermally modified wood is covered below because its values depend on the base species and treatment.
Red cedar
The Wood Database rates western red cedar as durable to very durable against decay. It is easy to work and has a strong, lingering scent when cut. It is soft and dents easily, and iron-based fasteners can stain it when it is wet. That staining is a reason to ask your maker what fasteners it uses (Wood Database).
White cedar
Northern white cedar is also rated durable to very durable, and resists termites and powder post beetles. It is one of the lightest and softest commercial U.S. woods, and it holds screws poorly. The page warns that the wood contains plicatic acid, which can irritate skin and, with long dust exposure, cause occupational asthma, so wear a dust mask when sanding or sawing it (Wood Database).
Thermally modified wood (ThermoWood and similar)
Thermal modification heats lumber above 180 degrees Celsius (356 degrees Fahrenheit) in an oxygen-free atmosphere. Wikipedia reports these effects (Wikipedia: Thermally modified wood):
- A softwood rated non-durable (class 5) can reach durability classes 1 to 3 under the European standard EN 350-2.
- Shrinking and swelling drop by 50 to 90 percent.
- Bending strength falls by up to 30 percent, and toughness and abrasion resistance also suffer.
- Sauna lumber was an original use, mainly in Finland.
Wikipedia is not a primary source, so check the treater's data sheet for the grade and base species. We also found no independent measurement of how it feels at sauna temperatures. Claims that it stays cooler to the touch come from sellers, not from tests we could verify.
Spruce
The Wood Database lists Sitka spruce heartwood as slightly resistant to non-resistant against decay. That is a weak rating for exterior walls. It is easy to work if knot-free. Its end grain has medium-sized resin canals (Wood Database). Sitka is one spruce. Other spruces differ.
Hemlock
Western hemlock is rated non-durable and susceptible to insect attack. It is harder and heavier than the cedars, and it shrinks more. The page lists resin canals as absent and notes it can cause skin and respiratory irritation (Wood Database). The page does not mention saunas, so we cannot back a sauna claim for it.
Pine
Eastern white pine is rated moderate to low in decay resistance. It is soft, easy to work, and has a faint resinous odor while being worked. Its end grain shows large, numerous resin canals (Wood Database). Pine is a large group, and other pines vary.
Aspen
Quaking aspen is rated non-durable and susceptible to insect attack. It has no characteristic odor listed, and it tends to warp as it dries. It also holds nails poorly and needs sharp cutters to avoid a fuzzy surface (Wood Database). A comment under the page says aspen is used for sauna benches in Scandinavia, but that is a reader comment and we do not treat it as a source.
How to use this table
- Exterior walls and roof. Pick from the top of the decay column: western red cedar or northern white cedar, or a thermally modified wood with documented durability. Rows rated non-durable are a weak fit for exposed boards.
- Interior walls and benches. The sources give no sauna-specific rating. Look at stability and workability, and ask your maker how it protects interior boards.
- Species. Ask for the exact species, not the family.
- Moisture. Wood moves with humidity. A lower shrinkage figure means less movement from wet to dry, so cedar is the steadier choice in the table.
- Dust. Cut cedar with a mask on and read the Wood Database safety note on plicatic acid.