Cedar vs hemlock vs spruce

The short version: cedar outdoors, hemlock indoors, spruce where budget dominates and the room is dry. The interesting part is why, and where the conventional answer is wrong.

Why cedar wins outdoors

Western red cedar contains natural compounds that resist decay and insects. That is the whole argument, and outdoors it is decisive: a structure that cycles between hot-and-wet and cold-and-frozen, exposed to rain and UV, is exactly the environment that destroys timber without that resistance.

Hemlock outdoors will need treatment, maintenance and vigilance, and will still have a shorter life. For an outdoor barrel or cube, the cedar premium is the cheapest insurance available in this category and we would pay it every time.

Why hemlock is fine indoors — and often better

Inside a conditioned room, cedar's rot resistance is solving a problem that does not exist. There is no rain, no freeze-thaw, no UV. What is left is aroma and appearance, and hemlock's near-total lack of scent is a genuine advantage for anyone who finds cedar's aroma strong — which, in an enclosed cabin at temperature, some people do.

This is the one place the conventional wisdom is wrong. Paying a cedar premium for an indoor infrared cabin in a dry basement buys you a smell you may or may not want. Hemlock is the sensible default there, which is why almost every indoor infrared cabin uses it.

Where spruce belongs, and its one real caveat

Spruce is the budget option and it works, with one caveat that matters: it has higher resin content than cedar or hemlock, and resin can weep at sauna temperatures. Where that happens it is sticky, it marks skin and towels, and it is not pleasant on a bench.

The practical rule: spruce is acceptable for walls and ceilings in a budget indoor build. Do not use it for benches. Bench timber is what your skin touches at 190 °F, and that is precisely where resin bleed is worst.

Clear versus rustic grade

Grade is a separate axis from species, and listings blur the two. Clear grade is knot-free and dimensionally stable. Rustic is the same species with knots left in, at considerably lower cost.

Knots move as timber cycles between hot-dry and cool-damp. Over years that means checking, occasional sap bleed and eventually splits. Rustic grade is not a disaster — it is a maintenance commitment, and it is a perfectly reasonable one to make on walls. On benches it is a false economy.

Watch the wording on listings: "rustic cedar" is a grade claim wearing a species word. It tells you the species and it tells you there are knots. Both are useful; neither is the other.

What about thermal conductivity?

All three of these are softwoods with broadly similar and low thermal conductivity, which is why they are used and why steel and stone benches are not. The differences between them are small enough that they should not drive your decision — rot resistance, resin and aroma are the axes that actually matter.

What does matter thermally is wall insulation, which is a separate component entirely and is rarely published in this category. A well-insulated hemlock room outperforms an uninsulated cedar one comfortably. See best saunas for cold climates.

Maintenance by species

1. 1Cedar outdoors: it will silver with UV exposure, which is cosmetic rather than structural. A UV-protective exterior finish keeps the color; leaving it to gray is a legitimate choice. Never seal the *interior* — sauna interiors stay bare so the wood can breathe.
2. 2Hemlock indoors: essentially nothing. Wipe benches down, let the room dry after use.
3. 3Spruce indoors: watch for resin bleed in the first year, particularly on anything near the heater. Scrape rather than solvent-clean.
4. 4All species: never varnish or seal the inside of a sauna. Sealed wood traps moisture and gets unpleasantly hot to touch.

Every pick on this page comes from published specifications, not from hardware we own. Read how we pick — including what spec-led ranking cannot tell you.