Best Sauna Wood and Timber: Which Type to Use and Why

Choosing the right sauna wood is the decision that quietly shapes how comfortable, durable and low-maintenance your sauna turns out to be. The timber has to cope with brutal swings from cold to over 80°C, tolerate steam without warping, and stay safe to touch when the room is hot. Get it right and the wood lasts decades and feels good against bare skin; get it wrong and you end up with sticky resin, cracked boards or a bench too hot to sit on. This guide compares the main sauna woods and explains which to use where, from the benches you touch to the outer cladding facing British weather.

Three different jobs, three different woods

The single most useful thing to understand is that a sauna is not built from one timber. Different parts have different demands, and matching the wood to the job is what separates a good build from a frustrating one.

• Benches and backrests are touched by bare skin, so they must stay relatively cool, be free of resin, and have no splinter-prone grain.
• Interior walls and ceiling need to handle heat and steam without bleeding resin or warping.
• Exterior cladding on an outdoor sauna faces rain, frost and UV, so durability and weather resistance matter more than feel.

Use one wood for all three and you compromise somewhere. The best builds mix them.

The best woods for benches and interior

Aspen (and thermo-aspen) is the standout choice for benches. It is pale, almost knot-free, low in resin, and crucially it does not get as hot to the touch as denser woods, so it stays comfortable against skin. It is also low-allergen. Thermally modified aspen adds even more stability. If you only upgrade one part of your sauna, make it aspen benches.

Alder is the classic Finnish sauna species. It has a warm reddish tone, stays pleasantly cool to sit on, and is traditional for a reason. It suits both benches and interior lining and is a safe, authentic pick.

Western red cedar is the premium all-rounder. It is naturally aromatic, resists moisture and decay, and is dimensionally stable so it barely moves through heat cycles. It is soft and costly, and the scent is a love-or-tolerate thing, but for a high-end interior it is hard to beat.

Nordic spruce is the traditional, affordable interior choice. It performs well in both dry and steam saunas and gives that pale Scandinavian look. It is the sensible budget option for walls and ceilings.

Why thermowood is worth understanding

You will see “thermowood” or “thermally modified” timber everywhere, and it is not a species but a treatment. The wood is heated to roughly 185 to 215°C in a low-oxygen kiln for a day or more, which drives out resins and changes its structure so it absorbs far less moisture, on the order of 40 to 60 per cent less than untreated wood. In practice that means it swells and shrinks much less through the sauna’s cold-to-hot cycles, so it stays flat and stable and needs little maintenance.

Thermo-modified pine, spruce and aspen give you the low-cost benefits of common softwoods without the resin problems that make raw versions unsuitable inside a sauna. It is the reason a thermowood interior can rival cedar for stability at a lower price. Manufacturers such as Thermory document the process, and the International ThermoWood Association sets the standards behind it.

The best wood for outdoor cladding

Cladding is a different problem: here you want weather resistance, not skin comfort. For a UK outdoor sauna the strong choices are thermally modified timber, Siberian larch and western red cedar, all of which are naturally durable and cope well with rain and frost. Larch and thermowood in particular are low-maintenance and stand up to the British climate. The important rule is that a wood good for the outside is not automatically good for the inside, which brings us to what to avoid.

Woods to avoid inside a sauna

Keep these out of the hot room, however common or cheap they are:

• Standard (untreated) pine, which bleeds resin at sauna temperatures and turns sticky.
• Siberian larch, excellent as exterior cladding but too resinous and dense for interior benches.
• Douglas fir, which likewise exudes resin when hot.

Resin bleed is the recurring theme. It creates sticky surfaces, can burn skin on contact with a hot spot, and ruins the clean feel of a sauna. Any resinous softwood belongs on the outside or in a thermally modified form, never as a raw interior bench.

Putting it together

A well-specified sauna often ends up with thermo-aspen or alder benches for comfort, spruce or thermowood lining the walls and ceiling, and thermowood, larch or cedar cladding on the outside of a garden cabin. Match each timber to its job rather than hunting for a single “best” wood. If you want a side-by-side of the interior options, see our sauna wood types compared, and if you are building from scratch, our how to build a sauna guide covers where each board goes. For heat source choices, the wood-fired sauna buying guide pairs naturally with the timber decision.

Frequently asked questions

What is the best wood for a sauna? There is no single best wood; the right choice depends on the job. Aspen or thermo-aspen is best for benches because it stays cool to the touch and is resin-free. Alder and western red cedar make excellent interiors, spruce is the affordable traditional lining, and thermowood or larch is best for outdoor cladding. Good saunas mix timbers rather than using one throughout.

Is cedar or thermowood better for a sauna? Both are excellent and stable. Western red cedar is aromatic, naturally moisture-resistant and premium-priced, while thermowood is thermally treated softwood that absorbs far less moisture and stays very stable at a lower cost. Choose cedar for the scent and top-end feel, thermowood for durability and value. Many builds use thermowood interiors and reserve cedar for a feature.

What wood should you avoid in a sauna? Avoid resinous softwoods inside the hot room: standard untreated pine, Siberian larch and Douglas fir all bleed sticky resin at sauna temperatures and can burn skin on hot spots. Larch is fine as exterior cladding but not for interior benches. If you want to use a softwood inside, choose a thermally modified version, which has had the resin driven out.

What is the best wood for sauna benches? Aspen, ideally thermo-aspen, is the top choice for benches because it is pale, knot-free, low in resin and does not get as hot to the touch as denser woods, so it stays comfortable against bare skin. Alder is the classic Finnish alternative and also stays cool. Both are far better underneath you than any resinous softwood.

What wood is best for outdoor sauna cladding in the UK? For UK weather, thermally modified timber, Siberian larch and western red cedar are the strongest cladding choices, all naturally durable and low-maintenance against rain, frost and UV. Larch and thermowood are especially hard-wearing. Remember that a good exterior cladding is not necessarily suitable for the interior, where skin comfort and resin matter more.

Does thermowood need treating or maintenance? Thermowood is prized precisely because it needs very little maintenance inside a sauna, since the treatment makes it dimensionally stable and resin-free. Exterior thermowood cladding will silver over time with UV exposure and can be oiled to keep its colour if you prefer, but structurally it is designed to be low-upkeep and weather-resistant.