If you're shopping for a sauna, the timber it's built from matters more than almost anything else. It decides how the sauna feels to sit in, how it copes with years of heat and steam, how safe it is to breathe in, and how long it lasts in a New Zealand backyard.

There's a lot of marketing noise out there, so here's a straight, fair comparison of the three woods you'll actually come across — Western Red Cedar, untreated (ordinary) spruce or pine, and thermally-treated Finnish spruce — and an honest explanation of why we build Bayther saunas from the last one.

What actually makes a good sauna wood?

Before comparing species, it helps to know what you're comparing them against. A sauna is one of the harshest environments you can put timber in: repeated cycles of dry heat, humidity from water on the rocks, and big temperature swings. Good sauna timber needs to:

• Stay cool enough to touch. A low-density softwood doesn't store and conduct heat the way a dense hardwood does, so benches and walls stay comfortable against bare skin.
• Handle moisture without warping, cracking or rotting. Dimensional stability is everything — timber that swells and shrinks will eventually cup, split and loosen.
• Resist rot and insects over a long life outdoors.
• Be safe to breathe. No resins bleeding out under heat, no chemical treatments off-gassing, and ideally a gentle aroma.
• Be responsibly sourced and hold together structurally for decades.

Western Red Cedar

Cedar is the wood most people picture when they think “premium sauna,” and for good reason. It's a genuinely lovely material.

The good: Cedar is light and low-density, so it doesn't get scorching to the touch. It has natural oils that give it real rot and insect resistance without any treatment, a beautiful reddish grain, and that famous warm, spicy aroma.

The trade-offs to know: Cedar is a soft timber, so it dents and marks fairly easily. Over years of humid use it can absorb moisture, darken unevenly and lose some of that aroma. Most cedar is Western Red Cedar milled in the Pacific Northwest of North America — it's slow-growing, increasingly drawn from mature and old-growth forests, and shipped a very long way, which pushes both the price and the environmental footprint up. And that lovely aroma isn't for everyone: the same aromatic oils that smell so good can irritate some people who are sensitive or have respiratory issues like asthma.

Cedar isn't a bad choice — it's a beautiful one. It just comes with real compromises on cost, softness, sustainability and, for some people, that strong scent.

Untreated spruce or pine (the wood that often needs fumigating)

At the budget end you'll find saunas built from plain, untreated spruce or pine. Fresh untreated softwood is cheap and it's the traditional Nordic starting point — but on its own it's the weakest performer of the three.

The catch: Untreated softwood is hygroscopic, meaning it readily takes on and gives off moisture. In a sauna that means more swelling, shrinking, cupping and cracking over time, and far less natural resistance to rot and mould. Raw spruce and pine also carry more resin, which can weep out of knots under high heat.

There's a less-talked-about issue too. Ordinary sawn timber shipped across borders is a biosecurity risk — it can carry insects and pathogens — so it very often has to be fumigated with methyl bromide, a toxic gas, to meet import rules. That's an accepted practice for things like structural timber and cladding. But methyl bromide is a nasty chemical (it's ozone-depleting and hazardous to handle), and personally we don't love the idea of it anywhere near a space you sit and breathe deeply in. More on how we avoid that below.

Thermally-treated Finnish spruce (what we build with)

• It stops rot and insects. Thermal treatment breaks down the sugars and moisture-loving compounds that fungi and insects feed on, so the timber is naturally far more resistant to decay — without any preservatives.
• It's far more stable. Thermally-modified wood absorbs much less moisture, so it moves, swells and cracks dramatically less than untreated timber through years of heat-and-steam cycles.
• It improves the way it handles heat. The process lowers the wood's density and moisture content and drives off resin, so it heats evenly, stays comfortable to the touch and won't bleed sticky sap onto your skin.
• It's built to last longer. Independent testing consistently shows well-made thermally-treated spruce outlasting untreated softwood by a wide margin — which is a big part of why so many serious European sauna makers have moved to it.
• It looks the part. Thermal treatment gives spruce a warm, even, honeyed-brown tone right through the board.

Thermally-treated spruce is also the honest, traditional Nordic answer. Spruce (and pine) is what Finland — the home of the sauna — has always built with, and thermal modification is the modern refinement of that heritage rather than a departure from it.

Why Bayther chooses thermally-treated Finnish spruce

We've built and used a lot of saunas, and for New Zealand conditions this is the timber we keep coming back to. Here's what tips it for us — beyond the material itself.

Continuous lengths — no finger joints, no glues

Completely toxin-free — and we really mean it

It also means our timber never needs chemical fumigation. Heat treatment is a recognised alternative to methyl bromide under the international ISPM 15 phytosanitary standard — and our spruce has already been heated well beyond that threshold, so there's no biosecurity reason to gas it. We keep tight control over our logistics network to make sure our timber is never fumigated with methyl bromide or any other toxic chemical at any point on its journey to your backyard. Fumigation is fine for fence posts and cladding; it has no place in a sauna — and it has no place in ours.

A lighter, gentler aroma

Thermally-treated spruce has a soft, subtle, pleasantly woody scent rather than cedar's intense aromatic hit. If you love a big cedar smell, that's a point for cedar — but if you (or someone in your household) find strong scents overwhelming or have any respiratory sensitivity, the gentler aroma of thermo-spruce is a real advantage.

A more sustainable choice

European spruce is fast-growing, abundant and grown in well-managed, certified Nordic forests, so it regenerates far quicker than slow-growing Western Red Cedar drawn from mature North American forests. And because thermal modification uses only heat — no chemical preservatives — the finished timber is cleaner from forest to backyard.

Cedar vs untreated spruce vs thermally-treated spruce: side by side

Here's how the three woods compare across what actually matters in a sauna:

So, which wood should you choose?

Here's the fair summary:

• Cedar — beautiful, aromatic and naturally durable, but soft, expensive, less sustainable, and its strong scent doesn't suit everyone.
• Untreated spruce or pine — cheap and traditional, but the least stable and least durable, and the type most likely to have been chemically fumigated.
• Thermally-treated Finnish spruce — exceptionally stable, rot- and insect-resistant, longer-lasting, gently scented, sustainably grown and completely toxin-free.

Common questions about sauna wood

What is the best wood for a sauna?

The best sauna woods are stable, low-density softwoods that stay cool to the touch and resist rot — principally thermally-treated Finnish spruce and Western Red Cedar. Thermally-treated spruce is the most stable and longest-lasting, while cedar is prized for its aroma. Untreated spruce or pine is cheapest but the least durable.

Is spruce or cedar better for a sauna?

Both are good. Cedar offers a strong natural aroma and a classic look; thermally-treated spruce offers better dimensional stability, a longer life, a gentler scent, lower cost and a smaller environmental footprint. For New Zealand conditions, Bayther builds with thermally-treated Finnish spruce.

Is thermally-treated wood safe for a sauna?

Yes. Thermal modification uses only heat and steam — no chemicals or preservatives — so there is nothing to off-gas when the sauna heats up. It also removes resin, so the timber won't weep sap onto your skin.

Does sauna wood need to be treated with chemicals?

No. Good sauna timber should never be chemically treated. Ordinary imported softwood is often fumigated with methyl bromide to meet biosecurity rules, but thermally-treated spruce is heat-treated well beyond the ISPM 15 standard, so it needs no fumigation at all.

Bayther is New Zealand-owned and our saunas are designed for our climate. If you'd like to talk it through, feel free to get in touch with Tim — or explore the range to see our thermally-treated spruce barrels for yourself.